SEARCH

SEARCH BY CITATION

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Pilus fibre structure
  5. Tfp and type II secretion
  6. Social gliding motility in Myxobacteria
  7. Twitching motility
  8. Pilus retraction
  9. Is PilT a motor?
  10. A sensory role for type IV pili
  11. Che homologues
  12. Pathogenesis
  13. Summary
  14. Acknowledgements
  15. References

Type IV pili (Tfp) mediate the movement of bacteria over surfaces without the use of flagella. These movements are known as social gliding in Myxococcus xanthus and twitching in organisms such as Pseudomonas aeruginosa and Neisseria gonorrhoeae. Tfp are localized polarly. Type IV pilins have a signature N-terminal domain, which forms a coiled-coil with other monomer units to polymerize a pilus fibre. At least 10 more proteins at the base of the fibre are conserved; they are related to the type II secretion system. Movements produced by Tfp range from short, jerky displacements to lengthy, smooth ones. Tfp also participate in cell–cell interactions, pathogenesis, biofilm formation, natural DNA uptake, auto-aggregation of cells and development. What is the means by which Tfp bring about the movement of cells?


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Pilus fibre structure
  5. Tfp and type II secretion
  6. Social gliding motility in Myxobacteria
  7. Twitching motility
  8. Pilus retraction
  9. Is PilT a motor?
  10. A sensory role for type IV pili
  11. Che homologues
  12. Pathogenesis
  13. Summary
  14. Acknowledgements
  15. References

For three decades, polar pili have been implicated in twitching and some forms of gliding motility in a variety of Gram-negative bacteria (Henrichsen, 1975; Kaiser, 1979). Remarkably, the pili borne by these motile bacteria are very similar to each other and have a common structure; they are known as type IV pili, abbreviated Tfp. They are distinguished from other pili by their polar location, sequence conservation of their pilins and pilus assembly proteins and their role in bacterial motility (Ottow, 1975; Strom and Lory, 1993). A diverse collection of reviews on Tfp and related systems can be found in Manning and Meyer (1997). The role of Tfp in motility has not been reviewed and is the subject of this paper. Particular attention is given to three organisms, including M. xanthus, for which there is extensive data on the genetics of motility and swarm patterns. In addition, P. aeruginosa and N. gonorrhoeae serve as model systems for Tfp biogenesis and structure.

Motility is based on Tfp in many bacteria. The well-studied M. xanthus, P. aeruginosa and N. gonorrhoeae have many properties in common. Recently, enteropathogenic Escherichia coli (EPEC; bundle-forming pili, BFP) and Synechocystis PCC6803 have been shown to have motility that depends on Tfp (Bieber et al., 1998; D. Bhaya and A. R. Grossman, unpublished). Bacteria whose motility is somewhat less well characterized and that have Tfp include N. meningitidis, Moraxella sp., Dichelobacter nodosus, Eikenella corrodens, Kingella denitrificans and Acinetobacter calcoaceticus (Henrichsen, 1983; Strom and Lory, 1993; Manning and Meyer, 1997). Henrichsen (1975; 1983) described still other bacteria with polarly localized pili (presumably type IV) and exhibiting twitching motility. Vibrio cholerae (toxin co-regulated pilus, TCP) and Branhamella catarrhalis have Tfp but lack reports of twitching (Strom and Lory, 1993; Manning and Meyer, 1997). Interestingly, E. coli K-12 contains a full complement of Tfp genes, but they are not expressed under laboratory conditions (Francetic et al., 1998).

Pilus fibre structure

  1. Top of page
  2. Abstract
  3. Introduction
  4. Pilus fibre structure
  5. Tfp and type II secretion
  6. Social gliding motility in Myxobacteria
  7. Twitching motility
  8. Pilus retraction
  9. Is PilT a motor?
  10. A sensory role for type IV pili
  11. Che homologues
  12. Pathogenesis
  13. Summary
  14. Acknowledgements
  15. References

The structure and dynamics of fibre assembly should have important implications for pilus function, if they are involved in cell movement. The Tfp fibre forms a three-layered helical structure of coiled α-helices surrounded by a β-sheet. These two inner layers are covered with the C-terminal regions of adjacent monomers. This structure is based on a 2.6 Å resolution X-ray crystal structure of the monomer protein pilin from N. gonorrhoeae (Parge et al., 1995). Anti-peptide antibodies have distinguished between buried and exposed regions of pilin and epitopes exposed only at the ends of the assembled fibre (Forest and Tainer, 1997). The N. gonorrhoeae fibre has five pilin monomers per helical turn, a rise of about 4 nm per monomer, and an outer diameter of about 6 nm. The helix parameters and diameter also agree with fibre diffraction on P. aeruginosa Tfp, which suggests that the proposed structure is representative of the Tfp in many organisms. The N-terminal amino acid sequence, which is highly conserved from one bacterial species to another, forms the innermost coil of staggered α-helices. This inner layer of the fibre has hydrophobically packed parallel α-helices. This hydrophobic packing and the flexibility of α-helices may allow pili to bend and to adopt twisted or bundled conformations. The middle layer of β-sheet is continuous from one pilin monomer to the next in the sense that any cross-section of the fibre would cut through 25 β-strands, and the β-sheet hydrogen bonding may provide much of the mechanical stability required for a fibre whose length of up to 4 μm approaches 600 times its diameter of 6 nm. It is generally believed that the pilus is assembled from its base, as a pool of pilin is found in the cell membrane. As there is no channel in the centre of the fibre, assembly from the tip is excluded.

Tfp and type II secretion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Pilus fibre structure
  5. Tfp and type II secretion
  6. Social gliding motility in Myxobacteria
  7. Twitching motility
  8. Pilus retraction
  9. Is PilT a motor?
  10. A sensory role for type IV pili
  11. Che homologues
  12. Pathogenesis
  13. Summary
  14. Acknowledgements
  15. References

The type II secretion system is related to Tfp; they share about 10 protein homologues (Pugsley, 1993; Russel, 1998). This may be significant, as there is a parallel relationship between flagellar motility and type III secretion systems. Proteins secreted by the type II system cross the inner membrane in a sec-dependent fashion. After folding, such proteins pass through a gated channel in the outer membrane, called a secretin. Mutants defective in type II secretion accumulate secretory proteins in the periplasm. Despite the sequence similarities between Tfp and the type II secretion system, substrate proteins follow different pathways as they cross the inner membrane. The unique prepilin signal sequence is neither necessary nor sufficient for pilin entry into the cytoplasmic membrane, although that sequence is required for pilus assembly (Pugsley, 1993; Strom and Lory, 1993). The prepilin signal sequence is cleaved on the cytoplasmic face of the membrane by a unique bifunctional peptidase, PilD, which also methylates the newly exposed N-terminal phenyalanine (Pepe and Lory, 1998). In V. cholerae, there is direct evidence that pilin secretion is sec independent (Kaufman et al., 1991). Additionally, none of the pil gene products is required for pilin insertion into the inner membrane. The absence of other requirements suggests that pilin may insert spontaneously into the membrane (Bernstein, 1998). Assembly of Tfp may be analogous to filamentous phage (Russel, 1998). It is thought that, in both cases, a helical polymer is polymerized from monomers in the inner membrane, and both polymers use an outer membrane secretin.

Pseudopilins, proteins homologous to type IV pilins, are used in type II secretion and in the related DNA uptake systems (Manning and Meyer, 1997; Chung et al., 1998). Pseudopilins produce no extracellular fibre, although pseudopilin monomers do associate with each other (Pugsley, 1996). Pseudopilins, like pilins, are processed by PilD or a homologue. It has been suggested that pseudopilins form a structure similar to Tfp between the inner and outer membrane involved in macromolecular transport (Pugsley, 1993; Mattick and Alm, 1995).

Social gliding motility in Myxobacteria

  1. Top of page
  2. Abstract
  3. Introduction
  4. Pilus fibre structure
  5. Tfp and type II secretion
  6. Social gliding motility in Myxobacteria
  7. Twitching motility
  8. Pilus retraction
  9. Is PilT a motor?
  10. A sensory role for type IV pili
  11. Che homologues
  12. Pathogenesis
  13. Summary
  14. Acknowledgements
  15. References

Myxobacteria move by gliding: gliding occurs at an interface (solid–liquid, solid–air, solid–solid) and is characterized by smooth motions in the interfacial plane that are directed along the long axis of the cell. Periodically, gliding myxobacterial cells reverse their direction. Gliding motility is found in a variety of bacteria, including the cytophaga, some cyanobacteria and mycoplasmas, not all of which have Tfp. The mechanism of gliding has not been elucidated. However, interesting possibilities have been proposed (Freese et al., 1997; Hoiczyk and Baumeister, 1998).

Jonathan Hodgkin made the seminal discovery in 1979 that two distinct genetic systems controlled gliding in M. xanthus; he called these gene sets adventurous (A) and social (S) motility genes (Hodgkin and Kaiser, 1979). A mutation in any gene of system A inactivates A-motility only; S-motility remains. Similarly, a mutation in any gene of system S inactivates S-motility, but A-motility remains. Fifty AS double mutants were constructed, involving 21 different A genes and nine different S genes, and all failed to swarm; they produced small dome-shaped colonies with sharp edges. Single AS cells also failed to move. Ten one-step non-swarming mutants mapped to the mglA (mutual gliding) gene, which encodes a Ras-like GTPase and is required for both A- and S-motility (Hodgkin and Kaiser, 1979; Hartzell, 1997).

A- and S-motility produce characteristically different swarm patterns at the colony edge, which arise from differences in cell behaviour. A-motile cells can move when they are far apart; S-motile cells cannot. Nevertheless, in both systems, most cells move in groups, and the rate of expansion of a circular swarm depends on the cell density (Kaiser and Crosby, 1983). The rate of expansion of an A-motile swarm (an A+S strain) responds sharply to changes at relatively low cell densities. In an S-motile swarm (an AS+ strain), the expansion rate increases slowly in the low cell density range and continues to change at densities that are 10-fold higher than the saturating density for A-motile swarms. Wild-type cells combine the cell density responses of A- and S-motility. The maximum rate of expansion in A+S+ cells (1.6 μm min−1) somewhat exceeds the sum of the maximum rates of A-motility and S-motility (0.5 and 0.6 μm min−1 respectively). Another difference between these systems is their response to changes in agar and nutrient concentrations (Shi and Zusman, 1993).

S-motility requires Tfp, but A-motility does not (Kaiser, 1979; Wu and Kaiser, 1995). Cells usually have their pili at one pole only and usually in a tuft of two to eight fibres. S-motility is lost when pili are shaved off by violent shear and reappears when the pili grow back (Rosenbluh and Eisenbach, 1992). S-motility depends strongly on interactions between cells. S-motile cells do not move at all when totally isolated (Kaiser and Crosby, 1983). Yet, individual cells that are separated by 1 or 2 μm (an average pilus length) from their nearest neighbour can move. S-motile cells swarm at their highest rate when the average cell–cell distance is less than about 1.5 μm. Apparently, cell bodies do not have to touch, although most cells are found in contact with others. At the advancing edge of an S-motile swarm, there are spearhead-like clusters of 50 or more cells, but no single cells (Fig. 1). Cells within a spearhead are found to be highly aligned end to end and side by side (Fig. 1B). Non-swarming M. xanthus strains, either AS or mgl, are unable to form fruiting bodies. Most A and S single mutants have defects in fruiting body development. Abnormal development is a very sensitive indicator of abnormal motility (Hodgkin and Kaiser, 1979; Wu et al., 1998).

image

Figure 1. . Arrangement of social gliding motility flares and cells. A. Phase-contrast microgragh of an S-motile colony edge. B. Fluorescent microgragh of image (A). Green fluorescent protein-labelled cells were mixed 1:10 with isogenic non-fluorescent cells. These cells are AS+; there are no isolated cells. M. xanthus cells are approximately 6 μm long, which is about 10 times their width. Note the highly ordered arrangement of cells. Even around curves, cells remain highly aligned end to end and side by side.

Download figure to PowerPoint

David Morandi (unpublished) carried out an extensive screen for S-motility mutants. S-motile (AS+) colonies are flat, and their spreading edges are smoothly serrated. From an AS+ strain, he isolated 130 AS mutants identified by their smooth, dome-shaped colonies with sharp non-serrated edges. He mapped many of these S mutations by linking insertions of transposon Tn5 to them. In all, about 160 S-motility mutants were generated by UV or by chemical mutagens, ensuring a broad spectrum of mutations. S-motility mutants are divided into three groups called tgl, pil (formerly sgl ) and dsp.

tgl (transient gliding) mutants lack S-motility and pili, but can be ‘stimulated’ transiently for both these defects when allowed to contact tgl+ cells (Kaiser, 1979; Wall and Kaiser, 1998). Stimulation is phenotypical only; cells remain genetically tgl. All seven point mutants map to a unique tgl locus (Rodriguez-Soto and Kaiser, 1997; D. Wall and D. Kaiser, unpublished). A tgl deletion mutant has the same phenotype as the point mutants, including its ability to be stimulated. In addition to tgl, five types of A-motility mutants are stimulatable, showing that stimulation is a property of both systems. The tgl gene product contains a type II signal sequence, suggesting that it is a lipoprotein. After the signal sequence, there are six tandem, but degenerate, tetratricopeptide repeats (TPRs; 34 amino acids per repeat). TPR motifs are involved in protein–protein interactions. Recent evidence suggests that Tgl stimulation involves an end-to-end interaction between donor and recipient cells (Wall and Kaiser, 1998). These end-to-end interactions are likely to be important because pili, Pil proteins and, hence, Tgl activity are located at the cell poles.

The largest group of S-motility genes are pil genes. Over 100 mutations have been mapped to a pil cluster that contains 17 genes, Fig. 2 (Wu and Kaiser, 1995; Wu et al., 1997; 1998; Wall et al., 1999; D. Wall and D. Kaiser, unpublished). Products of 14 of these genes are roughly 30% identical and 50% similar to proteins in P. aeruginosa and N. gonorrhoeae, including type IV pilin and pilus assembly proteins. When possible, the M. xanthus genes have been named after their homologues in P. aeruginosa and N. gonorrhoeae (e.g. pilQ in M. xanthus was formerly sglA). Within the Myxococcus cluster, pilG, pilH and pilI have no known homologues in other Tfp systems, yet pilH encodes an ABC transporter homologue with an ATP-binding cassette. To determine null phenotypes, in frame deletions and transposon insertions have been constructed for many of the pil genes (Fig. 2). Mutations in all but three genes abolish S-motility. Deletions or insertions in two of the genes, pilS and pilS2, which are homologous to sensor histidine kinases, have no apparent S-motility defect, although PilS is a negative regulator of pilA expression (Wu and Kaiser, 1997). The pilR2 gene has not yet been mutated. Mutations in all the other S-motility genes, except pilT, also abolish the production of pili. Most of the known S-motility genes are represented five or more times in the library of mutants (for examples, see Fig. 2). Mutational analysis and sequencing left and right of the pil cluster revealed no other pil homologues or S-motility genes.

image

Figure 2. . Genetic map of the pil cluster in M. xanthus. All 17 ORFs are transcribed from the left to the right. Most of these pil genes are named after their homologues in P. aeruginosa. The exceptions are pilG, H and I, which are unique to M. xanthus. The location of several Tn5 insertions is indicated by downward-pointing triangles and a four-digit Ω number. In frame deletion mutants are indicated by ▵ symbols below their corresponding gene. Filled-in triangles indicate that S-motility is inactivated by these gene disruptions, while open symbols indicates an S+ phenotype. The rough map position of many S-motility point mutations is shown. Numbers indicate the original DK strain that the mutation was isolated in and, hence, their allele name.

Download figure to PowerPoint

The third group of S-motility mutants is the dsp. Of the 21 dsp mutants (D. Morandi, unpublished), all map near Tn5Ω1407 (not located in the region shown in Fig. 2), and all retain their Tfp. Transductional linkage within this group suggests an approximately 10–15 kb dsp region. Unlike wild-type cells, the dsp mutants grow as dispersed cultures in liquid medium; they neither clump nor (auto-) aggregate, even though they are piliated (pil+). Auto-aggregation of cells in suspension is common for bacteria that have Tfp (Manning and Meyer, 1997; Wu et al., 1997; Bieber et al., 1998). Adsp double mutants are defective in A- and S-motility but, after prolonged incubation, they produce a narrow fringe around the colony, as if there were a limited or conditional motility associated with dsp. Double mutants dsp pilT, still pil+, show a sharp colony edge and hence lack S-motility (Wu et al., 1997). dsp mutants are depleted, but not totally lacking, in extracellular fibrils (Dana and Shimkets, 1993; Chang and Dworkin, 1994). Fibrils are peritrichous filaments composed of approximately equal amounts of polysaccharide and protein. It has been hypothesized that fibrils are involved in S-motility, perhaps by helping to hold cells together. However, for two reasons the relationship between S-motility and fibrils might be indirect. First, some mutants that are defective in fibril production have, in fact, been shown to retain S-motility (Chang and Dworkin, 1994; Ramaswamy et al., 1997). Secondly, no S-motility gene has yet been found that encodes a structural part of a fibril.

Linked to the dsp region is a new S-motility gene, sglK (Weimer et al., 1998). This gene is homologous to members of the DnaK/Hsp70 molecular chaperone family. The phenotype of sglK mutants resembles that of dsp. sglK mutants are pil+, defective in fibril production and form fringed colonies. Five mutants in our library map to sglK and have similar phenotypes. Additionally, five mutants are linked to Tn5Ω3416, which are pil+ and have a temperature-sensitive (Ts) phenotype for S-motility. About 20 other S-motility mutants do not map to the described loci, suggesting additional S-motility genes. Eight of these 20 mutants are conditionally defective: they are S on an agar surface open to air, but are S+ when placed between agar and plastic, or agar and agar.

To summarize, it may help to think of S-motility as the product of a dependent pathway:

pilin synthesis [RIGHTWARDS ARROW] pilus assembly [RIGHTWARDS ARROW] social gliding and its control.

Blocks at any stage would yield an S mutant. The first stage (pilin synthesis) includes the control of pilA expression by PilS/PilR, a two-component system (Wu and Kaiser, 1997), then membrane insertion of a pilin monomer and, finally, processing by the bifunctional PilD peptidase/N-terminal methylase. The pilin subunits are then polymerized into a long helical filament, presumably growing out from the inner membrane (note that other pili, e.g. Pap-pili, polymerize from the outer membrane). Polymerization requires PilB, C, M, N, O, P and Q. The PilQ secretin forms a channel in the outer membrane, which is the portal for pilus export. And finally, PilT is necessary for pilus function in social gliding. Diagrams of the structure of Tfp fibres and structural models have been published previously (Mattick and Alm, 1995; Forest and Tainer, 1997; Manning and Meyer, 1997).

Twitching motility

  1. Top of page
  2. Abstract
  3. Introduction
  4. Pilus fibre structure
  5. Tfp and type II secretion
  6. Social gliding motility in Myxobacteria
  7. Twitching motility
  8. Pilus retraction
  9. Is PilT a motor?
  10. A sensory role for type IV pili
  11. Che homologues
  12. Pathogenesis
  13. Summary
  14. Acknowledgements
  15. References

The term twitching was introduced by Lautrop to describe a type of surface translocation that did not depend on flagella. Twitching movements of individual cells appear as small, intermittent jerks that often change the direction of movement (Henrichsen, 1972). Like social gliding, twitching depends on humid agar and requires a minimum number of cells, about 100. Unlike gliding, twitching is not necessarily directed along the long axis of the cell (Henrichsen, 1972; 1975; 1983). Henrichsen surveyed some 1000 bacterial strains representing more than 50 species, classifying their surface translocations into five categories, the principal ones being twitching and gliding. About 20 species, some with several strains, were found to twitch. Importantly, he discovered a strong correlation between twitching and polar pili. All strains of the same species with polar pili exhibited twitching. Conversely, twitching was not found in variants of the strains that had lost their polar pili under cultivation. Twitching was also not found in piliated strains with peritrichous pili and, therefore, presumably not type IV. One limitation on those arguments is that it is not possible to determine whether cocci have polar pili, as their poles are not obvious. From his studies, Henrichsen concluded that twitching depends on polar pili.

Sequence conservation between P. aeruginosa and M. xanthus Tfp, which are required for motility in both organisms, clearly connects twitching and social gliding (Wu and Kaiser, 1995; Mattick et al., 1996; Wall et al., 1999). Similarities in the motility pattern and cell behaviours of twitching and S-motility have been pointed out (Darzins, 1994; 1995). Recently, the conditions for twitching in P. aeruginosa have been optimized to give swarm rates severalfold faster than those of M. xanthus (Darzins, 1993; Mattick et al., 1996). Twitching motility, it should be noted, includes a broad spectrum of movements from extensive displacements to those barely resolvable from Brownian motion. In contrast to the spreading of P. aeruginosa, EPEC cultures exhibit a restricted form of twitching that is manifested as jerky movements of cells in an aggregate (Bieber et al., 1998). Henrichsen (1983) also described other pathogens that have spatially limited forms of twitching. Perhaps because P. aeruginosa and M. xanthus are common in the soil, where they forage for food, they may have evolved a more sophisticated form of Tfp motility, which gives cells a greater spatial range. For example, these bacteria use signal transduction proteins that are similar to those used in chemotactic responses (Darzins and Russell, 1997; Ward and Zusman, 1997). As yet, there is no evidence for che-like genes in EPEC, nor does N. gonorrhoeae have che genes in its genome, although it does have other two-component systems.

Several genetic screens for identifying Tfp components in P. aeruginosa have been carried out. Loss of pili has been detected by loss of twitching. On agar plates, twitching cells produce large, rough, flat colonies with serrated edges, while non-twitching mutants (Twt) produce small, smooth, dome-shaped colonies with sharp edges. Using this phenotype, Mattick et al. (1996) found 147 Twt among 13 000 Tn5-B21 mutants. This library was refined to about 60 mutants using a more stringent subagar twitching assay. Lory, Darzins and their colleagues have isolated pil mutants by selecting for resistance to pilus-specific phage and screening resistant mutants for Twt (Strom and Lory, 1993; Darzins and Russell, 1997). Together, their efforts have identified about 34 genes. These genes can be divided into four groups: (i) five transcriptional regulators (pilS, R, fimS, algR and rpoN ); (ii) eight che-like genes (pilG, H, I, J, K, L, chpA and B ); (iii) 19 Tfp biogenesis genes (pilA, B, C, D, E, F, M, N, O, P, Q, V, W, X, Y1, Y2, Z, fimT and U ); and (iv) two pilus function genes (pilT and pilU ).

The first proposal for Tfp function in motility was made by Henrichsen (1975; 1983). Because pili and an air–water interface are hydrophobic, while the cell surface and the aqueous ionic phase are hydrophilic, Henrichsen suggested that twitching was the result of a random alternation, driven by thermal motion, between hydrophilic surface interactions and hydrophobic surface interactions. This proposed mechanism may have difficulty with pilT mutants. These mutants neither glide nor twitch, yet they express structurally normal pili, presumably with normal hydrophobic/hydrophilic properties, that are inactive (Whitchurch et al., 1990; Wu et al., 1997; Bieber et al., 1998; Wolfgang et al., 1998a). The failure of pilT mutants to twitch or glide implies that structurally normal pili can be inactive. The PilT protein, not thought to be part of the pilus fibre, is essential for activity. A different mechanism was proposed by Bradley (1980): that Tfp can retract forcefully and, if a pilus extending from one cell were attached to another, the pull of retraction would propel a twitch (or glide). The retraction model has many adherents.

Type IV pili, clearly essential for twitching and S-motile gliding, might play a motor role, a sensory role or both. Because the evidence available does not seem to exclude any of these possibilities, both functions are presented. The aim of the following discussion is hopefully to elicit experiments. In addition to retraction, pilus rotation is a theoretically conceivable motor. As (helical) Tfp from different cells bundle together, the rotation of one, the other or both members of a pair could generate motion in the same way that two interdigitated spirally threaded shafts translate past each other.

Pilus retraction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Pilus fibre structure
  5. Tfp and type II secretion
  6. Social gliding motility in Myxobacteria
  7. Twitching motility
  8. Pilus retraction
  9. Is PilT a motor?
  10. A sensory role for type IV pili
  11. Che homologues
  12. Pathogenesis
  13. Summary
  14. Acknowledgements
  15. References

The core of Bradley's retraction argument is based on studies with phage that bind specifically to pili in P. aeruginosa (Bradley, 1972; 1974). As seen by electron microscopy (EM), phage-infected cells most often have phage particles clustered on the surface of the cell pole. Among the phage-resistant mutants were non-piliated (pil) mutants and hyperpiliated mutants. Unlike the pil mutants, hyperpiliated mutants can adsorb phage particles. In contrast to wild-type cells, these hyperpiliated mutants were found to have phage attached along the length of pili and at various distances from the cell. Phage particles were rarely found at the cell surface on the hyperpiliated mutants. To explain how a phage could reach the cell surface of wild-type cells, but not of the hyperpiliated mutants, Bradley proposed that a phage binds at some position on a pilus; the pilus then retracts, bringing the phage to the cell surface (Fig. 3A). The pili of phage-resistant hyperpiliated mutants would be unable to retract.

image

Figure 3. . Bradley's retraction model. A. A phage particle is shown bound to a pilus, which then retracts. B. Pilus retraction/extension model for cell motility. Dobson et al. (1979) includes an EM micrograph suggesting pilus interactions between cells.

Download figure to PowerPoint

Later, Bradley extended the role of retractile pili to twitching motility (Bradley, 1980). He found that neither pil nor ‘non-retractile’ (hyperpiliated) mutants exhibit twitching. Moreover, twitching was blocked by (i) pilus specific antiserum and (ii) pilus-specific phage that were not lethal. From these and the earlier studies, he concluded that ‘fully functional retractile pili are the mechanical basis for twitching motility’ (Fig. 3B). Retraction may be related to the way certain pathogenic bacteria, i.e. Listeria, Shigella and Richettsia, move intracellularly by polymerizing a ‘comet tail’ of actin filaments (Ireton and Cossart, 1997). This system requires that the nucleator protein, e.g. ActA, be localized at one cell pole, so polymerization and, hence, force displacement is unidirectional. Polarly localized pili allow pulling/pushing forces along the longitudinal axis of the cell, and perhaps other directions as well, which would support either gliding or twitching motility. To test this, one would like to observe pilus function directly but, to date, this has not been possible and remains an important experimental challenge.

Is PilT a motor?

  1. Top of page
  2. Abstract
  3. Introduction
  4. Pilus fibre structure
  5. Tfp and type II secretion
  6. Social gliding motility in Myxobacteria
  7. Twitching motility
  8. Pilus retraction
  9. Is PilT a motor?
  10. A sensory role for type IV pili
  11. Che homologues
  12. Pathogenesis
  13. Summary
  14. Acknowledgements
  15. References

Mattick and colleagues revitalized interest in the retraction model when they showed that two of Bradley's hyperpiliated (non-retractile) mutants resulted from small deletions in the pilT gene (Whitchurch et al., 1990). pilT mutants in N. gonorrhoeae and M. xanthus also express pili, but lack twitching and S-motility (Wu et al., 1997; Wolfgang et al., 1998a). An analogous mutation in EPEC's bfpF gene, which has (limited) homology to pilT, is also hyperpiliated and lacks twitching (Bieber et al., 1998). EM and biochemical studies in all four organisms have revealed no differences in pilus structure between wild type and corresponding pilT mutants. These findings support the view that the motility defect of pilT mutants is not correlated with structural defects of the pilus filament, but instead with a functional defect, perhaps one at the base of the pilus. Moreover, pilT is homologous to pilB over large regions of the protein. Both PilT and PilB proteins contain nucleotide-binding motifs, Walker boxes, suggesting that they have NTPase activity and could thus be motors. Mutations in the Walker boxes of PilB or XcpR (PilB homologue) block pilus biogenesis and extracellular secretion respectively (Turner et al., 1993). As pilT mutants express pili, but lack motility, pilus extension is evidently not sufficient for motility. It is possible that only retraction or a dynamic balance between retraction and extension provides a force for bacterial movement, a concept embodied in the opposing arrows in Fig. 3B.

In P. aeruginosa, immediately downstream of pilT is the pilU gene (Whitchurch and Mattick, 1994). These proteins are closely related (60% similarity), including their nucleotide-binding motif. Like pilT, pilU mutants are hyperpiliated and lack twitching. However, unlike pilT, pilU mutants are at least partly sensitive to four different pilus-specific phages, including two that Bradley used. Although this observation raised some questions about retraction mechanics (Whitchurch and Mattick, 1994), these authors continue to support it (Mattick and Alm, 1995).

In N. gonorrhoeae, the PilC protein (homologue of PilY1 in P. aeruginosa) is associated with the cell surface and is involved in pilus-mediated attachment to host cells (Fussenegger et al., 1997). Recently, Koomey and colleagues have reported that pilT null mutations suppress the pilus defects associated with mutations in pilC (Wolfgang et al., 1998b). These results suggest an interaction between the PilC and PilT proteins, which would provide a link between the cell surface and pilus function. In N. gonorrhoeae, pilT, pilC and other pil genes are also required for natural competence for transformation (Fussenegger et al., 1997; Wolfgang et al., 1998a). Although pilT mutants assemble pili and contain normal pilus-associated proteins (PilQ and PilC), they are specifically blocked in DNA uptake across the outer membrane (Wolfgang et al., 1998a). Systems related to Tfp are involved in natural competence in other bacteria as well (Manning and Meyer, 1997; Chung et al., 1998).

PilT is perhaps the signature protein in Tfp motility, because it is one of the best conserved proteins between M. xanthus, P. aeruginosa and N. gonorrhoeae, and it is not found in the related type II secretion system. PilT parallels the MotA and B proteins of the flagellar motor, as mutations in pilT or mot result in an assembled structure, pilus or flagellum that fails to generate movement.

If PilT transduces energy for retraction, then PilB might transduce energy for pilus extension by polymerization. The ΔG for polymerization may be negative, as pilins spontaneously assemble filaments in vitro. Pilus depolymerization is implied by retraction, because Tfp filaments have not been found inside the cell. The free energy required for depolymerization might be partially offset by insertion of the hydrophobic pilin domains into the membrane, presumably a spontaneous process (Fussenegger et al., 1997). Mattick and colleagues have suggested that Tfp, type II secretion and DNA uptake systems transport macromolecules by means of a pilus-like ratchet that carries associated macromolecules in or out of the cell (Mattick and Alm, 1995). They propose that the ratchet would be driven by NTP-binding proteins (PilB, PilT or homologues). For type II secretion and for DNA uptake, this process is unidirectional, and these systems contain only one PilB homologue. For Tfp, this process would be bidirectional; the ratchet could push out for extension and pull in for retraction, powered by PilB and PilT/U respectively. Nunn and colleagues have repeatedly isolated suppressors of an xcpT (pseudopilin) Ts mutant in xcpR (pilB homologue) (Kagami et al., 1998). These authors suggest that XcpT and XcpR proteins interact and, indeed, cross-linked complexes have been detected between XcpT and PilA (Lu et al., 1997). A study of the interactions between pilin, PilB, PilT and PilU proteins seems likely to provide an insight into Tfp function.

A sensory role for type IV pili

  1. Top of page
  2. Abstract
  3. Introduction
  4. Pilus fibre structure
  5. Tfp and type II secretion
  6. Social gliding motility in Myxobacteria
  7. Twitching motility
  8. Pilus retraction
  9. Is PilT a motor?
  10. A sensory role for type IV pili
  11. Che homologues
  12. Pathogenesis
  13. Summary
  14. Acknowledgements
  15. References

Pili are required for twitching and for S-motile gliding movements. Perhaps they generate a translocating force, as described above. The requirement of Tfp for motility can be explained with similar consistency, having Tfp play an essential sensory role. Conjugal DNA transfer in E. coli from an F′ donor strain is initiated by a mating signal that appears to be carried through the F-pilus when the pilus tip of the donor cell contacts a recipient cell (Kingsman and Willetts, 1978). As S-motility is very sensitive to the density of cells, as described above, and is in fact limited to cell–cell distances of less than a pilus length, it is likely that pili are sense organs for detecting cells nearby. Ways in which a signal could pass from the tip of a fibre to its base include, in theory, the propagation of a helix dislocation (Macnab and Ornston, 1977) or a mechanical force such as tension, compression or flexion. Arriving at the base of the pilus, the force or dislocation could signal the appropriate movement response by the cell. The ability of Vibrio haemolyticus to activate the expression of many genes in response to a viscous drag on its polar flagellum provides a mechanical precedent for such a signalling possibility in a fibre composed of helically arranged subunits (McCarter et al., 1988; Kawagishi et al., 1996). Nor are sensory and motor functions mutually exclusive. Even if pili are motors for gliding or twitching, it would seem necessary to postulate a sensory role to trigger a retraction process taking place at the base of the pilus. A sensory role is also suggested by the presence of chemotaxis-like proteins in Tfp systems.

Che homologues

  1. Top of page
  2. Abstract
  3. Introduction
  4. Pilus fibre structure
  5. Tfp and type II secretion
  6. Social gliding motility in Myxobacteria
  7. Twitching motility
  8. Pilus retraction
  9. Is PilT a motor?
  10. A sensory role for type IV pili
  11. Che homologues
  12. Pathogenesis
  13. Summary
  14. Acknowledgements
  15. References

In enteric bacteria, chemotaxis (Che) proteins control the direction of bacterial swimming by regulating the direction of flagellar rotation (Parkinson, 1993). P. aeruginosa, like M. xanthus, uses Che homologues in its Tfp directed motility. Indeed, P. aeruginosa has two distinct Che systems; one controls flagellar based swimming, while the other supports Tfp-based twitching (Darzins and Russell, 1997). Mutations in homologues of cheY (pilG ), cheW (pilI ), two cheAs (pilL and chpA) and a gene encoding a methyl-accepting chemotaxis protein (MCP, pilJ ) block the production of pili. Mutations in pilK (cheR homologue) and chpB (cheB homologue) express pili and exhibit twitching. Interestingly, a mutant defective in a second cheY-like gene (pilH ) has pili, but modifies the pattern of twitching movements such that doughnut-shaped swarms are formed (Darzins, 1994), which are reminiscent of the ‘frizzy’ phenotype in M. xanthus. As pilG, pilI, pilJ, pilL and chpA mutants are pil, Darzins and Russell (1997) proposed a model in which these Che-like proteins regulate both pilus biogenesis and twitching. Proteins that may interact with this signal transduction system, probably through one or more of the CheY homologues, would include PilT and PilU. Perhaps of note, the pilT and pilU genes are linked to this che-like cluster of genes. It would also be of interest to know whether pilH, pilK or chpB mutations alter the frequency of reversal of cell movement.

In M. xanthus, the frizzy signal transduction system is involved in controlling the frequency of reversal of cell movement and, consequently, the swarm pattern (Ward and Zusman, 1997). The frz genes are homologous to the che genes and their twitching homologues. Null mutations in frz genes decrease the reversal frequency, while the frzD gain-of-function mutation in frzCD (encoding a MCP homologue) increases the reversal frequency. Several lines of evidence suggest that frz mutants are members of the S system and would thus function with Tfp. First, Afrz double mutants show defective S-motility. Secondly, frzD mutants completely lack S-motility, whether or not the strain has a functional A-motility system (D. Wall, unpublished). Preliminary data indicate that frzE and frzZ mutants have pili; the frzD mutant has not yet been examined. Recently, a second set of che-like genes have been found, which includes homologues of cheA, cheW, cheY and an MCP (Yang et al., 1998; W. Shi, personal communication). These genes map to the dsp region, and mutations in them lead to a dsp (S) phenotype. Rosenbluh and Eisenbach (1992) have reported that dsp mutants exhibit a hyporeversal frequency.

Pathogenesis

  1. Top of page
  2. Abstract
  3. Introduction
  4. Pilus fibre structure
  5. Tfp and type II secretion
  6. Social gliding motility in Myxobacteria
  7. Twitching motility
  8. Pilus retraction
  9. Is PilT a motor?
  10. A sensory role for type IV pili
  11. Che homologues
  12. Pathogenesis
  13. Summary
  14. Acknowledgements
  15. References

Tfp play an essential role in many bacterial infections, usually serving as an adhesin (Strom and Lory, 1993; Manning and Meyer, 1997). A putative receptor on eukaryotic cells for Neisseria Tfp is CD46 (Källstrom et al., 1998). Tfp may serve to permit infection in other ways. For example, Schoolnik and colleagues have shown that bfpF (the pilT homologue in EPEC) mutants are avirulent, even though they make pili, aggregate and colonize tissue monolayers (Bieber et al., 1998). These mutants are blocked in twitching. Engel and colleagues isolated a collection of pil mutants of P. aeruginosa that are no longer cytotoxic, yet are able to bind host cells (Kang et al., 1997). Three possible functions of Tfp were offered in explanation: (i) pilus retraction allows close contact with host cells, thus facilitating the transfer of bacterial toxins via a type III secretion system; (ii) twitching motility allows bacteria to spread in the infected tissue; (iii) Tfp transduce a signal indicating host cell adherence, which then triggers gene expression. Tfp also assist in the formation of biofilms (O'Toole and Kolter, 1998; Wall et al., 1999). Biofilms, in which bacteria are moving actively, may support tissue colonization and protect the bacteria against antibodies and antibiotics.

Summary

  1. Top of page
  2. Abstract
  3. Introduction
  4. Pilus fibre structure
  5. Tfp and type II secretion
  6. Social gliding motility in Myxobacteria
  7. Twitching motility
  8. Pilus retraction
  9. Is PilT a motor?
  10. A sensory role for type IV pili
  11. Che homologues
  12. Pathogenesis
  13. Summary
  14. Acknowledgements
  15. References

Recently, type IV pili have emerged as an efficient device for bacterial motility. The most highly conserved components include the pilin, the PilB, C, D, M, N, O, P, Q and T proteins. The function of and interactions between these proteins are being pursued actively. Understanding the functions of Tfp is still a challenge. Currently, the PilT protein is an attractive target for experiments. Another area ripe for investigation is the pattern of cell movement reflecting the behaviour of individual cells. S-motile cells are highly ordered and tightly packed side by side and end to end in dynamic arrays (Fig. 1). Isolated S-motile cells do not glide, suggesting that they require interactions with other cells. Similarly, twitching motility appears to require cell–cell contacts. Within an S-motile flare, cells move in concert in one direction along their longitudinal axis in ever-changing associations. Tfp, the only morphological and genetic structure involved in both twitching and S-motility, are located at the cell poles. Tgl stimulation of S-motility involves cell alignment and probably end-to-end cell contacts for the transfer of Tgl activity between cells (Wall and Kaiser, 1998). The polar localization of Tfp, essential end-to-end contacts between cells and the movement of cells along their longitudinal axis are key elements in S-motility. Understanding the molecular role of Tfp in social gliding and twitching is an important objective.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Pilus fibre structure
  5. Tfp and type II secretion
  6. Social gliding motility in Myxobacteria
  7. Twitching motility
  8. Pilus retraction
  9. Is PilT a motor?
  10. A sensory role for type IV pili
  11. Che homologues
  12. Pathogenesis
  13. Summary
  14. Acknowledgements
  15. References

We thank Michael Koomey and Wenyuan Shi for sharing their results before publication. Owing to space limitations, the reference list has been abridged, and we apologize to those authors who have made significant contributions to this field but have not been cited appropriately. This work was supported by a grant from the National Science Foundation (MCB 9423182 to D.K.). D.W. was a recipient of an American Cancer Society postdoctoral fellowship (PF-4138).

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Pilus fibre structure
  5. Tfp and type II secretion
  6. Social gliding motility in Myxobacteria
  7. Twitching motility
  8. Pilus retraction
  9. Is PilT a motor?
  10. A sensory role for type IV pili
  11. Che homologues
  12. Pathogenesis
  13. Summary
  14. Acknowledgements
  15. References
  • 1
    Bernstein, H.D. (1998) Membrane protein biogenesis: the exception explains the rules. Proc Natl Acad Sci USA 95: 1458714589.
  • 2
    Bieber, D., Ramer, S.W., Wu, C., Murray, W.J., Tobe, T., Fernandez, R., et al (1998) Type IV pili, transient bacterial aggregates and virulence in enteropathogenic Escherichia coli. Science 280: 21142118.
  • 3
    Bradley, D.E. (1972) Shortening of Pseudomonas aeruginosa pili after RNA-phage adsorption. J Gen Microbiol 72: 303319.
  • 4
    Bradley, D.E. (1974) The absorption of Pseudomonas aeruginosa pilus-dependent bacteriophages to a host mutant with nonretractile pili. Virology 58: 149163.
  • 5
    Bradley, D.E. (1980) A function of Pseudomonas aeruginosa PAO polar pili: twitching motility. Can J Microbiol 26: 146154.
  • 6
    Chang, B.Y. & Dworkin, M. (1994) Isolated fibrils rescue cohesion and development in the Dsp mutant of Myxococcus xanthus. J Bacteriol 176: 71907196.
  • 7
    Chung, Y.S., Breidt, F., Dubnau, D. (1998) Cell surface localization and processing of the ComG proteins, required for DNA binding during transformation of Bacillus subtilis. Mol Microbiol 29: 905913.
  • 8
    Dana, J.R. & Shimkets, L.J. (1993) Regulation of cohesion-dependent cell interactions in Myxococcus xanthus. J Bacteriol 175: 36363647.
  • 9
    Darzins, A. (1993) The pilG gene product, required for Pseudomonas aeruginosa pilus production and twitching motility, is homologous to the enteric, single-domain response regulator CheY. J Bacteriol 175: 59345944.
  • 10
    Darzins, A. (1994) Characterization of a Pseudomonas aeruginosa gene cluster involved in pilus biosynthesis and twitching motility: sequence similarity to the chemotaxis proteins of enterics and the gliding bacterium Myxococcus xanthus. Mol Microbiol 11: 137153.
  • 11
    Darzins, A. (1995) The Pseudomonas aeruginosa pilK gene encodes a chemotactic methyltransferase (CheR) homologue that is translationally regulated. Mol Microbiol 15: 703717.
  • 12
    Darzins, A. & Russell, M.A. (1997) Molecular genetic analysis of type-4 pilus biogenesis and twitching motility using Pseudomonas aeruginosa as a model system — a review. Gene 192: 109115.
  • 13
    Dobson, W.J., Mcurdy, H.D., MacRae, T.H. (1979) The function of fimbriae in Myxococcus xanthus. II. The role of fimbriae in cell–cell interactions. Can J Microbiol 25: 13591372.
  • 14
    Forest, K.T. & Tainer, J.A. (1997) Type-4 pilus structure: outside to inside and top to bottom — a minireview. Gene 192: 165169.
  • 15
    Francetic, O., Lory, S., Pugsley, A.P. (1998) A second prepilin peptidase gene in Escherichia coli K-12. Mol Microbiol 27: 763775.
  • 16
    Freese, A., Reichenbach, H., Lünsdorf, H. (1997) Further characterization an in situ localization of chain-like aggregates of the gliding bacteria Myxococcus fulvus and Myxococcus xanthus. J Bacteriol 179: 12461252.
  • 17
    Fussenegger, M., Rudel, T., Barten, R., Ryll, R., Meyer, T.F. (1997) Transformation competence and type-4 pilus biogenesis in Neisseria gonorrhoeae— a review. Gene 192: 125134.
  • 18
    Hartzell, P. (1997) Complementation of sporulation and motility defects in a prokaryote by a eukaryotic GTPase. Proc Natl Acad Sci USA 94: 98819886.
  • 19
    Henrichsen, J. (1972) Bacterial surface translocation: a survey and a classification. Bacteriol Rev 36: 478503.
  • 20
    Henrichsen, J. (1975) The occurrence of twitching motility among Gram-negative bacteria. Acta Pathol Microbiol Scand B 83: 171178.
  • 21
    Henrichsen, J. (1983) Twitching motility. Annu Rev Microbiol 37: 8193.
  • 22
    Hodgkin, J. & Kaiser, D. (1979) Genetics of gliding motility in Myxococcus xanthus (Myxobacterales): two gene systems control movement. Mol Gen Genet 171: 177191.
  • 23
    Hoiczyk, E. & Baumeister, W. (1998) The junctional pore complex, a prokaryotic secretion organelle, is the molecular motor underlying gliding motility in cyanobacteria. Curr Biol 8: 11611168.
  • 24
    Ireton, K. & Cossart, P. (1997) Host–pathogen interactions during entry and actin based movements of Listeria monocytogenes. Annu Rev Genet 31: 113138.
  • 25
    Kagami, Y., Ratliff, M., Surber, M., Martinez, A., Nunn, D.N. (1998) Type II protein secretion by Pseudomonas aeruginosa: genetic suppression of a conditional mutation in the pilin-like component XcpT by the cytoplasmic component XcpR. Mol Microbiol 27: 221233.
  • 26
    Kaiser, D. (1979) Social gliding is correlated with the presence of pili in Myxococcus xanthus. Proc Natl Acad Sci USA 76: 59525956.
  • 27
    Kaiser, D. & Crosby, C. (1983) Cell movement and its coordination in swarms of Myxococcus xanthus. Cell Motility 3: 227245.
  • 28
    Källstrom, H., Islam, M.S., Berggren, P., Jonsson, A. (1998) Cell signaling by the type IV pili of pathogenic Neisseria. J Biol Chem 273: 2177721782.
  • 29
    Kang, P.J., Hauser, A.R., Apodaca, G., Fleiszig, S.M.J., Wiener-Kronish, J., Mostov, K., et al (1997) Identification of Pseudomonas aeruginosa genes required for epithelial cell injury. Mol Microbiol 24: 12491262.
  • 30
    Kaufman, M.R., Seyer, J.M., Taylor, R.K. (1991) Processing of TCP pilin by TcpJ typifies a common step intrinsic to a newly recognized pathway of extracellular protein secretion by Gram-negative bacteria. Genes Dev 5: 18341846.
  • 31
    Kawagishi, I., Imagawa, M., Imae, Y., McCarter, L., Homma, M. (1996) The sodium-driven polar flagellar motor of marine Vibrio as the mechanosensor that regulates lateral flagellar expression. Mol Microbiol 20: 693699.
  • 32
    Kingsman, A. & Willetts, N. (1978) The requirements for conjugal DNA synthesis in the donor strain during Flac transfer. J Mol Biol 122: 287300.
  • 33
    Lu, H.M., Motley, S.T., Lory, S. (1997) Interactions of the components of the general secretion pathway: role of Pseudomonas aeruginosa type IV pilin subunits in complex formation and extracellular protein secretion. Mol Microbiol 25: 247259.
  • 34
    McCarter, L., Hilmen, M., Silverman, M. (1988) Flagellar dynamometer controls swarmer cell differentiation of V. parahaemolyticus. Cell 54: 345351.
  • 35
    Macnab, R.M. & Ornston, M.K. (1977) Normal-to-curly flagellar transitions and their role in bacterial tumbling. Stabilization of an alternative quaternary structure by mechanical force. J Mol Biol 112: 130.
  • 36
    Manning, P.A. & Meyer, T.F. (1997) Type-4 pili: biogenesis, adhesins, protein export and DNA import. Proceedings of a workshop. Gene 192: 1198.
  • 37
    Mattick, J.S. & Alm, R.A. (1995) Common architecture of type 4 fimbriae and complexes involved in macromolecular traffic. Trends Microbiol 3: 411413.
  • 38
    Mattick, J.S., Whitchurch, C.B., Alm, R.A. (1996) The molecular genetics of type-4 fimbriae in Pseudomonas aeruginosa— a review. Gene 179: 147155.
  • 39
    O'Toole, G.A. & Kolter, R. (1998) Flagellar and twitching motility are necessary for Pseudomonas aeruginosa biofilm development. Mol Microbiol 30: 295304.
  • 40
    Ottow, J.C.G. (1975) Ecology, physiology, and genetics of fimbriae and pili. Annu Rev Microbiol 29: 79108.
  • 41
    Parge, H.E., Forest, K.T., Hickey, M.J., Christensen, D.A., Gertzoff, E.D., Tainer, J.A. (1995) Structure of the fibre-forming protein pilin at 2.6 A resolution. Nature 378: 3238.
  • 42
    Parkinson, J.S. (1993) Signal transduction schemes of bacteria. Cell 73: 857871.
  • 43
    Pepe, J.C. & Lory, S. (1998) Amino acid substitution in PilD, a bifunctional enzyme of Pseudomonas aeruginosa. J Biol Chem 273: 1912019129.
  • 44
    Pugsley, A.P. (1993) The complete general secretory pathway in Gram-negative bacteria. Microbiol Rev 57: 50108.
  • 45
    Pugsley, A.P. (1996) Multimers of the precursor of a type IV pilin-like component of the general secretory pathway are unrelated to pili. Mol Microbiol 20: 12351245.
  • 46
    Ramaswamy, S., Dworkin, M., Downard, J. (1997) Identification and characterization of Myxococcus xanthus mutants deficient in calcofluor white binding. J Bacteriol 179: 28632871.
  • 47
    Rodriguez-Soto, J.P. & Kaiser, D. (1997) The tgl gene: social motility and stimulation in Myxococcus xanthus. J Bacteriol 179: 43614371.
  • 48
    Rosenbluh, A. & Eisenbach, M. (1992) Effect of mechanical removal of pili on gliding motility of Myxococcus xanthus. J Bacteriol 174: 54065413.
  • 49
    Russel, M. (1998) Macromolecular assembly and secretion across the bacterial cell envelope: type II protein secretion systems. J Mol Biol 279: 485499.
  • 50
    Shi, W. & Zusman, D.R. (1993) The two motility systems of Myxococcus xanthus show different selective advantages on various surfaces. Proc Natl Acad Sci USA 90: 33783382.
  • 51
    Strom, M.S. & Lory, S. (1993) Structure-function and biogenesis of the type IV pili. Annu Rev Genet 47: 565596.
  • 52
    Turner, L.R., Lara, J.C., Nunn, D.N., Lory, S. (1993) Mutations in the consensus ATP-binding sites of XcpR and PilB eliminate extracellular protein secretion and pilus biogenesis in Pseudomonas aeruginosa. J Bacteriol 175: 49624969.
  • 53
    Wall, D. & Kaiser, D. (1998) Alignment enhances the cell-to-cell transfer of pilus phenotype. Proc Natl Acad Sci USA 95: 30543058.
  • 54
    Wall, D., Kolenbrander, P.E., Kaiser, D. (1999) The Myxococcus xanthus pilQ (sglA) gene encodes a secretin homolog required for type IV pilus biogenesis, social motility and development. J Bacteriol 181: 2433.
  • 55
    Ward, M.J. & Zusman, D.R. (1997) Regulation of directed motility in Myxococcus xanthus. Mol Microbiol 24: 885893.
  • 56
    Weimer, R.M., Creighton, C., Stassinopoulos, A., Youderian, P., Hartzell, P.L. (1998) A chaperone in the Hsp70 family controls production of extracellular fibrils in Myxococcus xanthus. J Bacteriol 180: 53575368.
  • 57
    Whitchurch, C.B. & Mattick, J.S. (1994) Characterization of a gene, pilU, required for twitching motility but not phage sensitivity in Pseudomonas aeruginosa. Mol Microbiol 13: 10791091.
  • 58
    Whitchurch, C.B., Hobbs, M., Livingston, S.P., Krishnapillai, V., Mattick, J.S. (1990) Characterisation of a Pseudomonas aeruginosa twitching motility gene and evidence for a specialised protein export system widespread in eubacteria. Gene 101: 3344.
  • 59
    Wolfgang, M., Lauer, P., Park, H., Brossay, L., Hébert, J., Koomey, M. (1998a) PilT mutations lead to simultaneous defects in competence for natural transformation and twitching motility in piliated Neisseria gonorrhoeae. Mol Microbiol 29: 321330.
  • 60
    Wolfgang, M., Park, H., Hayes, S.F., VanPutten, J.P.M., Koomey, M. (1998b) Suppression of an absolute defect in type IV pilus biogenesis by loss-of-function mutations in pilT, a twitching motility gene in Neisseria gonorrhoeae. Proc Natl Acad Sci USA 95: 1497314978.
  • 61
    Wu, S.S. & Kaiser, D. (1995) Genetic and functional evidence that type IV pili are required for social gliding motility in Myxococcus xanthus. Mol Microbiol 18: 547558.
  • 62
    Wu, S.S. & Kaiser, D. (1997) Regulation of expression of the pilA gene of Myxococcus xanthus. J Bacteriol 179: 77487758.
  • 63
    Wu, S.S., Wu, J., Kaiser, D. (1997) The Myxococcus xanthus pilT locus is required for social gliding motility although pili are still produced. Mol Microbiol 23: 109121.
  • 64
    Wu, S.S., Wu, J., Cheng, Y.L., Kaiser, D. (1998) The pilH gene encodes an ABC transporter homologue required for type IV pilus biogenesis and social motility in Myxococcus xanthus. Mol Microbiol 29: 12491261.
  • 65
    Yang, Z., Yongzhi, G., Xu, D., Kaplan, H.B., Shi, W. (1998) A new set of chemotaxis homologues is essential for Myxococcus xanthus social motility. Mol Microbiol 30: 11231130.