SEARCH

SEARCH BY CITATION

Keywords:

  • kinesin;
  • cytoplasmic dynein;
  • kinectin;
  • microtubule transport;
  • membrane traffic;
  • Golgi;
  • endoplasmic reticulum;
  • residue transport;
  • saltatory movements

Abstract

  1. Top of page
  2. Abstract
  3. Vesicle movements are short and saltatory
  4. Control of motor activity
  5. Models of the organelle motor complex
  6. Coordination of vesicle traffic
  7. Control of vesicle motility
  8. Vesicles transported by motors
  9. Transport of non-vesicular cargoes
  10. Motor-linkage components
  11. Technological advances in defining active motor complexes
  12. Summary
  13. Acknowledgements
  14. References

The movements of intracellular cargo along microtubules within cells are often saltatory or of short duration. Further, calculations of the fraction of membrane vesicles that are moving at any period, indicate that active motor complexes are rare. From observations of normal vesicle traffic in cells, there appears to be position-dependent activation of motors and a balance of traffic in the inward and outward directions. In-vitro binding of motors to cargo is observed under many conditions but motility is not. Multi-component complexes appear to be involved in producing active organelle movements by a graded activation system that is highly localized in the cell. The basis of the activation of motility of the organelle motor complexes is still unknown but phosphorylation has been implicated in many systems. In the case of the motor-binding protein, kinectin, it has been linked to active organelle movements powered by conventional kinesin. From the coiled-coil structure of kinectin and the coiled-coil tail of kinesin, it is postulated that a coiled-coil assembly is responsible for the binding interaction. Many other cargoes are transported but the control of transport will be customized for each function, such as axonemal rafts or cytoskeletal complexes. Each function will have to be analyzed separately and motor activity will need to be integrated into the specific aspects of the function.

As the number of motor proteins defined by sequence motifs increases, there is increasing confusion about the exact functional role that any single motor might play in a cell [1,2]. The phenotype of many of the motor deletions has been hard to reconcile with the expected function of the motor. There is uncertainty about the nature of the cargo transported by any given motor and whether or not motors can bind promiscuously to different cargo in different in-vivo environments. In-vitro reconstitution of motility and some genetic screens have identified additional components that affect motility in vivo but additional components add to the complexity of the system. If we look at the roles of the motors in cell functions, we find that motors are active only occasionally and for short periods. Motor binding to complex cargoes such as vesicles is observed but motility often requires additional components. In-vitro studies of motor interactions with purified cargo components have not been well correlated with motile activity.

Vesicle movements are short and saltatory

  1. Top of page
  2. Abstract
  3. Vesicle movements are short and saltatory
  4. Control of motor activity
  5. Models of the organelle motor complex
  6. Coordination of vesicle traffic
  7. Control of vesicle motility
  8. Vesicles transported by motors
  9. Transport of non-vesicular cargoes
  10. Motor-linkage components
  11. Technological advances in defining active motor complexes
  12. Summary
  13. Acknowledgements
  14. References

Early descriptions of organelle movements along microtubules have indicated that the time of movement was short. For example, in a cell of 20 µm in radius, an organelle moving at 4–5 µm·s–1 would only move for 4–5 s to cover the distance from the cell center to the periphery. Even in axons where the organelles must cover up to a meter distance, the movement of organelles is characterized as saltatory, with rapid displacements followed by diffusion [3]. Some of the periods of stasis are the result of physical impediments to transport, others may be because of loss of binding to the microtubule or the result of loss of motor activity on the organelles. It has been possible to measure the fraction of active motors bound to latex beads in motility assays using laser tweezers to hold beads in the neighborhood of the microtubule [4,5]. With vesicle-motility assays, similar levels of motility have not been found and measurements of motile activity are almost always expressed in relative and not in absolute terms. Thus, it appears that vesicle-motile activity is relatively low.

The fraction of membranous organelles that is actively moving at any given time is quite small. Even with the most sensitive measures of motility by video-enhanced differential interference contrast microscopy, volumes of membrane equivalent to the plasma membrane are seen to move inward and outward every 24 minutes [6]. Because the plasma membrane is only 3–5% of the total membrane area in the cell (ER is about 50%), the fraction of membrane vesicles in a cell lysate that will be active is small. A rough calculation can be made for the dense microsome (ER) fraction by assuming that all of the movements seen in the cell were ER membranes and that the motor complex was active long enough to move the membrane over the radius of the cell (assume 10 s to cover 25 µm). With these assumptions, 10% of the ER membranes would be active for 10 s during a 24-min period. At any given time, only 0.07% of the ER vesicles would be active (multiply the percentage of active vesicles, 10%, by the fraction of time that they are moving, 10 s/1440 s = 0.007). Measurements of vesicle motility in vitro are consistent with these calculations and belie the difficulties in making the measurements. Higher fractions of motile vesicles are found in preparations of activated T-killer cell granules [7] or phagocytic particles [8]. Ionic conditions (low ionic strength) and activation protocols (kinase activation or phosphatase inhibition) have been used in many instances to increase the number of vesicle movements observed. There is some concern about whether or not those treatments created nonphysiological motor complexes similar to the attachment of motors to anionic latex beads or potentially anionic lipid surfaces [9]. Nonspecific binding of motors to membranes may be a normal part of cellular functions. Interruptions in the flow of membrane from one compartment to another result in the buildup of membrane in the first compartment, which could expose new motor sites or even bare lipid.

Control of motor activity

  1. Top of page
  2. Abstract
  3. Vesicle movements are short and saltatory
  4. Control of motor activity
  5. Models of the organelle motor complex
  6. Coordination of vesicle traffic
  7. Control of vesicle motility
  8. Vesicles transported by motors
  9. Transport of non-vesicular cargoes
  10. Motor-linkage components
  11. Technological advances in defining active motor complexes
  12. Summary
  13. Acknowledgements
  14. References

Because there are high concentrations of many motors in cytoplasm and many of the two-headed motors are processive, active movements could theoretically occur in the absence of cargo. In the case of kinesin, the native motor with light chains is largely inactive, but binding to carboxylated beads will activate the microtubule-dependent ATPase activity over 10-fold [10]. In the case of cytoplasmic dynein, the soluble protein will hydrolyze most nucleotide triphosphates whereas lipid-bound cytoplasmic dynein is a specific ATPase [11]. These findings are consistent with the early motility assays that relied upon glass or anionic surfaces to activate the motors and low ionic strength solutions to favor motor-substrate complexes [12]. The activation of the motors appears to involve more than just the modification of a single phosphorylation site. Rather the activities of the motors appear to be very dependent upon the conformation of the molecules. Thus, the ‘active motor complex’ could involve the orchestration of several factors, including motor binding to the cargo structure and assembly of cofactors.

Models of the organelle motor complex

  1. Top of page
  2. Abstract
  3. Vesicle movements are short and saltatory
  4. Control of motor activity
  5. Models of the organelle motor complex
  6. Coordination of vesicle traffic
  7. Control of vesicle motility
  8. Vesicles transported by motors
  9. Transport of non-vesicular cargoes
  10. Motor-linkage components
  11. Technological advances in defining active motor complexes
  12. Summary
  13. Acknowledgements
  14. References

The published models of the organelle motor complex emphasize either a multicomponent complex that is quite dynamic [13,14] or a stable complex that is resistant to salt and alkaline extraction conditions [9,15]. The former is based primarily upon in-vitro reconstitution studies from embryonic brains or cultured cell extracts whereas the latter primarily arises from analyses of axonal transport. The site of the activation of motility is critical in both types of models and the mechanism of activation is also important (see Fig. 1). At this point it is useful to consider the steps involved in membranous vesicle motility. First, the vesicle must be activated for motility, which would signal the motor binding and/or activation process. The active motor complex would then bind to a microtubule and move until it is spontaneously inactivated or reaches a region of cytoplasm where it would encounter an inactivation complex. In non-neuronal cells, there is good reason to have a soluble pool of motors. After a transport step, motors could diffuse back to where they could be used again. In neuronal processes, the distances are too great to count on diffusion and motors may then move on transported vesicles in an inactive form. We will focus here on the case of normal membrane traffic to arrive at a model for the most general process. Significant modifications for specialized processes such as axonal transport or regulated secretion are expected.

image

Figure 1. Position dependence of the assembly and disassembly of active motor complexes. Spontaneous inactivation is postulated to explain the saltatory nature of most movements and is consistent with the involvement of small G proteins in the transport [25].

Coordination of vesicle traffic

  1. Top of page
  2. Abstract
  3. Vesicle movements are short and saltatory
  4. Control of motor activity
  5. Models of the organelle motor complex
  6. Coordination of vesicle traffic
  7. Control of vesicle motility
  8. Vesicles transported by motors
  9. Transport of non-vesicular cargoes
  10. Motor-linkage components
  11. Technological advances in defining active motor complexes
  12. Summary
  13. Acknowledgements
  14. References

There are two aspects of membrane traffic in cells that have important implications for the motor mechanisms. First, most steps in membrane transport involve a unidirectional transport of some components through the cyclic transport of vesicular components. This is true for both the exocytic and endocytic pathways. For example, it is generally accepted that plasma-membrane proteins are transported from ER to Golgi and then from Golgi to the plasma membrane by a bidirectional flow of membrane between ER and Golgi and between Golgi and the plasma membrane. Salvage vesicles carry ER components from the Golgi back to the ER and resialylation of plasma membrane proteins has documented the bidirectional traffic between Golgi and plasma membrane. The transport vesicles can be likened to tractor trailer trucks in the everyday world which carry material from a site of manufacture to a site of assembly or distribution and then cycle back for another load.

The second important aspect of membrane traffic involves the nature of membrane flow. Because the lifetime of many membrane components is greater than the rate of flow of membrane through various compartments, the fluid lipids of the cell can be likened to water flowing through a multilevel fountain. The total mass of lipid is relatively constant but it can be collected in different pools by simply varying the rate of influx versus the rate of efflux. In the case of the 14° block of ER to Golgi transport, there is an expansion of an intermediate compartment and likewise in the 18° block of Golgi to plasma membrane transport, there is an expansion of the Golgi. Both the cyclic nature of membrane transport and the possibility of changing the size of a membrane compartment mean that transport steps are linked. In practical terms, the activation of one transport step will decrease the size of the donor compartment and increase the size of the acceptor compartment. This will favor the reverse transport process from the acceptor to the donor compartment. Rapidly a new steady state will be reached when the greater size of the acceptor results in the same number of vesicles moving in the reverse and forward directions (see Fig. 2). If this were the case in vivo, then the number of inward and outward vesicle movements would normally be equalized. Consistent with this hypothesis, there is a simultaneous activation of both kinesin and cytoplasmic dynein-dependent movements over a wide range of motile activities [6].

image

Figure 2. The motor activity and vesicle size dependence of normal bidirectional membrane traffic and formation of a new steady state. In (A), the normal steady state is depicted as an equal number of vesicles moving from the donor (e.g. ER) pool to the acceptor (e.g. Golgi) as moving in the reverse direction. (B) If Motor A is immediately activated, then the number of vesicles moving from the donor pool is increased, while the number from the acceptor pool is the same as before. (C) As a result of the increased transport from donor to acceptor, the size of the acceptor compartment is increased. A new steady state is established if the number of transported vesicles is sensitive to the size of the vesicle pool (shown in C). Thus, a decrease in the size of the donor pool will decrease the number of vesicles moving to the acceptor whereas the increase in acceptor pool size will increase the number of vesicles moving in the opposite direction. As illustrated, the activation of motor A does increase the level of transport in both directions if the pool size is related to level of transport.

Based upon these two considerations of normal vesicle traffic and the normal position dependence, a graded model of control of motility is proposed in which position, principles of mass action, and overall cell metabolic state interplay to control the level of motility for each motor. As seen in Fig. 1, a localized complex is needed to activate the vesicles for binding the appropriate motor and the amount of active motor should be dependent upon the size of the compartment (Fig. 2). One way to accomplish this is to imagine that vesicle budding is dependent upon motor activity [16]. An additional control element could be the metabolic activity of the cell, which would be expressed in changes in the level of phosphorylation of critical sites on the motors and perhaps on the membranes as well.

Control of vesicle motility

  1. Top of page
  2. Abstract
  3. Vesicle movements are short and saltatory
  4. Control of motor activity
  5. Models of the organelle motor complex
  6. Coordination of vesicle traffic
  7. Control of vesicle motility
  8. Vesicles transported by motors
  9. Transport of non-vesicular cargoes
  10. Motor-linkage components
  11. Technological advances in defining active motor complexes
  12. Summary
  13. Acknowledgements
  14. References

There is correlative evidence both in vivo and in vitro of phosphorylation control of vesicle motility [6,17]. Regulation of plus-end directed vesicle movements and kinesin motor activity correlates with phosphorylation of kinesin light chain in cultured mammalian cells [18]. In stimulation of zymogen-granule release, there is a dramatic increase in kinesin binding to granules and in kinesin phosphorylation [19]. The more phosphorylated forms of kinesin are moved most rapidly down the axons [20]. Kinesin-like protein phosphorylation is implicated in controlling motility [21]. However, in lobster leg axons, the increased phosphorylation by protein kinase A leads to inhibition of kinesin-dependent vesicle motility [22], which further differentiates axonal transport from normal membrane traffic. In terms of the kinases involved, cytoplasmic dynein is phosphorylated by casein kinase [23]. In the platelet, cytoplasmic dynein moves from a cytoplasmic to a membrane compartment in correlation with an initial phosphorylation followed by dephosphorylation [24]. Thus, there is ample evidence of changes in motile activity with changes in phosphorylation state; however, the critical phosphorylation sites have not been defined.

GTP appears to be involved as well, as motility requires the presence of GTP and motility levels are altered by GTPγS [25]. The only clear connection between the organelle motor complex and GTP is through the binding of small G proteins to kinectin. Kinectin interacts with the small G proteins as determined by yeast two-hybrid screens [26] and in-vitro peptide binding studies [27].

Vesicles transported by motors

  1. Top of page
  2. Abstract
  3. Vesicle movements are short and saltatory
  4. Control of motor activity
  5. Models of the organelle motor complex
  6. Coordination of vesicle traffic
  7. Control of vesicle motility
  8. Vesicles transported by motors
  9. Transport of non-vesicular cargoes
  10. Motor-linkage components
  11. Technological advances in defining active motor complexes
  12. Summary
  13. Acknowledgements
  14. References

A number of systems have been defined wherein motors are important for vesicular transport. Because membrane traffic can be driven by vesicle fusion and fission alone in the absence of motors, active motors are implicated primarily in accelerating transport or in highly asymmetric cells such as neurons. There is still some question about the exact role of kinesin and cytoplasmic dynein in various membrane trafficking events. Overexpression of individual subunits of proteins such as the kinesin light chains or dynamitin have caused alteration of membrane traffic, mitosis or cytoplasmic organization that suggest roles for the motors in these processes [7,28]. However, the secondary effects of overexpression make the interpretation of results difficult. Multiple cytoplasmic dyneins have been found in individual cells and multiple roles have been suggested for them [29–31].

Fast axonal transport of kinesin is observed with different isoforms moving in different, fast transport pools [15]. Although the fast axonal transport of kinesin is used to argue that kinesin is stably bound to vesicles, freeze substitution and labeling studies support the biochemical observations that most of the kinesin in cells is in a soluble form [32]. An anti-(kinesin light chain) antibody inhibits axonal transport [33].

Suppression of kinesin expression with antisense causes a dramatic decrease in insulin secretion [34] and in axonal transport in vitro[35]. Inhibition of kinesin function with a function-blocking antibody blocks Golgi dispersion in nocodazole [36]. Erythro-9-[3-2-hydroxynonyl)]adenine (EHNA) inhibits the movement of water channels from a vesicular compartment to the plasma membrane in response to vasopressin, which indicates that the vesicles are normally transported by cytoplasmic dynein [37,38]. Deletion of kinesin in fungal hyphae or in mice does not appear to alter the amount of microtubule-dependent motility as observed by video-enhanced differential interference contrast microscopy [39,40]. In kinesin deletions, there are however, major rearrangements of membrane compartments (hyphal assembly structures [39]). Cell wounding activates a repair mechanism that involves the kinesin-dependent movement of membrane to the wound site followed by calcium-stimulated fusion and an increase in cell size [41].

Mitochondrial movements to the periphery are clearly dependent upon kinesin as found in the kinesin knockout mouse [40], and the inhibition of movements is synergistic with an apoptotic pathway [42]. Loss of a cytoplasmic dynein light chain in flies gives an apoptotic phenotype and an early death [43]. Mitochondrial movements have been associated with a variety of different kinesin-family motors. In Drosophila, KLP67 A is found in association with mitochondria [44]. Kinesin light chains appear to specify the site of binding for kinesin since an antibody to one of the splice forms of the light chains specifically labels mitochondria [45].

Kinesin light-chain knockout flies showed blocks in axonal transport that mimicked the heavy-chain phenotype [46]. There are swellings in the fly axons of organelles that were not transported to the periphery [47,48].

Knockout mice lacking cytoplasmic dynein heavy chain (cDHC) were found to die before 8.5 days postcoitum. Cells from blastocysts had dispersed Golgi and endosomes, and lysosomes were not concentrated near the nucleus [49]. In studies of secretory vesicles shed from the Golgi of intestinal epithelia, cytoplasmic dynein binding and movement has been found. When vesicles were extracted with cold Triton X-100, cytoplasmic dynein binding and motility was not lost even after the removal of the majority of spectrin, ankyrin and kinectin [50] (K. Fath and D. Burgess, personal communication). A complex of dynactin with Huntingtin protein and Huntingtin-associated protein suggests that protein complexes can be transported independent of vesicles.

Endocytic vesicles contain a variety of cytoskeletal proteins including kinesin and cytoplasmic dynein as well as the filament proteins, actin and tubulin [51].

Interaction of cytoplasmic dynein with frog ER membranes was cell-cycle dependent, and release correlated with increased phosphorylation of the dynein light intermediate chain [17]. High molecular mass tropomyosin was found to induce vesicle movements to the center of the cell along with cytoplasmic dynein, indicating that tropomyosin has a role in binding cytoplasmic dynein to vesicles [52].

Transport of non-vesicular cargoes

  1. Top of page
  2. Abstract
  3. Vesicle movements are short and saltatory
  4. Control of motor activity
  5. Models of the organelle motor complex
  6. Coordination of vesicle traffic
  7. Control of vesicle motility
  8. Vesicles transported by motors
  9. Transport of non-vesicular cargoes
  10. Motor-linkage components
  11. Technological advances in defining active motor complexes
  12. Summary
  13. Acknowledgements
  14. References

Although there has been an emphasis on the transport of membranous vesicles because motors were first isolated from brain tissue where transport was defined, the transport of other macromolecular complexes appears to involve many other motor proteins or perhaps the same motors with other linkages. Many of the motors have been implicated in the critical process of mitosis, which will not be considered here. In addition, the transport of mRNA–protein complexes is critical as well as the movement of macromolecular complexes involved in the assembly of flagella and axons (slow axonal transport).

A well-studied case is the bidirectional transport of raft complexes in flagella driven in the anterograde direction by the trimeric molecules of kinesin II (sea urchin) [53], Fla10 (Chlamydomonas) [54] or Kif3 A/B and KAP3 complex (mouse and frog) [55,56] and the retrograde direction by dynein [57]. The phenomenon of active transport in Chlamydomonas axonemes has been described as the saltatory movement of phase-dense particles between the axonemal microtubules and the membrane [58]. Mutations in a kinesin-like protein (Fla10) in the axonemes block outward movement of protein rafts, indicating that the outward movement is driven by a kinesin. Recently, the rafts have been isolated as 17S complexes but the complexes do not contain the motor proteins [54,59]. In the sea-urchin sperm, kinesin II is shown by immunofluorescence to be in close proximity to the rafts [53,60]. Photoreceptors have similar protein complexes that may be involved the receptor assembly [61]. One of the three sea-urchin kinesin II subunits [62] is not a motor. In the mouse, the analog is KAP3. KAP3 is thought to link the motor to vesicles [63]. The MAP kinase kinase kinases, MLK2 and MLK3 have been found by yeast two-hybrid to interact with the kinesin family member KIF3 [64]. Also using a yeast two-hybrid method a Smg-GDS associating protein (involved in GDP--GTP exchange) was shown to bind to a protein associated with sea urchin kinesin II (KIF3) [65]. More recently a chromosome-binding protein was added to the complex [66]. Although kinesin II is found to be a plus-end directed motor in vitro, a direct connection between the motor and the raft complexes has been elusive.

A role for cytoplasmic dynein in slow axonal transport has been postulated [67,68]. Kinesin binding to vimentin was observed that may be involved in moving intermediate filament complexes on microtubules [69]. Transcription-factor interactions with kinesin-related proteins suggest that they may also be transported along microtubules [70].

Messenger RNA transport is also supported by the microtubule motors such as kinesin [71] and requires intact microtubule, but not actin, structure. The observation that kinesin message is enriched in squid axons indicates that the synthesis of motors may aid in the transport process [72]. In the insect ovarioles, there is saltatory transport from nurse cells to the oocyte on microtubules [73]. The microtubules are oriented with their minus ends toward the oocyte and cytoplasmic dynein has been implicated as the major motor involved in the transport. Fusion proteins of motor domains and β-galactosidase have defined the polarity of transport events in Drosophila embryos (which can explain some of the polarization of mRNA in the oocyte) and indicate that minus-end motors are capable of moving to the distal portions of dendrites [74].

Yeast motor complexes have been defined from genetically interacting proteins and the complexes of 20 or more proteins are seen as functional units in spindle-pole separation [75]. The molecular basis of the organization of microtubules and their movements are being clarified through genetic analysis of motor and MAP mutants [76–79]. Dynamic measurements of yeast microtubules shows that they are moved rapidly and undergo major organizational changes [80].

Motor-linkage components

  1. Top of page
  2. Abstract
  3. Vesicle movements are short and saltatory
  4. Control of motor activity
  5. Models of the organelle motor complex
  6. Coordination of vesicle traffic
  7. Control of vesicle motility
  8. Vesicles transported by motors
  9. Transport of non-vesicular cargoes
  10. Motor-linkage components
  11. Technological advances in defining active motor complexes
  12. Summary
  13. Acknowledgements
  14. References

The mechanism of linkage of motors to cargo in an active motor complex is not well understood. In the case of kinesin, a membrane protein, kinectin, has been identified [13] and even soluble forms of kinectin are present that could be used for linking kinesin to other cargo sites [81]. In the case of cytoplasmic dynein, the cofactor dynactin is postulated to link the complex to the actin-binding protein, spectrin, through the actin-related proteins that form part of the dynactin complex [14]. However, neither dynactin nor spectrin are membrane proteins and the nature of the membrane binding site for cytoplasmic dynein is very unclear [50]. The inhibition of cytoplasmic dynein motility by an anti-kinectin antibody indicates that kinectin may have a role in cytoplasmic dynein motility [13]. Kinectin interacts with the small G proteins, as determined by yeast two-hybrid screens [26] as well as in-vitro peptide binding studies [27]. There are multiple splice variants of kinectin in mouse [52]. The ribosome receptor shares several domains with kinectin and with the ER protein ES/130 [82]. A mitochondrial distribution defect is found in Dictyostelium as a result of a knockout of CluA, which is a novel protein with a series of 42-amino-acid repeats [83]. The membrane site and the mechanism of motor activation on the membrane are still uncertain.

Mini-chromosomes were used to define the portions of the DNA responsible for interaction with specific motors by looking for transmission of the chromosomes to daughter cells in motor mutant backgrounds [84]. In-vitro assembly assays have been developed for identifying interacting partners [85]. Exquisite control of the motors, both plus and minus-end directed, must occur for the proper positioning of the spindle in yeast [86]. Nuclear movements to the bud site involve a feedback between Kip3 and cytoplasmic dynein [43]. Cytoplasmic dynein and dynactin complexes form with NuMA that are responsible for moving NuMA to spindle poles [87]. A kinesin-related protein, KLP38B, appears to bind to chromatin and participate in chromosome alignment on bipolar spindles [88] as well as meiosis [89]. It also binds to PP1 [90] and the fly deletion shows a deficiency in cytokinesis [91]. Another kinesin-related protein, costal2, has been found in the signaling complex for hedgehog [92]. Another kinesin-related protein binds to DNA [93]. How any of these phenomena are accomplished at a molecular level in vivo has yet to be established.

Technological advances in defining active motor complexes

  1. Top of page
  2. Abstract
  3. Vesicle movements are short and saltatory
  4. Control of motor activity
  5. Models of the organelle motor complex
  6. Coordination of vesicle traffic
  7. Control of vesicle motility
  8. Vesicles transported by motors
  9. Transport of non-vesicular cargoes
  10. Motor-linkage components
  11. Technological advances in defining active motor complexes
  12. Summary
  13. Acknowledgements
  14. References

Visualization of the motors during cellular motile events has been difficult. Recent technological advances have enabled the detection of gold-tagged antigens in deep-etched preparations of Xenopus eggs [94]. There, Eg5 has been localized to microtubules in the vegetal pole during the time when the egg cytoplasm is rotating [94]. Antibody labeling of the motors does not show distributional changes that correlate with function [95].

An interesting approach to the analysis of the full complement of proteins in the active motor complex is to use a saturating genetic screen. In an analysis of over 1000 ropy mutants, of the ropy phenotype in Aspergillus, which found both cytoplasmic dynein and dynactin [96], they have found 23 complementation groups. One of the mutants, nudc, has the interesting phenotype of an altered cell wall, suggesting that a motor complex may have a significant role in cell wall deposition as has been proposed [97].

Summary

  1. Top of page
  2. Abstract
  3. Vesicle movements are short and saltatory
  4. Control of motor activity
  5. Models of the organelle motor complex
  6. Coordination of vesicle traffic
  7. Control of vesicle motility
  8. Vesicles transported by motors
  9. Transport of non-vesicular cargoes
  10. Motor-linkage components
  11. Technological advances in defining active motor complexes
  12. Summary
  13. Acknowledgements
  14. References

We have proposed a graded model for the control of vesicle motility in normal membrane traffic. To maintain a balance of membrane flow at steady state, the level of transport should be related to the size of the membrane compartment. Position dependent activation must occur and the level of motility must be sensitive to overall level of cellular metabolic activity. Vesicle–motor interactions appear to be altered in axonal transport in a manner consistent with the longer duration of that process. Other transport events will be responsive to the different nature of the cargo being transported, as in the cases of axonemal rafts or cytoskeletal assemblies. In most functions, motor-dependent transport is catalytic and motor–cargo interactions are only one aspect of complex multifaceted membrane functions.

References

  1. Top of page
  2. Abstract
  3. Vesicle movements are short and saltatory
  4. Control of motor activity
  5. Models of the organelle motor complex
  6. Coordination of vesicle traffic
  7. Control of vesicle motility
  8. Vesicles transported by motors
  9. Transport of non-vesicular cargoes
  10. Motor-linkage components
  11. Technological advances in defining active motor complexes
  12. Summary
  13. Acknowledgements
  14. References
  • 1
    Nakagawa, T., Tanaka, Y., Matsuoka, E., Kondo, S., Okada, Y., Noda, Y., Kanai, Y. & Hirokawa, N. ( 1997) Identification and classification of 16 new kinesin superfamily (KIF) proteins in mouse genome. Proc. Natl Acad. Sci. USA 94, 96549659.
  • 2
    Yamashita, R. & May, G. ( 1998) Motoring along the hyphae, molecular motors and the fungal cytoskeleton. Curr. Opin. Cell Biol. 10, 7479.
  • 3
    Allen, R., Metuzals, D., Tasaki, I., Brady, S. & Gilbert, S. ( 1982) Fast axonal transport in squid giant axon. Science 218, 11271129.
  • 4
    Block, S., Goldstein, L. & Schnapp, B. ( 1990) Bead movement by single kinesin molecules studied with optical tweezers. Nature 348, 348352.
  • 5
    Wang, Z., Khan, S. & Sheetz, M. ( 1995) Single cytoplasmic dynein molecule movements, characterization and comparison with kinesin. Biophys. J. 69, 20112023.
  • 6
    Hamm-Alvarez, S., Kim, P. & Sheetz, M. ( 1993) Regulation of vesicle transport in CV-1 cells and extracts. J. Cell Sci. 106, 955966.
  • 7
    Burkhardt, J., Echeverri, C., Nilsson, T. & Vallee, R. ( 1997) Overexpression of the dynamitin (p50) subunit of the dynactin complex disrupts dynein-dependent maintenance of membrane organelle distribution. J. Cell Biol. 139, 469484.
  • 8
    Blocker, A., Severin, F., Burkhardt, J., Bingham, J., Yu, H., Olivo, J., Schroer, T., Hyman, A. & Griffiths, G. ( 1997) Molecular requirements for bidirectional movement of phagosomes along microtubules. J. Cell. Biol. 137, 113129.
  • 9
    Muresan, V., Godek, C., Reese, T. & Schnapp, B. ( 1996) Plus-end motors override minus-end motors during transport of squid axon vesicles on microtubules. J. Cell Biol. 135, 383397.
  • 10
    Jiang, M. & Sheetz, M. ( 1995) Cargo-activated ATPase activity of kinesin. Biophys. J. 68, 283S284S.
  • 11
    Ferro, K. & Collins, C. ( 1995) Microtubule-independent phospholipid stimulation of cytoplasmic dynein ATPase activity. J. Biol. Chem 270, 44924496.
  • 12
    Vale, R., Schnapp, B., Reese, T. & Sheetz, M. ( 1985) Organelle, bead, and microtubule translocations promoted by soluble factors from the squid giant axon. Cell 40, 559569.
  • 13
    Sheetz, M. & Yu, H. ( 1996) Regulation of kinesin and cytoplasmic dynein-driven organelle motility. Semin. Cell Dev. Biol. 7, 329334.
  • 14
    Vallee, R. & Sheetz, M. ( 1996) Targeting of motor proteins. Science 271, 15391544.
  • 15
    Elluru, R., Bloom, G. & Brady, S. ( 1995) Fast axonal transport of kinesin in the rat visual system: functionality of kinesin heavy chain isoforms. Mol. Biol. Cell. 6, 2140. (Erratum published in 6, 1261.)
  • 16
    Lippincott-Schwartz, J. ( 1998) Cytoskeletal proteins and Golgi dynamics. Curr. Opin. Cell Biol. 10, 5259.
  • 17
    Niclas, J., Allan, V. & Vale, R. ( 1996) Cell cycle regulation of dynein association with membranes modulates microtubule-based organelle transport. J. Cell Biol. 133, 585593.
  • 18
    Lindesmith, L., McIlvain, J., Argon, Y. & Sheetz, M. ( 1997) Phosphotransferases associated with the regulation of kinesin motor activity. J. Biol. Chem 272, 2292922933.
  • 19
    Marlowe, K., Farshori, P., Torgerson, R., Anderson, K., Miller, L. & McNiven, M. ( 1998) Changes in kinesin distribution and phosphorylation occur during regulated secretion in pancreatic acinar cells. Eur J. Cell Biol. 75, 140152.
  • 20
    Lee, K. & Hollenbeck, P. ( 1995) Phosphorylation of kinesin in vivo correlates with organelle association and neurite outgrowth. J. Biol. Chem 270, 56005605.
  • 21
    Vernos, I. & Karsenti, E. ( 1996) Motors involved in spindle assembly and chromosome segregation, Curr. Opin. Cell Biol. 8, 49.
  • 22
    Okada, Y., Sato-Yoshitake, R. & Hirokawa, N. ( 1995) The activation of protein kinase A pathway selectively inhibits anterograde axonal transport of vesicles but not mitochondria transport or retrograde transport in vivo. J. Neurosci. 15, 30533064.
  • 23
    Karki, S., Tokito, M. & Holzbaur, E. ( 1997) Casein kinase II binds to and phosphorylates cytoplasmic dynein. J. Biol. Chem 272, 58875891.
  • 24
    Rothwell, S. & Calvert, V. ( 1997) Activation of human platelets causes post-translational modifications to cytoplasmic dynein. Thromb. Haemost. 78, 910918.
  • 25
    Bloom, G. & Goldstein, L. ( 1998) Cruising along microtubule highways, how membranes move through the secretory pathway. J. Cell Biol. 140, 12771280.
  • 26
    Hotta, K., Tanaka, K., Mino, A., Kohno, H. & Takai, Y. ( 1996) Interaction of the Rho family small G proteins with kinectin, an anchoring protein of kinesin motor. Biochem. Biophys. Res. Commun. 225, 6974.
  • 27
    Alberts, A., Bouquin, N., Johnston, L. & Treisman, R. ( 1998) Analysis of RhoA-binding proteins reveals an interaction domain conserved in heterotrimeric G protein beta subunits and the yeast response regulator protein Skn7. J. Biol. Chem 273, 86168622.
  • 28
    Echeverri, C., Paschal, B., Vaughan, K. & Vallee, R. ( 1996) Molecular characterization of the 50-kD subunit of dynactin reveals function for the complex in chromosome alignment and spindle organization during mitosis. J. Cell Biol. 132, 617633.
  • 29
    Criswell, P. & Asai, D. ( 1998) Evidence for four cytoplasmic dynein heavy chain isoforms in rat testis. Mol. Biol. Cell 9, 237247.
  • 30
    Criswell, P., Ostrowski, L. & Asai, D. ( 1996) A novel cytoplasmic dynein heavy chain, expression of DHC1b in mammalian ciliated epithelial cells. J. Cell Sci. 109, 18911898.
  • 31
    Vaisberg, E., Grissom, P. & McIntosh, J. ( 1996) Mammalian cells express three distinct dynein heavy chains that are localized to different cytoplasmic organelles. J. Cell Biol. 133, 831842.
  • 32
    Moreira, J., Dodane, V. & Reese, T. ( 1998) Immunoelectronmicroscopy of soluble and membrane proteins with a sensitive postembedding method. J. Histochem. Cytochem. 46, 847854.
  • 33
    Stenoien, D. & Brady, S. ( 1997) Immunochemical analysis of kinesin light chain function. Mol. Biol. Cell 8, 675689.
  • 34
    Meng, Y., Wilson, G., Avery, M., Varden, C. & Balczon, R. ( 1997) Suppression of the expression of a pancreatic beta-cell form of the kinesin heavy chain by antisense oligonucleotides inhibits insulin secretion from primary cultures of mouse beta-cells. Endocrinology 138, 19791987.
  • 35
    Amaratunga, A., Leeman, S., Kosik, K. & Fine, R. ( 1995) Inhibition of kinesin synthesis in vivo inhibits the rapid transport of representative proteins for three transport vesicle classes into the axon. J. Neurochem. 64, 23742376.
  • 36
    Minin, A. ( 1997) Dispersal of Golgi apparatus in nocodazole-treated fibroblasts is a kinesin-driven process. J. Cell Sci. 110, 24952505.
  • 37
    Marples, D., Barber, B. & Taylor, A. ( 1996) Effect of a dynein inhibitor on vasopressin action in toad urinary bladder. J. Physiol. (London) 490, 767774.
  • 38
    Marples, D., Schroer, T., Ahrens, N., Taylor, A., Knepper, M. & Nielsen, S. ( 1998) Dynein and dynactin colocalize with AQP2 water channels in intracellular vesicles from kidney collecting duct. Am. J. Physiol. 274, F384F394.
  • 39
    Seiler, S., Nargang, F., Steinberg, G. & Schliwa, M. ( 1997) Kinesin is essential for cell morphogenesis and polarized secretion in Neurospora crassa. EMBO J. 16, 30253034.
  • 40
    Tanaka, Y., Kanai, Y., Okada, Y., Nonaka, S., Takeda, S., Harada, A. & Hirokawa, N. ( 1998) Targeted disruption of mouse conventional kinesin heavy chain, kif5B, results in abnormal perinuclear clustering mitochondria. Cell 93, 11471158.
  • 41
    Miyake, K. & McNeil, P. ( 1995) Vesicle accumulation and exocytosis at sites of plasma membrane disruption. J. Cell Biol. 131, 17371745.
  • 42
    De Vos, K., Goossens, V., Boone, E., Vercammen, D., Vancompernolle, K., Vandenabeele, P., Haegeman, G., Fiers, F. & Grooten, J. ( 1998) The 55-kDa tumor necrosis factor receptor induces clustering of mitochondria through its membrane-proximal region. J. Biol. Chem 273, 96739680.
  • 43
    DeZwaan, T., Ellingson, E., Pellman, D. & Roof, D. ( 1997) Kinesin-related KIP3 of Saccharomyces cerevisiae is required for a distinct step in nuclear migration. J. Cell Biol. 138, 10231040.
  • 44
    Pereira, A., Dalby, B., Stewart, R., Doxsey, S. & Goldstein, L. ( 1997) Mitochondrial association of a plus end-directed microtubule motor expressed during mitosis in Drosophila. J. Cell Biol. 136, 10811090.
  • 45
    Khodjakov, A., Lizunova, E., Minin, A., Koonce, M. & Gyoeva, F. ( 1998) A specific light chain of kinesin associates with mitochondria in cultured cells. Mol. Biol. Cell 9, 333343.
  • 46
    Gindhart, J., Desai, C., Beushausen, S., Zinn, K. & Goldstein, L. ( 1998) Kinesin light chains are essential for axonal transport in Drosophila. J. Cell Biol. 141, 443454.
  • 47
    Hurd, D. & Saxton, W. ( 1996) Kinesin mutations cause motor neuron disease phenotypes by disrupting fast axonal transport in Drosophila. Genetics 144, 10751085.
  • 48
    Hurd, D., Stern, M. & Saxton, W. ( 1996) Mutation of the axonal transport motor kinesin enhances paralytic and suppresses Shaker in Drosophila. Genetics 142, 195204.
  • 49
    Harada, A., Takei, Y., Kanai, Y., Tanaka, Y., Nonaka, S. & Hirokawa, N. ( 1998) Golgi vesiculation and lysosome dispersion in cells lacking cytoplasmic dynein. J. Cell Biol. 141, 5159.
  • 50
    Fath, K., Trimbur, G. & Burgess, D. ( 1997) Molecular motors and a spectrin matrix associate with Golgi membranes in vitro. J. Cell Biol. 139, 11691181.
  • 51
    Pol, A., Ortega, D. & Enrich, C. ( 1997) Identification of cytoskeleton-associated proteins in isolated rat liver endosomes. Biochem. J. 327, 741746.
  • 52
    Leung, E., Print, C., Parry, D., Closey. D., Lockhart, P., Skinner, S., Batchelor, D. & Krissansen, G. ( 1996) Cloning of novel kinectin splice variants with alternative C-termini, structure, distribution and evolution of mouse kinectin. Immunol. Cell Biol. 74, 421433.
  • 53
    Morris, R. & Scholey, J. ( 1997) Heterotrimeric kinesin-II is required for the assembly of motile 9+2 ciliary axonemes on sea urchin embryos. J. Cell Biol. 138, 10091022.
  • 54
    Cole, D., Diener, D., Himelblau, A., Beech, P., Fuster, J. & Rosenbaum, J. ( 1998) Chlamydomonas kinesin-II-dependent intraflagellar transport (IFT), IFT particles contain proteins required for ciliary assembly in Caenorhabditis elegans sensory neurons. J. Cell Biol. 141, 9931008.
  • 55
    Nakajima, T., Miura, I., Kashiwagi, A. & Nakamura, M. ( 1997) Molecular cloning and expression of the KIF3A gene in the frog brain and testis. Zool. Sci. 14, 917921.
  • 56
    Yamazaki, H., Nakata, T., Okada, Y. & Hirokawa, N. ( 1995) KIF3A/B, a heterodimeric kinesin superfamily protein that works as a microtubule plus end-directed motor for membrane organelle transport. J. Cell Biol. 130, 13871399.
  • 57
    Pazour, G., Wilkerson. C. & Witman, G. ( 1998) A dynein light chain is essential for the retrograde particle movement of intraflagellar transport (IFT). J. Cell Biol. 141, 979992.
  • 58
    Kozminski, K., Johnson, K., Forscher, P. & Rosenbaum, J. ( 1993) A motility in the eukaryotic flagellum unrelated to flagellar beating. Proc. Natl Acad. Sci. USA 90, 55195523.
  • 59
    Piperno, G. & Mead, K. ( 1997) Transport of a novel complex in the cytoplasmic matrix of Chlamydomonas flagella. Proc. Natl Acad. Sci. USA 94, 44574462.
  • 60
    Henson, J., Cole, D., Roesener, C., Capuano, S., Mendola, R. & Scholey, M. ( 1997) The heterotrimeric motor protein kinesin-II localizes to the midpiece and flagellum of sea urchin and sand dollar sperm. Cell Motil. Cytoskeleton 38, 2937.
  • 61
    Muresan, V., Bendala-Tufanisco, E., Hollander, B. & Besharse, J. ( 1997) Evidence for kinesin-related proteins associated with the axoneme of retinal photoreceptors. Exp. Eye Res. 64, 895903.
  • 62
    Wedaman, K., Meyer, D., Rashid, D., Cole, D. & Scholey, J. ( 1996) Sequence and submolecular localization of the 115-kD accessory subunit of the heterotrimeric kinesin-II (KRP85/95) complex. J. Cell Biol. 132, 371380.
  • 63
    Yamazaki, H., Nakata, T., Okada, Y. & Hirokawa, N. ( 1996) Cloning and characterization of KAP3, a novel kinesin superfamily-associated protein of KIF3A/3B. Proc. Natl Acad. Sci. USA 93, 84438448.
  • 64
    Nagata, K., Puls, A., Futter, C., Aspenstrom, P., Schaefer, E., Nakata, T., Hirokawa, N. & Hall, A. ( 1998) The MAP kinase kinase kinase MLK2 co-localizes with activated JNK along microtubules and associates with kinesin superfamily motor KIF3. EMBO J. 17, 149158.
  • 65
    Shimizu, K., Kawabe, H., Minami, S., Honda, T., Takaishi, K., Shirataki, H. & Takai, Y. ( 1996) SMAP, an Smg GDS-associating protein having arm repeats and phosphorylated by Src tyrosine kinase. J. Biol. Chem 271, 2701327017.
  • 66
    Shimizu, K., Shirataki, H., Honda, T., Minami, S. & Takai, Y. ( 1998) Complex formation of SMAP/KAP3, a KIF3A/B ATPase motor-associated protein, with a human chromosome-associated polypeptide. J. Biol. Chem 273, 65916594.
  • 67
    Dillman, 3 rd, J., Dabney, L., Karki, S., Paschal, B., Holzbaur, E. & Pfister, K. ( 1996) Functional analysis of dynactin and cytoplasmic dynein in slow axonal transport. J. Neurosci. 16, 67426752.
  • 68
    Dillman, 3 rd, J., Dabney, L. & Pfister, K. ( 1996) Cytoplasmic dynein is associated with slow axonal transport. Proc. Natl Acad. Sci. USA 93, 141144.
  • 69
    Liao, G. & Gundersen, G. ( 1998) Kinesin is a candidate for cross-bridging microtubules and intermediate filaments: selective binding of kinesin to detyrosinated tubulin and vimentin. J. Biol. Chem 273, 97979803.
  • 70
    Kalderon, D. ( 1997) Hedgehog signalling, Ci complex cuts clasps. Curr. Biol. 7, R759R762.
  • 71
    Carson, J., Worboys, K., Ainger, K. & Barbarese, E. ( 1997) Translocation of myelin basic protein mRNA in oligodendrocytes requires microtubules and kinesin. Cell Motil. Cytoskeleton 38, 318328.
  • 72
    Chun, J., Gioio, A., Crispino, M., Giuditta, A. & Kaplan, B. ( 1996) Differential compartmentalization of mRNAs in squid giant axon. J. Neurochem. 67, 18061812.
  • 73
    Harrison, R. & Huebner, E. ( 1997) Unipolar microtubule array is directly involved in nurse cell-oocyte transport. Cell Motil. Cytoskeleton 36, 355362.
  • 74
    Clark, I., Jan, L. & Jan, N. ( 1997) Reciprocal localization of Nod and kinesin fusion proteins indicates microtubule polarity in the Drosophila oocyte epithelium neuron muscle Development 124, 461470.
  • 75
    Geiser, J., Schott, E., Kingsbury, T., Cole, N., Totis, L., Bhattacharyya, G., He, L. & Hoyt, M. ( 1997) Saccharomyces cerevisiae genes required in the absence of the CIN8-encoded spindle motor act in functionally diverse mitotic pathways. Mol. Biol. Cell 8, 10351050.
  • 76
    Huyett, A., Kahana, J., Silver, P., Zeng, X. & Saunders. W. ( 1998) The Kar3p and Kip2p motors function antagonistically at the spindle poles to influence cytoplasmic microtubule numbers. J. Cell Sci. 111, 295301.
  • 77
    Manning, B., Padmanabha, R. & Snyder, M. ( 1997) The Rho-GEF Rom2p localizes to sites of polarized cell growth and participates in cytoskeletal functions in Saccharomyces cerevisiae. Mol. Biol. Cell 8, 18291844.
  • 78
    Pidoux, A., LeDizet, M. & Cande, W. ( 1996) Fission yeast pkl1 is a kinesin-related protein involved in mitotic spindle function. Mol. Biol. Cell 7, 16391655.
  • 79
    Saunders, W., Hornack, D., Lengyel, V. & Deng, C. ( 1997) The Saccharomyces cerevisiae kinesin-related motor Kar3p acts at preanaphase spindle poles to limit the number and length of cytoplasmic microtubules. J. Cell Biol. 137, 417431.
  • 80
    Shaw, S., Yeh, E., Maddox, P., Salmon, E. & Bloom, K. ( 1997) Astral microtubule dynamics in yeast, a microtubule-based searching mechanism for spindle orientation and nuclear migration into bud. J. Cell Biol. 139, 985994.
  • 81
    Kumar, J. , Erickson, H. & Sheetz, M. ( 1998) Ultra-structural and biochemical properties of the 120 kDa form of kinectin in chicken. J. Biol. Chem.273, 3173831743.
  • 82
    Print, C., Morris, C., Spurr, N., Rooke, L. & Krissansen, G. ( 1996) The CG-1 gene, a member of the kinectin and ES/130 family, maps to human chromosome band 14q22. Immunogenetics 43, 227229.
  • 83
    Zhu, Q., Hulen, D., Liu, T. & Clarke, M. ( 1997) The cluA-mutant of Dictyostelium identifies a novel class of protein required for dispersion of mitochondria. Proc. Natl Acad. Sci. USA 94, 73087313.
  • 84
    Cook, K., Murphy, T., Nguyen, T. & Karpen, G. ( 1997) Identification of trans-acting genes necessary for centromere function in Drosophila melanogaster using centromere-defective minichromosomes. Genetics 145, 737747.
  • 85
    Desai, A., Deacon, H., Walczak, C. & Mitchison, T. ( 1997) A method that allows the assembly of kinetochore components onto chromosomes condensed in clarified Xenopus egg extracts. Proc. Natl Acad. Sci. USA 94, 1237812383.
  • 86
    Cottingham, F. & Hoyt, M. ( 1997) Mitotic spindle positioning in Saccharomyces cerevisiae is accomplished by antagonistically acting microtubule motor proteins. J. Cell Biol. 138, 10411053.
  • 87
    Merdes, A., Ramyar, K., Vechio, J. & Cleveland, D. ( 1996) A complex of NuMA and cytoplasmic dynein is essential for mitotic spindle assembly. Cell 87, 447458.
  • 88
    Molina, I., Baars, S., Brill, J., Hales, K., Fuller, M. & Ripoll, P. ( 1997) A chromatin-associated kinesin-related protein required for normal mitotic chromosome segregation in Drosophila. J. Cell Biol. 139, 13611371.
  • 89
    Ruden, D., Cui, W., Sollars, V. & Alterman, M. ( 1997) A Drosophila kinesin-like protein, Klp38B, functions during meiosis, mitosis and segmentation. Dev. Biol. 191, 284296.
  • 90
    Alphey, L., Parker, L., Hawcroft, G., Guo, Y., Kaiser, K. & Morgan, G. ( 1997) KLP38B, a mitotic kinesin-related protein that binds PP1. J. Cell Biol. 138, 395409.
  • 91
    Ohkura, H., Torok, T., Tick, G., Hoheisel, J., Kiss, I. & Glover. D. ( 1997) Mutation of a gene for a Drosophila kinesin-like protein, Klp38B, leads to failure of cytokinesis. J. Cell Sci. 110, 945954.
  • 92
    Robbins, D., Nybakken, K., Kobayashi, R., Sisson, J., Bishop, J. & Therond, P. ( 1997) Hedgehog elicits signal transduction by means of a large complex containing the kinesin-related protein costal2. Cell 90, 225234.
  • 93
    Tokai, N., Fujimoto-Nishiyama, A., Toyoshima, Y., Yonemura, Y., Tsukita, S., Inoue, J. & Yamamota, T. ( 1996) Kid, a novel kinesin-like DNA binding protein, is localized to chromosomes and the mitotic spindle. EMBO J. 15, 457467.
  • 94
    Chang, P., LeGuellec, K. & Houliston, E. ( 1996) Immunodetection of cytoskeletal structures and the Eg5 motor protein on deep-etch replicas of Xenopus egg cortices isolated during the cortical rotation. Biol. Cell 88, 8998.
  • 95
    Lin, S., Pfister, K. & Collins, C. ( 1996) Comparison of the intracellular distribution of cytoplasmic dynein and kinesin in cultured cells: motor protein location does not reliably predict function. Cell Motil. Cytoskeleton 34, 299312.
  • 96
    Bruno, G., Tinsley, J., Minke, P. & Plamann, M. ( 1996) Genetic interactions among cytoplasmic dynein, dynactin, and nuclear distribution mutants of Neurospora crassa. Proc. Natl Acad. Sci. USA 93, 47754780.
  • 97
    Chiu, Y., Xiang, X., Dawe, A. & Morris, N. ( 1997) Deletion of nudC, a nuclear migration gene of Aspergillus nidulans, causes morphological and cell wall abnormalities and is lethal. Mol. Biol. Cell 8, 17351749.