Homologous hydroxyproline-rich glycoproteins (HRGPs) of the plant extracellular matrix include extensins, repetitive proline-rich proteins (RPRPs), some nodulins, gum arabic glycoprotein (GAGP), arabinogalactan-proteins (AGPs), and chimeric proteins such as potato lectin which contain an extensin module fused to a lectin. The key to the role of HRGPs in cell wall self-assembly and cell extension lies in their chemistry, which is dependent on extensive post-translational modifications (PTMs): hydroxylation, glycosylation, and cross-linking. Repetitive peptide motifs characterize HRGPs. One or more repetitive peptide motifs and their variants, singly or in combination, may constitute functional sites involved in various aspects of cell wall assembly, as follows:
- (i)X—Hypn including Ser—Hyp4 (arabinosylation site, molecular rigidity, and reptation).
- (ii)Pro—Hyp—Val—Tyr—Lys and variants (putative intermolecular cross-links, adhesion, cohesion, and possible β-turns).
- (iii)Tyr—X—Tyr—Lys (intramolecular isodityrosine [IDT] cross-links increase molecular rigidity and hydrophobicity).
- (iv)(Glyco)peptide palindromes (centrosymmetric domains: putative self-assembly nucleation sites).
- (v)Ionic interaction sites (protein—protein and protein-carbohydrate cross-links).
- (vi)Hyp and Ser glycosylation sites (enhance conformational stability and molecular recognition).
- (vii)Extensin modules in chimeric proteins (e.g. solanaceous lectins).
Rules for the post-translational modifications are emerging:
- (i)Hydroxylation of proline residues may depend on multiple, sequence-specific prolyl hydroxylases rather than on a single (polyproline-II) conformation-dependent enzyme. Furthermore, Lys—Pro, Tyr—Pro, and Phe—Pro are not hydroxylated, while Pro—Val is always.
- (ii)Contiguity of Hyp residues probably determines the extent of Hyp glycosylation, blocks of tetrahydroxyproline (Hyp4) being the most highly arabinosylated, while single non-contiguous Hyp residues are rarely arabinosylated, although they are likely attachment sites for the larger arabinogalactan substituents of gum arabic glycoprotein and arabinogalactan-proteins.
- (iii)While intramolecular cross-links involve IDT, unidentified intermolecular cross-links most likely involve the Val—Tyr—Lys motif (perhaps also Val—Lys—Pro—Tyr—His—Pro), probably as an adduct between Tyr and Lys catalyzed in vitro by a pl 4.6 extensin cross-linking peroxidase. Thus, we can classify HRGPs functionally as either cross-linking or non-cross-linking, i.e. CL- or NCL-extensins.
Their protistan origin obscures the phylogenetic affinities of a single extensin-HRGP family due to their sequence divergence. We propose a phylogenetic series ranging from the minimally glycosylated basic RPRPs to the highly glycosylated acidic AGPs. Furthermore, based on similarities between dicots and gymnosperm extensins, and their marked difference from graminaceous monocot extensins, graminaceous monocot and dicot lines may have diverged as early as the progymnosperms. The origin of the embryophytes (land plants) is an even more open question, the nearest extant algal relatives being unknown. The Charophyta, frequently suggested as ancestral to the embryophytes, seem unlikely as the Charalean cell wall of Chara and Nitella lacks hydroxyproline.