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 beta-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 pI 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.(ABSTRACT TRUNCATED AT 400 WORDS)

Cytokinesis partitions the cell by a cleavage furrow in animals but by a new cross wall in plants. How this new wall assembles at the molecular level and connects with the mother cell wall remains unclear. A lethal Arabidopsis embryogenesis mutant designated root-, shoot-, hypocotyl-defective (rsh) provides some clues: RSH encodes extensin AtEXT3, a structural glycoprotein located in the nascent cross wall or ''cell plate'' and also in mature cell walls. Here we report that electron micrographs of rsh mutant cells lacking RSH extensin correspond to a wall phenotype typified by incomplete cross wall assembly. Biochemical characterization of the purified RSH glycoprotein isolated from wild-type Arabidopsis cell cultures confirmed its identity as AtEXT3: a (hydroxy)proline-rich glycoprotein comprising 11 identical amphiphilic peptide repeats with a 28-residue periodicity: SOOOOKKHYVYKSOOOOVKHYSOOOVYH (O ‫؍‬ Hyp), each repeat containing a hydrophobic isodityrosine cross-link motif (YVY, underlined). Atomic force microscopy of RSH glycoprotein imaged its propensity for self-assembly into a dendritic scaffold. Extensin peroxidase catalyzed in vitro formation of insoluble RSH gels with concomitant tyrosine cross-linking, hence this likelihood in muro. We conclude that self-assembling amphiphiles of lysine-rich RSH extensin form positively charged scaffolds in the cell plate. These react with negatively charged pectin to create an extensin pectate coacervate that may template further orderly deposition of the new cross wall at cytokinesis. cytokinesis ͉ (hydroxy)proline-rich glycoprotein ͉ nanotechnology

Plant cell walls are comprised largely of the polysaccharides cellulose, hemicellulose, and pectin, along with ;10% protein and up to 40% lignin. These wall polymers interact covalently and noncovalently to form the functional cell wall. Characterized cross-links in the wall include covalent linkages between wall glycoprotein extensins between rhamnogalacturonan II monomer domains and between polysaccharides and lignin phenolic residues. Here, we show that two isoforms of a purified Arabidopsis thaliana arabinogalactan protein (AGP) encoded by hydroxyproline-rich glycoprotein family protein gene At3g45230 are covalently attached to wall matrix hemicellulosic and pectic polysaccharides, with rhamnogalacturonan I (RG I)/homogalacturonan linked to the rhamnosyl residue in the arabinogalactan (AG) of the AGP and with arabinoxylan attached to either a rhamnosyl residue in the RG I domain or directly to an arabinosyl residue in the AG glycan domain. The existence of this wall structure, named ARABINOXYLAN PECTIN ARABINOGALACTAN PROTEIN1 (APAP1), is contrary to prevailing cell wall models that depict separate protein, pectin, and hemicellulose polysaccharide networks. The modified sugar composition and increased extractability of pectin and xylan immunoreactive epitopes in apap1 mutant aerial biomass support a role for the APAP1 proteoglycan in plant wall architecture and function.

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