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Extensin
Extensins are a family of flexuous, rodlike, hydroxyproline-rich glycoproteins (HRGPs) of the plant cell wall,〔Lamport, D.T.A. (1965) Advances in Botanical Research 2:151-218 The protein component of primary cell walls〕 discovered by Derek T.A. Lamport at the University of Cambridge.〔Lamport,D.T.A.; Northcote,D.H. (1960) Nature 188:665-666 Hydroxyproline in primary cell walls of higher plants〕
They are highly abundant proteins. There are around 20 extensins in ''Arabidopsis thaliana''. They form crosslinked networks in the young cell wall. Typically they have two major diagnostic repetitive peptide motifs, one hydrophilic and the other hydrophobic, with potential for crosslinking. Extensins are thought to act as self-assembling amphiphiles〔Rapaport,H. (2006) Ordered peptide assemblies at interfaces. Supramolecular Chemistry, 18, 445-454.〕 essential for cell-wall assembly and growth by cell extension and expansion. The name "extensin" encapsulates the hypothesis that they are involved in cell extension,〔Lamport,D.T.A. (1963) Oxygen fixation into hydroxyproline of plant cell wall protein. J.Biol.Chem., 238, 1438-1440.〕 and some scientists consider it a misnomer.
==Hydrophilic motif==
This pentapeptide consists of serine (Ser) and four hydroxyprolines (Hyp): Ser-Hyp-Hyp-Hyp-Hyp.〔Lamport,D.T.A. (1973): The glycopeptide linkages of extensin, O-D-galactosyl serine and O-L-arabinosyl hydroxyproline. In: Biogenesis of plant cell wall polysaccharides, Anonymouspp. 149-164. Academic Press Inc., New York.〕〔Lamport,D.T.A. (1977): Structure, biosynthesis and significance of cell wall glycoproteins. In: Recent Advances in Phytochemistry, edited by F.A.Loewus, et al, pp. 79-115. Plenum Publishing Corp., New York.〕〔Fong,C., Kieliszewski,M.J., de Zacks,R., Leykam,J.F., and Lamport,D.T.A. (1992) A gymnosperm extensin contains the serine-tetrahydroxyproline motif. Plant Physiol., 99, 548-552.〕 Hydroxyproline is unusual not only as a cyclic amino acid that restricts peptide flexibility but as an amino acid with no codon, being encoded as proline. Polypeptides targeted for secretion are subsequently hydroxylated by direct addition of molecular oxygen to proline at C-4.〔 Extensin hydroxyproline is uniquely glycosylated with short chains of L-arabinose〔Lamport,D.T.A. (1967) Hydroxyproline-O-glycosidic linkage of the plant cell wall glycoprotein extensin. Nature, 216, 1322-1324.〕 that further rigidify〔van Holst,G.-J. and Varner,J.E. (1984) Reinforced polyproline II conformation in a hydroxyproline-rich glycoprotein from carrot root. Plant Physiol., 74, 247-251.〕 and increase hydrophilicity. Generally the serine has a single galactose attached.〔Lamport,D.T.A., Katona,L., and Roerig,S. (1973) Galactosyl serine in extensin. Biochem.J., 133, 125-131.〕
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