Extensin

Extensin was discovered by Derek T.A. Lamport at Cambridge University, they are hydroxyproline-rich glycoproteins (HRGPs) of the plant cell wall.

Extensins defined: A family of flexuous rodlike hydroxyproline-rich glycoproteins (HRGPs) arguably among the most abundant proteins on planet earth. Extensins number about twenty in Arabidopsis. 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. Hence the current view of extensins as self-assembling amphiphiles 10 essential for cell wall assembly and growth by cell extension and expansion. The name extensin encapsulates a working hypothesis, namely that they are involved in cell extension 4. Some consider that a misnomer.

1.	The Ser-Hyp-Hyp-Hyp-Hyp hydrophilic motif 6, 7 3. [Ser = serine; Hyp = hydroxyproline] This pentapeptide consists of serine and four hydroxyprolines. Hydroxyproline is unusual not only as a cyclic amino acid that restricts peptide flexibility but as an amino acid with no codon. The genome encodes it as proline. Polypeptides targeted for secretion are subsequently hydroxylated by direct addition of molecular oxygen to proline at C-4 4. Extensin hydroxyproline is uniquely glycosylated with short chains of L-arabinose 5 that further rigidify 12 and increase hydrophilicity. Generally the serine has a single galactose attached 9.

2.	The hydrophobic tyrosine crosslinking motif. Two tyrosines separated by a single amino acid, typically valine or another tyrosine, form a short intra-molecular diphenylether crosslink 1. This can be crosslinked further by the enzyme extensin peroxidase 2, 8, 11 to form an inter-molecular bridge between extensin molecules and thus form networks and sheets.

Reference List

1. 	Epstein,L. and Lamport,D.T.A. (1984) An intramolecular linkage involving isodityrosine in extensin. Phytochem., 23, 1241-1246. 2. 	Everdeen,D.S., Kiefer,S., Willard,J.J., Muldoon,E.P., Dey,P.M., Li,X.-B., and Lamport,D.T.A. (1988) Enzymic crosslinkage of monomeric extensin precursors in vitro. Plant Physiol., 87, 616-621. 3. 	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. 4. 	Lamport,D.T.A. (1963) Oxygen fixation into hydroxyproline of plant cell wall protein. J.Biol.Chem., 238, 1438-1440. 5. 	Lamport,D.T.A. (1967) Hydroxyproline-O-glycosidic linkage of the plant cell wall glycoprotein extensin. Nature, 216, 1322-1324. 6. 	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. 7. 	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. 8. 	Lamport,D.T.A. (1989): Extensin peroxidase ties the knots in the extensin network. In: Cell Separation in Plants, edited by D.J.Osborne, et al, pp. 101-113. Springer-Verlag, Berlin. 9. 	Lamport,D.T.A., Katona,L., and Roerig,S. (1973) Galactosyl serine in extensin. Biochem.J., 133, 125-131. 10. 	Rapaport,H. (2006) Ordered peptide assemblies at interfaces. Supramolecular Chemistry, 18, 445-454. 11. 	Schnabelrauch,L.S., Kieliszewski,M.J., Upham,B.L., Alizedeh,H., and Lamport,D.T.A. (1996) Isolation of pI 4.6 extensin peroxidase from tomato cell suspension cultures and identification of Val-Tyr-Lys as putative intermolecular cross-link site. Plant J., 9, 477-489. 12. 	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.