Extensin

Extensins are a family of flexuous, rodlike, hydroxyproline-rich glycoproteins (HRGPs) of the plant cell wall.[1][2]

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[3] 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.[4]

Hydrophilic motif

This pentapeptide consists of serine (Ser) and four hydroxyprolines (Hyp): Ser-Hyp-Hyp-Hyp-Hyp.[5][6][7] 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.[4] Extensin hydroxyproline is uniquely glycosylated with short chains of L-arabinose[8] that further rigidify[9] and increase hydrophilicity. Generally the serine has a single galactose attached.[10]

Hydrophobic tyrosine crosslinking motif

Two tyrosines separated by a single amino acid, typically valine or another tyrosine, form a short intra-molecular diphenylether crosslink.[11] This can be crosslinked further by the enzyme extensin peroxidase[12][13][14] to form an inter-molecular bridge between extensin molecules and thus form networks and sheets.

References
  1. Lamport, D.T.A. (1965) Advances in Botanical Research 2:151-218 The protein component of primary cell walls
  2. Lamport,D.T.A.; Northcote,D.H. (1960) Nature 188:665-666 Hydroxyproline in primary cell walls of higher plants
  3. Rapaport,H. (2006) Ordered peptide assemblies at interfaces. Supramolecular Chemistry, 18, 445-454.
  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. (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.
  6. 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.
  7. 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.
  8. Lamport,D.T.A. (1967) Hydroxyproline-O-glycosidic linkage of the plant cell wall glycoprotein extensin. Nature, 216, 1322-1324.
  9. 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.
  10. Lamport,D.T.A., Katona,L., and Roerig,S. (1973) Galactosyl serine in extensin. Biochem.J., 133, 125-131.
  11. Epstein,L. and Lamport,D.T.A. (1984) An intramolecular linkage involving isodityrosine in extensin. Phytochemistry, 23, 1241-1246.
  12. 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.
  13. 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.
  14. 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.

Further reading
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