# Extensin

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{{short description|Family of glycoproteins found in plants}}
'''Extensins ''' are a family of flexuous, rodlike, [hydroxyproline](/source/hydroxyproline)-rich [glycoprotein](/source/glycoprotein)s (HRGPs) of the [plant cell wall](/source/plant_cell_wall).<ref>Lamport, D.T.A. (1965) Advances in Botanical Research 2:151-218 The protein component of primary cell walls</ref><ref>Lamport,D.T.A.; Northcote,D.H. (1960) Nature 188:665-666 Hydroxyproline in primary cell walls of higher plants</ref>

They are highly abundant proteins. There are around 20 extensins in ''[Arabidopsis thaliana](/source/Arabidopsis_thaliana)''. They form crosslinked networks in the young cell wall. Typically they have two major diagnostic repetitive peptide motifs, one [hydrophilic](/source/hydrophilic) and the other [hydrophobic](/source/hydrophobic), with potential for crosslinking. Extensins are thought to act as [self-assembling](/source/self-assembly) [amphiphile](/source/amphiphile)s<ref>Rapaport,H. (2006) Ordered peptide assemblies at interfaces.  Supramolecular Chemistry, 18, 445-454.</ref> 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.<ref name=Lamport_1963>Lamport,D.T.A. (1963) Oxygen fixation into hydroxyproline of plant cell wall protein.  J.Biol.Chem., 238, 1438-1440.</ref>

==Hydrophilic motif==
This pentapeptide consists of [serine](/source/serine) (Ser) and four [hydroxyproline](/source/hydroxyproline)s (Hyp): Ser-Hyp-Hyp-Hyp-Hyp.<ref>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.</ref><ref>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.</ref><ref>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.</ref> 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](/source/proline). Polypeptides targeted for [secretion](/source/secretion) are subsequently hydroxylated by direct addition of molecular oxygen to proline at C-4.<ref name=Lamport_1963 /> Extensin hydroxyproline is uniquely glycosylated with short chains of [L-arabinose](/source/L-arabinose)<ref>Lamport,D.T.A. (1967) Hydroxyproline-O-glycosidic linkage of the plant cell wall glycoprotein extensin.  Nature, 216, 1322-1324.</ref> that further rigidify<ref>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.</ref> and increase hydrophilicity. Generally the serine has a single galactose attached.<ref>Lamport,D.T.A., Katona,L., and Roerig,S. (1973) Galactosyl serine in extensin.  Biochem.J., 133, 125-131.</ref>

==Hydrophobic tyrosine crosslinking motif==
Two [tyrosine](/source/tyrosine)s separated by a single amino acid, typically valine or another tyrosine, form a short intra-molecular diphenylether crosslink.<ref>Epstein,L. and Lamport,D.T.A. (1984) An intramolecular linkage involving isodityrosine in extensin.  Phytochemistry, 23, 1241-1246.</ref> This can be crosslinked further by the enzyme [extensin peroxidase](/source/extensin_peroxidase)<ref>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.</ref><ref>Lamport,D.T.A. (1989): Extensin peroxidase ties the knots in the extensin network. In: Cell Separation in Plants, edited by [D.J. Osborne](/source/Daphne_Osborne), et al, pp. 101-113. Springer-Verlag, Berlin.</ref><ref>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.</ref> to form an inter-molecular bridge between extensin molecules and thus form networks and sheets.

==References==
{{reflist}}

==Further reading==
*[http://www3.interscience.wiley.com/cgi-bin/fulltext/119281263/PDFSTART Kieliszewski M, Lamport DTA (1994) Extensin: repetitive motifs, functional sites, post-translational codes, and phylogeny ''Plant Journal'' 5: 157–172]{{dead link|date=January 2025|bot=medic}}{{cbignore|bot=medic}}

Category:Plant proteins
Category:Structural proteins
Category:Glycoproteins

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Adapted from the Wikipedia article [Extensin](https://en.wikipedia.org/wiki/Extensin) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Extensin?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.