Expansins are unusual proteins that mediate cell wall extension in plants. They are believed to act as a sort of chemical grease, allowingpolymers to slide past one another by disrupting non-covalent hydrogenbonds that hold many wall polymers to one another. This process is notdegradative and hence does not weaken the wall, which could otherwiserupture under internal pressure during growth.Sequence comparisons indicate at least four distinct expansin cDNAs inrice and at least six in Arabidopsis thaliana. The proteins are highly conserved insize and sequence (75-95% amino acid sequence similarity between any pairwise comparison), and phylogenetic trees indicate that this multigenefamily formed before the evolutionary divergence of monocotyledons and dicotyledons. Sequence and motif analyses show no similarities to knownfunctional domains that might account for expansin action on wall extension[1]. It is thought that several highly-conserved tryptophans may function in expansin binding to cellulose, or other glycans. The high conservation of the family indicates that the mechanism by which expansins promote wallextensin tolerates little variation in protein structure.

1. Molecular cloning and sequence analysis of expansins--a highly conserved, multigene family of proteins that mediate cell wall extension in plants. Proc. Natl. Acad. Sci. U.S.A. 92, 9245-9

Expansins are unusual proteins that mediate cell wall extension in plants. They are believed to a
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