Plant Science

Constructing a Scaffold

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Science  29 Feb 2008:
Vol. 319, Issue 5867, pp. 1162
DOI: 10.1126/science.319.5867.1162b

Plant cells partition at cytokinesis by forming a new cell wall. These walls are composed of interpenetrating networks of the polysaccharides cellulose and pectin and of (hydroxy)proline-rich glycoproteins, notably the extensins. Cannon et al. show by electron microscopy that the Arabidopsis mutant rsh is defective in cell wall assembly and that the defect is likely due to the absence of the rsh-encoded extensin protein, AtEXT3. This protein contains 11 identical amphiphilic motifs that, besides being rich in hydroxyproline, contain an isodityrosine (Idt) cross-link motif (YXY) and an HYS motif. In vitro, extensin peroxidase catalyzed tyrosine cross-linking between purified AtEXT3 monomers and led to the inference that the monomers were offset such that intermolecular cross-linking occurred between Idt and HYS, rather than between two Idt motifs. Atomic force micro scopy imaging shows that AtEXT3 forms a dendritic network displaying both end-on and lateral adhesion. The alternating hydrophilic and hydrophobic modules of AtEXT3 may induce like-to-like self-association with cross-linking stabilizing the network and favoring a staggered alignment that would permit two-dimensional growth. The authors suggest that such a positively charged extensin network may serve as a template for the orderly deposition of negatively charged pectin during cell wall assembly. — VV

Proc. Natl. Acad. Sci. U.S.A. 105, 2226 (2008).

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