DISRUPTION OF HYDROGEN-BONDING BETWEEN PLANT-CELL WALL POLYMERS BY PROTEINS THAT INDUCE WALL EXTENSION

Plant cell enlargement is controlled by the ability of the constrainin g cell wall to expand. This ability has been postulated to be under th e control of polysaccharide hydrolases or transferases that weaken or rearrange the load-bearing polymeric networks in the wall. We recently identified a family of wall proteins, called expansins, that catalyze the extension of isolated plant cell walls. Here we report that these proteins mechanically weaken pure cellulose paper in extension assays and stress relaxation assays, without detectable cellulase activity ( exo- or endo- type). Because paper derives its mechanical strength fro m hydrogen bonding between cellulose microfibrils, we conclude that ex pansins can disrupt hydrogen bonding between cellulose fibers. This co nclusion is further supported by experiments in which expansin-mediate d wall extension (i) was increased by 2 M urea (which should weaken hy drogen bonding between wall polymers) and (ii) was decreased by replac ement of water with deuterated water, which has a stronger hydrogen bo nd. The temperature sensitivity of expansin-mediated wall extension su ggests that units of 3 or 4 hydrogen bonds are broken by the action of expansins. In the growing cell wall, expansin action is likely to cat alyze slippage between cellulose microfibrils and the polysaccharide m atrix, and thereby catalyze wall stress relaxation, followed by wall s urface expansion and plant cell enlargement.