EXPANSIN MODE OF ACTION ON CELL-WALLS - ANALYSIS OF WALL HYDROLYSIS, STRESS-RELAXATION, AND BINDING

The biochemical mechanisms underlying cell wall expansion in plants ha ve long been a matter of conjecture. Previous work in our laboratory i dentified two proteins (named ''expansins'') that catalyze the acid-in duced extension of isolated cucumber cell walls. Here we examine the m echanism of expansin action with three approaches. First, we report th at expansins did not alter the molecular mass distribution or the visc osity of solutions of matrix polysaccharides. We conclude that expansi ns do not hydrolyze the major pectins or hemicelluloses of the cucumbe r wall. Second, we investigated the effects of expansins on stress rel axation of isolated walls. These studies show that expansins account f or the pH-sensitive and heat-labile components of wall stress relaxati on. In addition, these experiments show that expansins do not cause a progressive weakening of the walls, as might be expected from the acti on of a hydrolase. Third, we studied the binding of expansins to the c ell wall and its components. The binding characteristics are consisten t with this being the site of expansin action. We found that expansins bind weakly to crystalline cellulose but that this binding is greatly increased upon coating the cellulose with various hemicelluloses. Xyl oglucan, either solubilized or as a coating on cellulose microfibrils, was not very effective as a binding substrate. Expansins were present in growing cell walls in low quantities (approximately 1 part in 5000 on a dry weight basis), suggesting that they function catalytically. We conclude that expansins bind at the interface between cellulose mic rofibrils and matrix polysaccharides in the wall and induce extension by reversibly disrupting noncovalent bonds within this polymeric netwo rk. Our results suggest that a minor structural component of the matri x, other than pectin and xyloglucan, plays an important role in expans in binding to the wall and, presumably, in expansin action.