Mechanism of action
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The exact mechanism by which expansins cause loosening and extension of cell walls is still enigmatic. On the one hand, there is evidence that they disrupt noncovalent bonding between cellulose microfibrils and matrix glucans that stick to the microfibril. Thus, some of our whimsical images of expansins as weavils (below) or crabs that tear apart hydrogen bonding between wall polysaccharides. On the other hand, no hydrolytic or other enzymatic activity has been found to account for expansin's unique effects on the cell wall. 

The image at left shows a simplified model of how expansins might interact with other wall components. Wall polysaccharides are secreted to the cell surface and then assemble into a load-bearng structure. The action of expansins is hypothesized to cause a transient release of short segment of matrix glycans attached to cellulose microfibrils, with the result that the cellulose and matrix polymers slide relative to one another. Wall hydrolases such as endoglucanase cut matrix glucans into shorter segments. This may lead to weakening, but not creep, of the cell wall. Transglycosylases, such as XET, can recombine glycans into shorter or longer pieces, depending upon conditions within the wall. Proton-ATPases in the plasma membrane (star symbol) may lower the wall pH, thereby activating expansins and other enzymes with acidic optima and inactivating wall enzymes with neutral pH optima. For graphical simplicity, pectins and structural proteins are not shown in this figure, but one should image them filling the space between microfibrils.
Here's another picture of how expansin might act. One domain (here labeled as CBD-like) may restrict expansin mobility by binding to the surface of cellulose microfibrils. The movement of expansin along the cellulose surface may disrupt loosely-bound matrix polymers, with the result that the wall polymers move, or creep, resulting in turgor-driven wall extension. 

ACTIVITY:  Two related actions are characteristic and diagnostic for expansins: 

  1. They induce long-term, irreversible extension (creep) of plant cell walls.
                            (More about wall creep)
  2. They enhance stress relaxation of walls over a wide time range (<0.1 to >100 s). 
                            (More about wall stress relaxation)

Both of these actions are pH dependent, with an acidic optimum. No enzymatic activity has been detected with expansins and their mechanism of action remains enigmatic.

 

This page was last updated on 03/22/06.

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