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.
Two related actions are characteristic and diagnostic
- They induce long-term, irreversible extension (creep) of plant cell walls.
(More about wall creep)
- 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.