Expansins are plant cell
wall proteins first discovered in studies of plant cell enlargement. They have unique
"loosening" effects on plant cell walls and are thought to function in
cell growth, cell wall disassembly, cell separation, pollen tube penetration (in grasses,
at least) and leaf primordium initiation. The first studies of expansin proteins, their
genes, and their action on cell walls began in the Cosgrove lab, but other groups are now
adding new insights about expansins.
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.
PROTEINS: Two classes of expansin proteins are currently recognized, called alpha-expansins and beta-expansins.