Plant cell expansion is regulated by wall relaxation and yielding, which is thought to be catalyzed by elusive "wall loosening" enzymes. By employing a reconstitution approach, we initially found that a crude
protein extract from the
cell walls of growing cucumber seedlings possessed the ability to induce the extension of
isolated cell walls. This activity was restricted to the
growing region of the stem and could induce the extension of
isolated cell walls from various dicots and monocots, but was less effective on grass
coleoptile walls. Sequential HPLC
fractionation of the active wall extract revealed two proteins with molecular masses of 29 and 30 kD, as measured by SDS-PAGE, associated with such activity. Each
protein, by itself, could induce wall extension without detectable hydrolytic breakdown of the wall. We proposed the name "expansins" for this class of proteins. Expansins have been isolated from various
plant sources including oat, cucumber, broccoli, celery, tomato, cotton, cabbage, and corn, and also from
snail and its
feces. These proteins weaken the intermolecular bonds between
plant wall polysaccharides. They decrease the
mechanical strength of commercial products made from polysaccharides, such as paper, and therefore present a novel approach in developing new technologies in industries which make use of such polysaccharides, such as in the paper industry, in the applications of
polysaccharide gums and related products. These proteins moreover present a novel approach in the control of
plant growth.