The gelsolin/fragmin family protein identified in the higher plant Mimosa pudica

Mimosa pudica L. rapidly closes its leaves and bends its petioles downward when mechanically stimulated. It has been suggested that the actin cytoskel eton is involved in the bending motion since both cytochalasin B and phallo idin inhibit the motion. In order to clarify the mechanism by which the act in cytoskeleton functions in the motion, we attempted to find actin-modulat ing proteins in the M. pudica plant by DNase I-affinity column chromatograp hy. The EGTA-eluate from the DNase I column contained proteins with apparen t molecular masses of 90- and 42-kDa. The 42-kDa band consisted of two clos ely migrating components: the slower migrating component was actin while th e faster migrating components was a distinct protein. The eluate showed an activity to sever actin filaments and to enhance the rate of polymerization of actin, both in a Ca(2+)dependent manner. Microsequencing of the faster migrating 42-kDa protein revealed its similarity to proteins in the gelsoli n/fragmin family. Our results provide the first biochemical evidence for th e presence in a higher plant of a gelsolin/fragmin family actin-modulating protein that severs actin filament in a Ca2+-dependent manner.