P140MDIA, A MAMMALIAN HOMOLOG OF DROSOPHILA-DIAPHANOUS, IS A TARGET PROTEIN FOR RHO SMALL GTPASE AND IS A LIGAND FOR PROFILIN

Rho small GTPase regulates cell morphology, adhesion and cytokinesis t hrough the actin cytoskeleton, We have identified a protein, pl40mDia, as a downstream effector of Rho, It is a mammalian homolog of Drosoph ila diaphanous, a protein required for cytokinesis, and belongs to a f amily of formin-related proteins containing repetitive polyproline str etches. pl40mDia binds selectively to the GTP-bound farm of Rho and al so binds to profilin, pl40mDia, profilin and RhoA are co-localized in the spreading lamellae of cultured fibroblasts. They are also co-local ized in membrane ruffles of phorbol ester-stimulated sMDCK2 cells, whi ch extend these structures in a Rho-dependent manner, The three protei ns are recruited around phagocytic cups induced by fibronectin-coated beads. Their recruitment is not induced after Rho is inactivated by mi croinjection of botulinum C3 exoenzyme, Overexpression of pl40mDia in COS-7 cells induced homogeneous actin filament formation. These result s suggest that Rho regulates actin polymerization by targeting profili n via pl40mDia beneath the specific plasma membranes.