OVEREXPRESSION OF COFILIN STIMULATES BUNDLING OF ACTIN-FILAMENTS, MEMBRANE RUFFLING, AND CELL-MOVEMENT IN DICTYOSTELIUM

Cofilin is a low molecular weight actin-modulating protein whose struc ture and function are conserved among eucaryotes. Cofilin exhibits in vitro both a monomeric actin-sequestering activity and a filamentous a ctin-severing activity. To investigate in vivo functions of cofilin, c ofilin was overexpressed in Dictyostelium discoideum cells. An increas e in the content of D. discoideum cofilin (d-cofilin) by sevenfold ind uced a co-overproduction of actin by threefold. In cells overexpressin g d-cofilin, the amount of filamentous actin but not that of monomeric actin was increased. Overexpressed d-cofilin co-sedimented with actin filaments, suggesting that the sequestering activity of d-cofilin is weak in vivo. The overexpression of d-cofilin increased actin bundles just beneath ruffling membranes where d-cofilin was co-localized. The overexpression of d-cofilin also stimulated cell movement as well as m embrane ruffling. We have demonstrated in vitro that d-cofilin transfo rmed latticework of actin filaments cross-linked by alpha-actinin into bundles probably by severing the filaments. D. discoideum cofilin may sever actin filaments in vivo and induce bundling of the filaments in the presence of cross-linking proteins so as to generate contractile systems involved in membrane ruffling and cell movement.