CONTROL OF ACTIN DYNAMICS IN CELL MOTILITY

Actin polymerization plays a major role in cell movement. The controls of actin sequestration/desequestration and of filament turnover are t wo important features of cell motility. Actin binding proteins use pro perties derived from the steady-state monomer-polymer cycle of actin i n the presence of ATP, to control the F-actin/G-actin ratio and the tu rnover rate of actin filaments. Capping proteins and profilin regulate the size of the pools of F-actin and unassembled actin by affecting t he steady-state concentration of ATP-G-actin. At steady state, the tre admilling cycle of actin filaments is fed by their disassembly from th e pointed ends. It is regulated in two different ways by capping prote ins and ADF, as follows. Capping proteins, in decreasing the number of growing barbed ends, increase their individual rate of growth and cre ate a ''funneled'' treadmilling process. ADF/cofilin, in increasing th e rate of pointed-end disassembly, increases the rate of filament turn over, hence the rate of barbed-end growth. Ln conclusion, capping prot eins and ADF cooperate to increase the rate of actin assembly up to va lues that support the rates of actin-based motility processes. (C) 199 7 Academic Press Limited.