Control of actin dynamics in cell motility¹

Abstract

Actin polymerization plays a major role in cell movement. The controls of actin sequestration/desequestration and of filament turnover are two important features of cell motility. Actin binding proteins use properties derived from the steady-state monomer-polymer cycle of actin in the presence of ATP, to control the F-actin/G-actin ratio and the turnover rate of actin filaments. Capping proteins and profilin regulate the size of the pools of F-actin and unassembled actin by affecting the steady-state concentration of ATP-G-actin. At steady state, the treadmilling cycle of actin filaments is fed by their disassembly from the pointed ends. It is regulated in two different ways by capping proteins and ADF, as follows. Capping proteins, in decreasing the number of growing barbed ends, increase their individual rate of growth and create a “funneled” treadmilling process. ADF/cofilin, in increasing the rate of pointed-end disassembly, increases the rate of filament turnover, hence the rate of barbed-end growth. In conclusion, capping proteins and ADF cooperate to increase the rate of actin assembly up to values that support the rates of actin-based motility processes.

Introduction

It is now well accepted that cell locomotion and, more generally, changes in cell shape in response to stimuli are powered by actin polymerization (Condeelis, 1993). Physical analyses show that actin polymerization can provide a protrusive force sufficient to overcome the resistance of the cell membrane Cortese et al 1989, Mogilner and Oster 1996. In recent years, two aspects of the involvement of actin polymerization in motility have given rise to intense investigations.

First, a shape change of the cell in response to stimuli often necessitates a massive polymerization of actin filaments. The stimulation of blood platelets, neutrophils or chemotactic amoebae, is rapidly followed by a large increase in the cellular amount of F-actin. The increase in the F-actin pool correlates with an identical decrease in the pool of “sequestered actin”, i.e. G-actin in complex with proteins that prevent actin from polymerizing, such as thymosin-β4 and profilin. The mechanism of control of the F-actin/G-actin ratio in cells is a key issue in motility.

Another fascinating feature of cell motility is the use of rapid actin filament turnover to generate movement. Continuous assembly of actin filaments at the leading edge of locomoting cells builds up the protrusive filopodial or lamellipodial extensions of the cytoplasm that determine the direction of movement. While net polymerization occurs at the front, net depolymerization occurs at the rear of the lamella. The rate of movement is 1 to 10 μm/min. For the advance of the lamellipodium to be driven by actin polymerization, the rate of filament growth at the leading edge would have to be as fast as 10 to 100 subunits per second. A key issue is to understand by which mechanism a cell can “maintain high rates of net polymerization and net depolymerization simultaneously at different sites in its cytoplasm” (Fechheimer & Zigmond, 1993). In such a steady regime of locomotion, the overall cellular F-actin content remains constant. The actin subunits coming from filaments depolymerizing at the rear of the lamella are recycled into new filaments assembled at the front in the seemingly rapid treadmilling process observed in locomoting keratocytes Wang 1985, Small 1995. Early evidence for the autonomy of the lamella as a motile machine has been provided (Euteneuer & Schliwa, 1984).

Bacterial pathogens such as Listeria monocytogenes or Shigella flexneri (Higley & Way, 1997, for a review) mimic the dynamic behavior of actin filaments at the leading edge. They elicit their own propulsion in the cytoplasm by inducing actin polymerization at their surface. The movement can be monitored in vitro in acellular extracts Theriot et al 1994, Marchand et al 1995, which provide a basis for identifying the cellular components of the motile machinery involved in actin nucleation at the plasma membranes and eventually reconstituting actin-based motile processes in a controlled medium.

In this short review, we will survey the principles of actin polymerization that are used by different actin binding proteins either to regulate actin desequestration, thus eliciting massive assembly of filaments, or to control actin filament turnover, thus mediating the forward movement of the leading edge.