Distinct Patterns of Actin Organization Regulated by the Small GTP-binding Proteins Rac and Rho

Many extracellular signals stimulate cellular responses that involve changes in the organization of actin microfilaments. For example, directional motility induced by chemoattractants requires precise spatial control of actin polymerization and depolymerization (Cooper 1991). Actin reorganization is also an early response to several growth factors, although it is not known what role such alterations play in cell growth control (Burridge et al. 1988). As yet, the signal transduction pathways linking transmembrane receptors to the actin cytoskeleton have not been delineated, but the targets are likely to include some of the many actin-binding proteins that can modulate actin polymerization and organization (Vandekerckhove 1990; Bretscher 1991).