Rho GTPases: molecular switches that control the organization and dynamicsof the actin cytoskeleton

The actin cytoskeleton plays a fundamental role in all eukaryotic cells-it is a major determinant of cell morphology and polarity and the assembly and disassembly of filamentous actin structures provides a driving force for d ynamic processes such as cell motility, phagocytosis, growth cone guidance and cytokinesis. The ability to reorganize actin filaments is a fundamental property of embryonic cells during development; the shape changes accompan ying gastrulation and dorsal closure, for example, are dependent on the pla sticity of the actin cytoskeleton, while the ability of cells or cell exten sions, such as axons, to migrate within the developing embryo requires rapi d and spatially organized changes to the actin cytoskeleton in response to the external environment. Work in mammalian cells over the last decade has demonstrated the central role played by the highly conserved Rho family of small GTPases in signal transduction pathways that link plasma membrane rec eptors to the organization of the actin cytoskeleton.