Protrusion of lamellipodia and filopodia from the cell surface requires that actin polymerize locally. Actin polymerization is initiated by numerous agonists, including growth factors, chemoattractants, extracellular matrix, and phagocytic particles. The signaling pathways from the corresponding receptors converge on Rho family GTPases, especially Rac and Cdc42, which induce actin polymerization through a family of proteins called WASP (Wiskott-Aldrich Syndrome protein) (Higgs and Pollard 1999). In mammals, the family includes WASP (specific to hematopoietic cells), N-WASP (neural WASP, which is actually ubiquitous), and at least four forms of WAVE (WASP verprolin homologous protein). The conserved COOH terminus of these proteins stimulates the Arp2/3 complex to nucleate actin filaments, which then elongate at their free barbed ends (Machesky et al. 1999).
Two papers in this issue (Higgs and Pollard 2000: Rohatgi et al. 2000)...
