What initiates actin polymerization?

For those working on the actin cytoskeleton, a major theme of the 39th annual meeting of the American Society for Cell Biology [] (Washington DC, December 11-15, 1999) was the elucidation of how actin polymerization is initiated. The emphasis was on the regulation and localization of the Arp2/3 complex, which over the last two years has been shown to stimulate actin nucleation, and on the identification of additional proteins that interact with actin and Arp2/3 in a variety of organisms.