Reorganization of the actin cytoskeleton is thought to play an important role in endocytosis, but the precise role of actin rearrangements and the specific actin-associated proteins involved have remained obscure. Kathryn Ayscough and co-workers now implicate two budding yeast proteins in the process: Sla1p, an adaptor that binds to the yeast homologue of the actin regulator Wiskott-Aldrich syndrome protein (WASP); and Sla2p, a yeast homologue of huntingtin-interacting protein (HIP1) that localizes to clathrin-coated pits (seep. 2551). The authors show that Sla1p and Sla2p interact in vitro, map the regions of the proteins responsible and demonstrate that the two proteins exist as a complex in vivo. They also show that sla1 and sla2 mutations impair endocytosis and that a truncated Slap1p construct containing the Slap2-interacting region inhibits both endocytosis and subsequent vesicle trafficking.
Interestingly, Ayscough and co-workers find that the proteins have opposing effects on actin dynamics, using the actin-disrupting drug latrunculin A to show that Sla1p destabilizes F-actin whereas Sla2p stabilizes it. They therefore propose that Sla1p and Sla2p facilitate cycles of actin assembly and disassembly required for reorganization of the cytoskeleton during endocytosis.