Protein complexes containing CYFIP/Sra/PIR121 coordinate Arf1 and Rac1 signalling during clathrin-AP-1-coated carrier biogenesis at the TGN.

Actin dynamics is a tightly regulated process involved in various cellular events including biogenesis of clathrin-coated, AP-1 (adaptor protein 1)-coated transport carriers connecting the trans-Golgi network (TGN) and the endocytic pathway. However, the mechanisms coordinating coat assembly, membrane and actin remodelling during post-TGN transport remain poorly understood. Here we show that the Arf1 (ADP-ribosylation factor 1) GTPase synchronizes the TGN association of clathrin-AP-1 coats and protein complexes comprising CYFIP (cytoplasmic fragile-X mental retardation interacting protein; Sra, PIR121), a clathrin heavy chain binding protein associated with mental retardation. The Rac1 GTPase and its exchange factor beta-PIX (PAK-interacting exchange factor) activate these complexes, allowing N-WASP-dependent and Arp2/3-dependent actin polymerization towards membranes, thus promoting tubule formation. These phenomena can be recapitulated with synthetic membranes. This protein-network-based mechanism facilitates the sequential coordination of Arf1-dependent membrane priming, through the recruitment of coats and CYFIP-containing complexes, and of Rac1-dependent actin polymerization, and provides complementary but independent levels of regulation during early stages of clathrin-AP1-coated carrier biogenesis.