Bidirectional crosstalk between actin dynamics and endocytosis

This chapter covers the cell biology evidence establishing the significance of actin dynamics for the correct execution of endocytosis and emphasizes the molecular connections between the endocytic and actin machineries. It also describes results showing how endocytosis itself controls various forms of actin dynamics, especially those required for the spatial restriction of signaling events. The minimal requirement to support cycles of actin polymerization and depolymerization have been identified through in vitro reconstitution experiments. These experiments showed that five different types of purified proteins are sufficient to generate forces for motility, including de novo actin nucleators and their regulators (Arp2/3 complex and N-WASP), actin severing/depolymerizing factors, barbed end cappers, and an ATP-actin monomeric binding protein (profilin). In mammalian cells, critical components of the actin dynamics machinery such as the Arp2/3 complex, N-WASP, and the capping protein Eps8 are also transiently recruited to clathrin-positive endocytic sites, and their ablation impairs clathrin mediated endocytosis to various extents. Rab5 is a master regulator of endocytosis and endosomal dynamics. Rab5, however, is also involved in the control of actin dynamics. This dual function is underscored by a complex system of regulation, in which several activators of Rab5 (including the GEFs RIN1, Rabex-5, and Alsin) are also involved in the control of actin dynamics. RIN1 is an effector of Ras and binding to Ras enhances the Rab5 specific GEF activity of RIN1 that mediates EGFR mediated Rab5 activation via Ras in vivo. A more direct way of achieving Rab5 activation by surface receptors is through Rabex-5 that possesses two ubiquitin binding domains, which bind ubiquitinated proteins including EGFR.