Actin dynamics and endocytosis in yeast and mammals

Abstract

Tight regulation of the actin cytoskeleton is critical for many cell functions, including various forms of cellular uptake. Clathrin-mediated endocytosis (CME) is one of the main methods of uptake in many cell types. An intact and properly regulated actin cytoskeleton is required for CME in Saccharomyces cerevisiae. Yeast CME requires the proper regulation of actin polymerization, filament cross-linking, and filament disassembly. Recent studies also point to a role for F-BAR and BAR-domain containing proteins in linking the processes of generating and sensing plasma membrane curvature with those regulating the actin cytoskeleton. Many of these same proteins are conserved in mammalian CME. However, until recently the requirement for actin in mammalian CME was less clear. Several recent studies in mammalian cells provide new support for an actin requirement in the invagination and late stages of CME. This review focuses on the regulation of the actin cytoskeleton during CME in yeast and the emerging evidence for a role for actin during mammalian CME.

Figure 1

Major models for actin assembly during endocytosis derived from the results of numerous works described and referenced herein. In both A and B the initial curvature of the membrane is generated by clathrin and additional endocytic proteins, such as F-BAR containing Syp1. Syp1 can inhibit WASp/Las17-Arp2/3 mediated actin assembly and may serve to keep actin polymerization inhibited during early steps of CME. As invagination proceeds, the changing membrane curvature may be sensed by other BAR domain containing proteins, for example Bzz1, which is recruited along with other endocytic proteins and regulators of Arp2/3. In model A, a ring of NPFs activate Arp2/3 which nucleates an actin network that flows away from the plasma membrane. Proteins of the endocytic coat link the membrane to this flowing network and this provides the force to invaginate further into the cell. The amphiphysin proteins, Rvs161 and Rvs167, drive membrane scission. In model B, actin is nucleated along the sides of the membrane tubule, perhaps in response to the loss of Syp1, and the recruitment of Bzz1, which can promote actin nucleation. The growing barbed ends of the actin push on the membrane tubule, squeezing it, driving elongation and assisting the amphiphysin proteins during membrane scission. In this model, the actin network is in a position to drive movement after the vesicle has been freed from the membrane. The best models for how actin polymerization is utilized during clathrin-mediated endocytosis in mammals are similar to the model presented in B [64, 65]