Functions of actin in endocytosis

Abstract

Endocytosis is a fundamental eukaryotic process required for remodelling plasma-membrane lipids and protein to ensure appropriate membrane composition. Increasing evidence from a number of cell types reveals that actin plays an active, and often essential, role at key endocytic stages. Much of our current mechanistic understanding of the endocytic process has come from studies in budding yeast and has been facilitated by yeast's genetic amenability and by technological advances in live cell imaging. While endocytosis in metazoans is likely to be subject to a greater array of regulatory signals, recent reports indicate that spatiotemporal aspects of vesicle formation requiring actin are likely to be conserved across eukaryotic evolution. In this review we focus on the 'modular' model of endocytosis in yeast before highlighting comparisons with other cell types. Our discussion is limited to endocytosis involving clathrin as other types of endocytosis have not been demonstrated in yeast.

Fig. 1

Actin in Saccharomyces cerevisiae and the analysis of endocytic actin patches. a Actin is visualized in budding yeast cells using rhodamine phallodin. Two main structures are visualized, cortical actin patches (spots marked with arrows) and actin cables that run along the mother bud axis. b Kymographs generated from movies to show the behaviour of different proteins that assembly and disassemble from the endocytic complex. Sla1, Las17 and Ent1 arrive relatively early in the process and remain non-motile for the majority of their lifetime. Abp1 is used as a marker of actin’s arrival at the complex, shortly before invagination is observed. Rvs167 is an amphiphysin thought to be involved in scission; it arrives shortly after inward movement has commenced

Fig. 2

Schematic depicting stages defined in yeast endocytosis. a Work by many laboratories has led to the development of a general model for the key stages of the endocytic process in S. cerevisiae. Stage 1 defines the phase during which the endocytic coat becomes assembled at the plasma membrane. Embedded within this structure, but held inactive, are proteins important for the initial stages of actin polymerization. Stage 2 is the slow movement or invagination stage. This corresponds to the time during which actin is polymerizing and forming a framework necessary to ensure the inward growth of the membrane. Stage 3 represents the scission and movement of the newly formed vesicle away from the membrane. The rate of movement is increased following scission. b Actin regulatory proteins are essential for invagination, as is the function of actin. During this stage actin organization is regulated by many proteins. Initially, the WASP homologue Las17 plays a key role in driving Arp2/3-dependent polymerization to form a loose network of F-actin at the site. This F-actin is required to recruit strong actin polymerization factors—the type I myosins. During the invagination stage, Las17 is possibly held inactive by a protein Bbc1. Dynamic regulation of actin filaments requires the activity of capping protein to ensure the funnelling of new actin monomer to filament ends and cofilin to drive depolymerization of actin. Actin bundling proteins Sac6 and Scp1 function together to form higher order F-actin structures, such as bundles or a cross-linked mesh, that provide a strong framework necessary to support inward growth of membrane

Fig. 3

Schematic comparing actin function in mammalian and yeast endocytosis. Actin has been shown to play roles at multiple distinct stages in endocytosis as shown. In mammalian cells, the exact stages of actin’s involvement may vary according to cargo and cell type, although a role in scission and vesicle translocation appear to be widespread. In yeast, actin is required for most stages, with the possible exception of site selection, as it is well documented that actin arrives after the site has been selected