Molecular requirements for the internalisation step of endocytosis: insights from yeast

Molecular genetic studies of endocytosis using the unicellular eukaryote Sa ccharomyces cerevisiae (budding yeast) have led to the identification of ma ny cellular components, both proteins and lipids, required for this process . While initially, many of these requirements (e.g. for actin, various acti n-associated proteins, the ubiquitin conjugation system, and for ergosterol and sphingolipids) appeared to differ from known requirements for endocyto sis in higher eukaryotes (e.g. clathrin, AP-2, dynamin), it now seems that endocytosis in higher and lower eukaryotes share many requirements. Often, what were initially identified as actin cytoskeleton-associated proteins in S. cerevisiae, are now revealing themselves as clathrin-coated pit- and ve sicle-associated proteins in higher eukaryotes. So rather than delineating two endocytic pathways, one actin-based and one clathrin-based. the combine d studies on higher and lower eukaryotes are revealing interesting interpla y in both systems between the actin cytoskeleton, clathrin coats. and lipid s in the formation of endocytic vesicles at the plasma membrane. Recent res ults from the yeast system show that the Arp2/3p complex, Wiskott-Aldrich s yndrome protein (WASP), and WASP-interacting protein (WIP), proteins involv ed in the nucleation step of actin filament assembly, play a major role in the formation of endocytic vesicles. This discovery suggests models whereby endocytic vesicles may be actively pushed from the plasma membrane and int o the cell by newly forming and rapidly extending actin filaments. (C) 2001 Published by Elsevier Science B.V.