Eisosomes mark static sites of endocytosis

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Abstract
Endocytosis is the process by which a cell's plasma membrane encircles a substance and then pinches off an intracellular vesicle in which to trap and transport the substance. It facilitates the cellular uptake of nutrients and macromolecules, and recycles plasma membrane components. Until now no mechanism to specify where in the cell's membrane endocytosis will occur had been identified: it remained possible that sites of endocytosis might be chosen at random. But now a protein assembly to do the job has been identified in yeast. The complex, named the eisosome (for portal body), forms immobile structures at the cell cortex that co-localize with sites of endocytosis. Endocytosis functions to recycle plasma membrane components, to regulate cell-surface expression of signalling receptors and to internalize nutrients in all eukaryotic cells. Internalization of proteins, lipids and other cargo can occur by one of several pathways that have different, but often overlapping, molecular requirements1,2,3,4,5. To mediate endocytosis, effectors assemble transiently underneath the plasma membrane, carry out the mechanics of membrane deformation, cargo selection and vesicle internalization, and then disassemble. The mechanism by which endocytosis initiates at particular locations on the plasma membrane has remained unknown. Sites of endocytosis might be formed randomly, induced by stochastic protein and/or lipid clustering. Alternatively, endocytosis might initiate at specific locations. Here we describe large immobile protein assemblies at the plasma membrane in the yeast Saccharomyces cerevisiae that mark endocytic sites. These structures, termed eisosomes (from the Greek ‘eis’, meaning into or portal, and ‘soma’, meaning body), are composed primarily of two cytoplasmic proteins, Pil1 and Lsp1. A plasma membrane protein, Sur7, localizes to eisosomes. These structures colocalize with sites of protein and lipid endocytosis, and their components genetically interact with known endocytic effectors. Loss of Pil1 leads to clustering of eisosome remnants and redirects endocytosis and endocytic effector proteins to these clusters.