Shedding light on the dynamics of endocytosis and viral budding

Endocytosis is used by eukaryotic cells to perform a wide range of functions, including the uptake of extracellular nutrients and the regulation of cell-surface receptors, as well as by toxins, viruses, and microorganisms to gain entry into cells (1). Endocytosis actually encompasses many different processes, such as phagocytosis of large (>250 nm) particles as well as pinocytosis of large volumes of fluid (2). One of the most important endocytic mechanisms is a receptor-mediated process whereby the plasma membrane binds specific macromolecules and smaller particles by means of specialized receptors, invaginates around those particles, and then pinches off to form small vesicles. Receptor-mediated endocytosis had been thought to be assisted by specific proteins, either clathrin or caveolin, polymerizing into a spherical shell around the invagination (3). Recently, however, evidence has arisen for a different, clathrin- and caveolin-independent route by which endocytosis may occur (4, 5). The understanding and quantitative analysis of the mechanisms underlying receptor-mediated endocytosis have important implications for not only viral pathogenesis but also the delivery of macromolecules and nanoparticles for intracellular imaging and targeted therapies (6).