ENDOCYTOSIS AND MOLECULAR SORTING

Endocytosis in eukaryotic cells is characterized by the continuous and regulated formation of prolific numbers of membrane vesicles at the p lasma membrane. These vesicles come in several different varieties, ra nging from the actin-dependent formation of phagosomes involved in par ticle uptake, to smaller clathrin-coated vesicles responsible for the internalization of extracellular fluid and receptor-bound ligands. In general, each of these vesicle types results in the delivery of their contents to lysosomes for degradation. The membrane components of endo cytic vesicles, on the other hand, are subject to a series of highly c omplex and iterative molecular sorting events resulting in their targe ting to specific destinations. In recent years, much has been learned about the function of the endocytic pathway and the mechanisms respons ible for the molecular sorting of proteins and lipids. This review att empts to integrate these new concepts with long-established views of e ndocytosis to present a more coherent picture of how the endocytic pat hway is organized and how the intracellular transport of internalized membrane components is controlled. Of particular importance are emergi ng concepts concerning the protein-based signals responsible for molec ular sorting and the cytosolic complexes responsible for the decoding of these signals.