ENDOCYTOSIS

Mammalian cells take up extracellular material by a variety of differe nt mechanisms that are collectively termed endocytosis. Endocytic mech anisms serve many important cellular functions including the uptake of extracellular nutrients, regulation of cell-surface receptor expressi on, maintenance of cell polarity, and antigen presentation. Endocytic pathways are also utilized by viruses, toxins, and symbiotic microorga nisms to gain entry into cells. One of the best-characterized endocyti c mechanisms is receptor-mediated endocytosis via clathrin-coated pits . This type of endocytosis constitutes the major emphasis of this revi ew, with a brief discussion of other endocytic mechanisms and their co mparison with the receptor-mediated pathway. This review describes and evaluates critically current understanding of the mechanisms of entry of plasma membrane components such as the receptor-ligand complexes a nd membrane lipids as well as the extracellular fluid into cells. The intracellular sorting and trafficking of these molecules upon internal ization are also described. The roles of endocytosis in physiological and pathological processes are discussed. These include maintenance of cell polarization, antigen presentation, glucose transport, atheroscl erosis, Alzheimer's disease, and the endocytosis of toxins and viruses .