Recycling of Cell-surface Receptors: Observations from the LDL Receptor System

During receptor-mediated endocytosis, extracellular ligands bind to plasma-membrane receptors and rapidly enter cells when the membrane invaginates and pinches off to form an endocytic vesicle. In many systems the receptors cluster in discrete regions of the plasma membrane, called coated pits, that invaginate to form clathrin-coated vesicles. The internalized vesicles rapidly lose their clathrin coats and eventually fuse with lysosomes (Goldstein et al. 1979; Pearse 1980).