Abstract
Receptor-mediated endocytosis is a process used by cells to rapidly and efficiently clear occupied receptors from the cell surface. Occupied receptors, and to some extent unoccupied receptors, concentrate in specialized regions of the plasma membrane which have been identified as clathrin-coated pits. This high concentration of receptors within a small percentage of the cell surface area allows rapid clearance of receptors (t sub(1/2) similar to 5 min) with a minimum of plasma membrane turnover. Since receptors and ligands enter the same endocytic vesicles but reach different intracellular destinations, it is evident that sorting must occur within the cell. Research has shown that alpha sub(2)M remains in prelysosomal endocytic vesicles for approximately 20 min after endocytosis. Sorting must occur while ligands are still in endocytic vesicles.