Receptor mediated endocytosis of polypeptide hormones: mechanism and significance.

Polypeptide hormones bind to specific receptors on the surface of cells. Under certain conditions they localize to specific microdomains of the membrane, i.e., microvilli and coated pits. At physiologic temperature the ligand is internalized by a process of adsorptive endocytosis. This process involves several intracellular membrane bounded structures including coated vesicles, non-coated vesicles and lysosomal structure. These events provide a simple and general mechanism for removal of the ligand from the cell surface in order to terminate its signal. Linked to this process is a mechanism for surface receptor regulation. Thus, the concentration of hormone receptors on the cell surface is a function of the synthetic rate, internalization rate and the rate of recycling of surface membrane.