Clathrin-coated pits as specialized regions of the plasma membrane, which are responsible for the selective uptake of extracellular material (Roth and Porter, 1964). These are depressions in the plasma membrane from which so-called "coated vesicles" are formed during receptor-mediated endocytosis. Clathrin-coated pits can occupy up to 2% of the membrane surface of a eukaryotic cell. Endocytosis of such membrane sections is a rapid and continuous process. In cultured fibroblasts, for example, it is observed that 50% of the surface proteins and lipids are internalized per hour. Nevertheless, the membrane surface remains constant, because in return lipids and proteins are recycled or newly incorporated into the membrane by exocytosis.
From a clathrin-coated pits, a "clathrin-coated vesicle" is finally formed by further accumulation of clathrin molecules in a complex interaction with other proteins. Dynamin is involved in the constriction of the cell membrane. This wraps itself around the vesicle like a ruff. Under GTP hydrolysis, the vesicle finally detaches. The clathrin sheath has the effect of inducing invaginations of the cell membrane through the structural peculiarity of its molecules, which are then the starting sites of the later vesicle formation. However, the clathrin envelope also contributes to selective cargo uptake through various interactions.