Receptor-mediated endocytosis. a. This diagram shows the steps in receptor-mediated endocytosis, a transport mechanism that allows selected molecules to enter the cell. CD Cargo receptors recognize and bind specific molecules that come in contact with the plasma membrane. Cargo receptor-molecule complexes are recognized by adaptin, a protein that helps select and gather appropriate complexes in specific areas of the plasma membrane for transport into cells. (D Clathrin molecules then bind to the adaptin-cargo receptor-molecule complex to assemble into a shallow basket-like cage, forming a coated pit.(3)Clathrln interactions then assist the plasma membrane to change shape to form a deep depression, a fully formed coated pit that then pinches off from the plasma membrane as a coated vesicle @ (I.e., budding from the membrane). Selected cargo proteins and their receptors are thus pulled from the extracellular space into the lumen of a forming coated vesicle. After budding and internalization of the vesicle, the coat proteins are removed © and recycled for further use. ©The uncoated vesicle travels to Its destination to fuse with a cytoplasmic organelle, b. Electron micrograph of the cytoplasmic surface of the plasma membrane of A431 cells prepared by the quick-freeze deep-etch technique. This image shows coated pits and clathrin-coated vesicles in different stages of their formation. Note that the coated pits and clathrin-coated vesicles are formed in areas devoid of actin filaments. The small uniform pinocy-totic vesicles do not have a clathrin coat and are located in close proximity to actin filaments, x200,000. (Courtesy of Dr. John E. Heuser, Washington University School of Medicine.)
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