Receptor-Mediated Endocytosis and the Sorting of Internalized Proteins
■ Some extracellular ligands that bind to specific cell-surface receptors are internalized, along with their receptors, in clathrin-coated vesicles whose coats also contain AP2 complexes.
■ Sorting signals in the cytosolic domain of cell-surface receptors target them into clathrin/AP2-coated pits for internalization. Known signals include the Asn-Pro-X-Tyr, Tyr-X-X-$, and Leu-Leu sequences (see Table 17-2).
■ The endocytic pathway delivers some ligands (e.g., LDL particles) to lysosomes where they are degraded. Most re-ceptor-ligand complexes dissociate in the acidic milieu of the late endosome; the receptors are recycled to the plasma membrane, while the ligands are sorted to lysosomes (see Figure 17-28).
■ Iron is imported into cells by an endocytic pathway in which Fe3+ ions are released from ferrotransferrin in the late endosome. The receptor-apotransferrin complex is recycled to the cell surface where the complex dissociates, releasing both the receptor and apotransferrin for reuse.
■ Endocytosed membrane proteins destined for degradation in the lysosome are incorporated into vesicles that bud into the interior of the endosome. Multivesicular endo-somes, which contain many of these internal vesicles, can fuse with the lysosome to deliver the vesicles to the interior of the lysosome (see Figure 17-31).
■ A portion of the cytoplasm or an entire organelle (e.g., peroxisome) can be enveloped in a flattened membrane and eventually incorporated into a double-membraned au-tophagic vesicle. Fusion of the outer vesicle membrane with the lysosome delivers the enveloped contents to the interior of the lysosome for degradation.
■ Some of the cellular components (e.g., ESCRT complexes) that mediate inward budding of endosomal membranes are used in the budding and pinching off of enveloped viruses such as HIV from the plasma membrane of virus-infected cells (see Figure 17-32).
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