Chapter 7 Endocytosis

Abstract

At the start of the 1980s (Goldstein et al., 1979) it was known that receptor mediated endocytosis was a very efficient and specific means of taking proteins into cells. In addition, several high affinity receptors able to concentrate in coated pits, and recycle through the endocytic pathway, had been characterized (Steinman et al., 1983). At that time, the molecules regulating endocytosis, such as clathrin, adaptin and ATP-dependent proton pumps were beginning to be isolated. Since then, a rapid expansion of recombinant DNA technology, and the development of assays that reconstitute endocytosis in vitro , have allowed major advances in the understanding of endocytosis to be made. Recombinant DNA methods have revealed the primary amino acid sequences of many receptors that enter cells by endocytosis. Internalization or, sorting motifs, have been identified in the cytoplasmic tails of receptors. Internalization motifs bind to adaptin complexes, and adaptin seeds the assembly of clathrin coats on the cytoplasmic face of the plasma membrane. Taken together, these data provide a molecular basis for the clustering of receptors in coated pits. Structural rearrangements within the clathrin lattice then cause the invagination of the coated pit, and the formation of the coated vesicle. Clathrin coats are removed by an uncoating ATPase, and uncoated vesicles fuse with endosomes. The lumen of endosomes is acidic. This acid environment, which is maintained by an ATP-dependent proton pump, is essential for the physical dissociation of receptor-ligand complexes. Sorting of receptors and ligands is further enhanced by the movement of receptors into tubular endosomal elements that recycle receptors to the plasma membrane. Membrane fusion throughout the endocytic pathway requires ATP, GTP, Ca 2+ , and cytosolic proteins. Many of these cytosolic proteins are required for membrane fusion in general, and are used in vesicular transport steps throughout the cell. Specific cytosolic proteins, called small GTP-binding proteins, or rabs, however, regulate the specificity of membrane fusion. Rabs 4 and 5 bind GTP and act as molecular switches that regulate the formation and recycling of endocytic vesicles. The formation of coated vesicles, and their fusion with early compartments of the endocytic pathway, may also require dynamin. Fusion of endosomes with lysosomes may require the additional help of microtubules. A specialized form of endocytosis, called potocytosis, has been characterized recently. Ligands taken up by potocytosis bind to GPI-anchored receptors that concentrate in specialized domains of the plasma membrane called caveolae. Caveolae are coated with caveolin and invaginate to form a membrane compartment that is sealed off from the extracellular fluid. Caveolae do not pinch off from the cell surface and concentrate and process molecules within specialized domains of the plasma membrane. Caveolae may be used to take up low molecular weight ligands, such as vitamins, into cells, or for the localized generation of extracellular signaling molecules.