Once generated at the plasma membrane, most endocytic vesicles fuse with a common receiving compartment, the early endosome, where internalized cargo is sorted: some cargo molecules are returned to the plasma membrane, either directly or via a recycling endosome, and others are designated for degradation by inclusion in a late endosome. Late endosomes form from a bulbous, vacuolar portion of early endosomes by a process called endosome maturation. This conversion process changes the protein composition of the endosome membrane, patches of which invaginate and become incorporated within the organelles as intralumenal vesicles, while the endosome itself moves from the cell periphery to a location close to the nucleus. As an endosome matures, it ceases to recycle material to the plasma membrane and irreversibly commits its remaining contents to degradation: late endosomes fuse with one another and with lysosomes to form endolysosomes, which degrade their contents, as discussed earlier (Figure 13–47).
Each of the stages of endosome maturation—from the early endosome to the endolysosome—is connected through bidirectional vesicle transport pathways to
Figure 13–47 Endosome maturation: the endocytic pathway from the plasma membrane to lysosomes. endocytic vesicles fuse near the cell periphery with an early endosome, which is the primary sorting station. Tubular portions of the early endosome bud off vesicles that recycle endocytosed cargo back to the plasma membrane—either directly, or indirectly via recycling endosomes. recycling endosomes can store proteins until they are needed. conversion of early endosomes to late endosomes is accompanied by loss of the tubular projections. Membrane proteins destined for degradation are internalized in intralumenal vesicles. The developing late endosome, or multivesicular body, moves on microtubules to the cell interior. fully matured late endosomes no longer send vesicles to the plasma membrane, and they fuse with one another and with endolysosomes and lysosomes to degrade their contents. each stage of endosome maturation is connected via transport vesicles with the Tgn, providing a continuous supply of newly synthesized lysosomal proteins.
0.1 µm the TGN. These pathways allow insertion of newly synthesized materials, such as lysosomal enzymes arriving from the ER, and the retrieval of components, such as the M6P receptor, back into the early parts of the secretory pathway. We next discuss how the cell uses and controls the various features of endocytic trafficking.
pinocytic vesicles form from coated pits in the plasma Membrane
Virtually all eukaryotic cells continually ingest portions of their plasma membrane in the form of small pinocytic (endocytic) vesicles. The rate at which plasma membrane is internalized in this process of pinocytosis varies between cell types, but it is usually surprisingly high. A macrophage, for example, ingests 25% of its own volume of fluid each hour. This means that it must ingest 3% of its plasma membrane each minute, or 100% in about half an hour. Fibroblasts endocytose at a somewhat lower rate (1% of their plasma membrane per minute), whereas some amoebae ingest their plasma membrane even more rapidly. Since a cell’s surface area and volume remain unchanged during this process, it is clear that the same amount of membrane being removed by endocytosis is being added to the cell surface by the converse process of exocytosis. In this sense, endocytosis and exocytosis are linked processes that can be considered to constitute an endocytic– exocytic cycle. The coupling between exocytosis and endocytosis is particularly strict in specialized structures characterized by high membrane turnover, such as a nerve terminal.