Uptake and Intracellular Processing of Cell Surface Receptors
Uptake of macromolecules at the cell surface is an essential function that occurs in all eucaryotic cells at some stage in their lifetime. It can be achieved by a variety of processes, but all of them probably involve the invagination and subsequent pinching off of a localized segment of the plasma membrane to form an intracellular vesicle. This form of uptake is readily observed in living cells during phagocytosis and pinocytosis, but these are activities found only in certain specialized cell types. The most widespread form of endocytosis occurs below the resolution limit of the light microscope, where profiles observed by electron microscopy suggest that invaginations in the size range 50–200 nm are responsible. This form of uptake, generally referred to as “micropinocytosis,” certainly includes a variety of processes that differ in their selectivity and purpose. Tracer studies suggest that the processes of micropinocytic uptake are responsible for a steady, continuous rate of internalization that, in cultured fibroblasts, for example, internalizes less than 50% of the plasma membrane every hour (Steinman et al., 1976, 1983).
KeywordsEpidermal Growth Factor Receptor Epidermal Growth Factor Transferrin Receptor Intracellular Processing Coated Vesicle
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