Receptor-mediated potocytosis

E. J. Smart, D. C. Foster, Y. S. Ying, B. A. Kamen, and R. G. Anderson. Protein kinase C activators inhibit receptor-mediated potocytosis by preventing internalization of caveolae. J. Cell Biol. 124 307-313 (1994). 

The folate receptor facilitates the cellular uptake of folate and 5-methyltetrahydrofolate via receptor-mediated endocytosis at caveolae (caveolae are plasma membrane invaginations distinct from the classical clathrin-coated pits) (6). It has been hypothesized that the folate receptor is functionally coupled to an anion transporter to mediate cytosolic folate delivery by a process defined as potocytosis (6). More recent studies suggest that folate receptor endocytosis also occurs at clathrin-coated pits (7). Studies by Low and coworkers at Purdue University have shown that folate conjugates are also taken up by the folate receptor (8-10), but not by the reduced folate carrier. Figure 2 illustrates an endocytic pathway of the type envisioned for folate conjugates. Unfortunately, the subcellular transport pathway of the folate conjugates has been only partially characterized and may well be affected by the properties of the molecule attached to folate. 

Receptor-Mediated Endocytosis. Mammalian cells have developed an assortment of mechanisms to facilitate the internalization of specific substrates and target these to defined locations inside the cytoplasm. Collectively, these processes of membrane deformations are termed "endocytosis," consisting of phagocytosis, pinocytosis, receptor-mediated endocytosis (clathrin-mediated), and potocytosis [nonclathrin (caveolin)-mediated RME]. The emphasis of this section is receptor-mediated endocytosis in the intestinal tract, but the interested student may consult alternative reviews covering the complete spectrum of endo-cytotic processes in other cell types (17,18). 

Vesicular transport occurs when a membrane completely surrounds a compound, particle, or cell and encloses it into a vesicle. When the vesicle fuses with another membrane system, the entrapped compounds are released. Endocytosis refers to vesicular transport into the cell, and exocytosis to transport out of the cell. Endocytosis is further classified as phagocytosis if the vesicle forms around particulate matter (such as whole bacterial cells or metals and dyes from a tattoo), and pinocy-tosis if the vesicle forms around fluid containing dispersed molecules. Receptor-mediated endocytosis is the name given to the formation of clathrin-coated vesicles that mediate the internalization of membrane-bound receptors in vesicles coated on the intracellular side with subunits of the protein clathrin (Eig. 10.14). Potocytosis is the name given to endocytosis that occurs via caveolae (small invaginations or caves ), which are regions of the cell membrane with a unique lipid and protein composition (including the protein caveolin-1). 

---