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The uptake of small molecules (less than 1 kilodalton) by eukaryotic cells is generally thought to occur by movement across the membrane through water-filled channels, carriers, or transporters (1). In contrast, the uptake of macromolecules such as low density lipoproteins, transferring, and protein toxins occurs through receptor-mediated endocytosis (2). Studies on receptor-dependent folate transport (3-9) have drawn our attention to the existence of a mechanism for small molecule transport that embodies certain features of receptor-mediated endocytosis. This pathway uses caveolae rather than clathrin-coated pits as an uptake vehicle. Caveolae, working together with glycosylphosphatidylinositol (GPI)-anchored membrane proteins that are able to concentrate select molecules at these sites, may take up a variety of different small molecules. We call this process potocytosis.
Caveolae (10) and plasmalemmal vesicles (11) are interchangeable terms for the same membrane feature. They are recognized in electron microscopic images as invaginated pits or vesicles that have a uniform diameter of ==50 nm. The cytoplasmic surface of each caveolae is decorated with a coat material composed of delicate filaments arranged into striations on the membrane surface (2, 12). Both invaginated and noninvaginated caveolae are frequently seen, which suggests that the shape of caveolae can change as a consequence of the mechanical activity of the striated coat (2). The organization of the coat is disrupted by sterol-binding drugs (13), and the number of visible caveolae is reduced in cholesterol-depleted cells (9). In addition, the sterol-binding drug filipin causes the formation of cholesterol precipitates over caveolae membrane (14), which indicates that there is a high concentration of cholesterol in this membrane domain.
Insight into the role of caveolae in potocytosis was gained from studying the function of the folate receptor. Certain cultured cells express a high affinity receptor for 5-methyltetrahydrofolate on their surfaces (3). The cloning of the receptor complementary DNA (7) and the biochemical analysis of the pure protein (15) have shown that this receptor is an integral membrane protein anchored by GPI. Folate receptors occur in dense clusters on the cell surface in association with caveolae (8). Each cluster contains up to 700 receptor molecules with an estimated density of ==30,000 molecules per square micrometer (8). Cholesterol-binding drugs such as filipin or nystatin and depletion of cholesterol from the plasma membrane cause the folate...