Surface distribution and recycling of the low density lipoprotein receptor as visualized with antireceptor antibodies

For more than 45 years biologists have known that cells internalize extracellular fluid and its contents by fluid-phase endocytosis, an inefficient process for the uptake of molecules that are present at low concentrations. More recently, animal cells have been found to express surface receptors that bind, and thereby concentrate, certain molecules so that they can be taken up in large amounts even when they are present at low concentrations outside the cell. Ligands that are internalized in this way include plasma transport proteins, hormones, and other macromolecules that fulfill nutritional and regulatory roles in cells. In many cases this process, called receptor-mediated endocytOsis, takes place in specialized regions of the plasma membrane called coated pits (reviewed in references 1 and 2). One of the most extensively studied model systems for receptor-mediated endocytosis is the low density lipoprotein (LDL) receptor system. LDL, the major cholesterol carrying protein in human plasma, delivers cholesterol to cells (3). Delivery is accomplished by cell surface receptors that bind LDL and lead to its internalization by receptor-mediated en-docytosis. The LDL is carried to lysosomes where it is degraded to liberate cholesterol, which is used for the synthesis of plasma membranes (most cells), steroid hormones (steroid-secreting cells), and bile acids (hepatocytes). In 1976 we described the mechanism of internalization of LDL in human fibroblasts. Using LDL that was covalently coupled to ferritin as a specific electron microscopic probe, we showed that LDL binds to receptors that are associated with coated pits on the cell surface (4). Coated pits (5), which represent 2% of the cell surface, contain about 70% of the LDL receptors (4,6). Immediately after binding, these pits invaginate and the LDL-ferritin is internalized into coated endocytic vesicles (6). Within 1 min, the endocytic vesicles lose their coat and become larger and irregularly shaped, apparently as a This work was presented in a Symposium on Mechanisms of Endocy-tosis at the Twenty-first Annual Meeting of The American Society for PERSPECTIVES result of fusion with other endocytic vesicles. We called these newly formed vesicles endocytic vesicles or endosomes (6); others have used the terms sorting endocytic vesicles (7) or receptosomes (8). After 5 to 10 min, LDL-ferritin can be detected in lysosomes. By quantitative analysis, the kinetics of binding and uptake of LDL-ferritin are the same as those observed with 125 1-LDL, a biochemical probe for binding, internalization, and degradation (9). On the basis of the combined morphological …