Occurrence of low density lipoprotein receptors within large pits on the surface of human fibroblasts as demonstrated by freeze-etching

The surface distribution of specific receptor sites for plasma low density lipoprotein (LDL) in cultured human fibroblasts was studied with the technique of freeze-fracturing/deep-etching. Through the use of LDL covalently linked to ferritin as a visual probe, the receptor sites were observed to be concentrated within large pits on the cell surface. These large pits corresponded to the coated pits previously identified as the sites of LDL-ferritin binding by transmission electron microscopy (TEM). On the true surface of the cell, these large pits appeared as shallow depressions of irregular contour. These large pits were also identified within the fracture plane of the membrane where they could be distinguished from a population of smaller pits that corresponded to small flask-shaped invaginations seen in TEM. The membrane that composed the large pits appeared to differ from the remainder of the plasma membrane in that it contained twice the number of intramembrane particles per μm² of surface and in that these intramembrane particles were of larger mean diameter. The current data lend support to the hypothesis that the large coated pits of the plasma membrane represent discrete regions where the membrane is structurally adapted to carry out the adsorptive endocytosis of receptor-bound macromolecules.