Display of low density lipoprotein receptors is clustered, not dispersed, in fibroblast and hepatocyte plasma membranes

Abstract

Although the principal details of low density lipoprotein (LDL) uptake by receptor-mediated endocytosis and its subsequent intracellular fate have been thoroughly investigated, an aspect of this mechanism that continues to provoke controversy concerns the manner of display of LDL receptors upon their initial insertion at the cell surface. While our studies based on electron microscopy of platinum/carbon replicas of gold-labeled cells have previously suggested a clustered display pattern, others have concluded, before and since, that LDL receptors are inserted individually at random widely dispersed sites in the plasma membrane. In this article, we present a series of experiments designed to discriminate between these competing hypotheses. In addition to the use of LDL-colloidal gold complexes, visualized electron microscopically, on cells subjected to a variety of experimental procedures, these experiments include the application of anti-apolipoprotein B-100 antibodies, anti-LDL-receptor antibodies, and direct visualization of native (unlabeled) LDL molecules at the cell surface. All results point to a loose-cluster arrangement, not one involving widely dispersed individual units, as the initial display pattern of newly inserted LDL receptors. A comparison of LDL and beta-very low density lipoprotein receptor distribution in fibroblasts and hepatocytes suggests that this cluster pattern is a characteristic of the LDL (apolipoprotein B/E) receptor across cell types, but that the closely related apolipoprotein E receptor differs in that it is inserted individually in a highly dispersed state, in common with a variety of other receptor types.