| Abstract: | In this investigation the membrane-perturbing effect of filipin, a polyene antibiotic which reacts specifically with cholesterol or cholesterol-like compounds in cell membranes, has been exploited to study the distribution of coated pits in cultured human skin fibroblasts. The coated pits, showing no filipin-cholesterol complexes, occurred singly or in clusters without apparent localization of either type to a particular region of the fibroblast membrane. Colloidal gold, conjugated to low-density lipoprotein, has proven to be an excellent marker, allowing the localization of low-density lipoprotein receptors on the surface of cultured cells. A pattern similar to that for the coated pits in the plasma membrane fracture faces was observed in the distribution of gold-low-density lipoprotein conjugates in surface replicas, indicating that the low-density lipoprotein receptors are associated with these coated pits. It was shown that there is an apparent heterogeneity in the distribution of low-density lipoprotein receptors, from cell to cell and even among different areas of the same cell membrane. The binding capacity for gold-low-density lipoprotein complexes, as represented by the extent of surface labeling, was directly related to the cell variety within the culture or to the particular experimental procedure. The observation of differences in the distribution of gold-low-density lipoprotein conjugates, even among adjacent coated pits, provides evidence for various stages of activity of the low-density lipoprotein receptors corresponding to incorporation, mobility, and internalization. |
