Ultrastructural visualization of low-density lipoproteins during receptor binding and cellular endocytosis.

Human fibroblasts possess surface receptors which have a high affinity for low-density lipoproteins (LDL). However, previous studies have not provided direct ultrastructural visualization of LDL bound to the receptor. To permit direct observation of unlabeled LDL during receptor binding and cellular endocytosis, we examined several fixative regimens which employ lipophilic stains. Staining with tannic acid, an oxidized form of ruthenium red, or potassium ferrocyanide imparted sufficient contrast to individual molecules of LDL to permit high-resolution electron microscopy of receptor binding and endocytosis. The LDL molecule was observed in immediate contact with the receptor and the coated vesicle, indicating that receptor-ligand binding occurs by short-range interactions.