A Cell-Free Analysis of the Pathway

A wide variety of ligands enter eukaryotic cells by being enclosed within vesicles that arise by invagination of the plasma membrane (Silverstein et al., 1977). In certain cases the ligands are selectively concentrated for uptake by first binding to specific receptors at the plasma membrane, a process known as receptor-mediated endocytosis. Once internalised the fate of the receptor-ligand complex is dependent upon the nature of the ligand (see Wileman et al., 1985), and the best-studied outcome is that where the receptor is recycled back to the plasma membrane and the ligand is delivered to lysosomes for degradation. This pathway is schematically illustrated in Fig. 1. The ligands circulating in the extracellular medium bind to high-affinity receptors on the cell surface and the receptor-ligand complex becomes concentrated in coated pits, specialised regions of the plasma membrane which possess a characteristic cytoplasmic coat. As the coat grows, the pit invaginates and pinches off to form a coated vesicle. Soon after entering the cell the vesicle begins to uncoat and the coat subunits, termed triskelions, are presumed to recycle back to the plasma membrane to form another coat. The uncoated, or partially uncoated, vesicle fuses with an endosome, a prelysosomal compartment with an acidic interior similar to the lysosome but lacking