Abstract
Alpha 2-macroglobulin is internalized into cultured fibroblasts by receptor-mediated endocytosis. This ligand binds initially to diffusely distributed receptors on the cell surface which cluster rapidly into bristle-coated pits. Within a few minutes at 37 degrees C, these complexes are internalized into uncoated cytoplasmic vesicles, called receptosomes, which move about in the cell by saltatory motion. These vesicles interact with the Golgi-endoplasmic reticulum-lysosome system in the cell to deliver the ligand to newly formed lysosomes within 30--60 min.