Receptor-mediated endocytosis proceeds by transfer of receptor-ligand complexes 
from clathrin-coated pits at the cell surface to uncoated endocytic vesicles 
termed receptosomes (or endosomes). These vesicles have now been purified more 
than 37-fold based on their content of newly internalized epidermal growth 
factor. 125I-labeled EGF was bound to human KB carcinoma cells at 4 degrees C, 
and then the cells were warmed to 37 degrees C for 8 min and disrupted. The 
purification scheme involved density gradient centrifugation on colloidal silica 
and sucrose and gel filtration on Sephacryl S-1000. Relative to homogenate, 
receptosomes are enriched 4.3-fold in their cholesterol content and depleted in 
enzyme markers for plasma membranes (2- to 3-fold) and lysosomes (9-fold). 
Receptosomes have a polypeptide composition that is different from plasma 
membrane, lysosome, and other homogenate fractions. They are enriched in 
transferrin receptors (30-fold) and in unidentified Mr 70,000-75,000 
glycoprotein(s); they contain phosphomannosyl receptors. They do not contain 
detectable amounts of clathrin.