Ligands internalized through coated or noncoated invaginations follow a common intracellular pathway

Cholera toxin (CT) represents a class of ligands that binds preferentially to noncoated pits on the cell surface. In the present study, we have investigated the mechanism of endocytosis of this class of ligand and compared it to the classic coated pit mechanism. When either CT coupled to colloidal gold particles (CT-gold) or 125I-labeled CT were incubated with 3T3 L1 fibroblasts at 4 degrees C, both ligands bound in a preferential fashion to small noncoated pits on the cell surface. CT-gold surface-labeled cells were then incubated at 22 degrees C. The labeled ligand progressively moved into noncoated vesicles and a tubulovesicular compartment composed of a network of tubules and vesicles closely associated with multivesicular bodies but distinct from the Golgi complexes. The ligand next passed into multivesicular bodies. By contrast, alpha 2-macroglobulin (alpha 2m)-gold initially localized preferentially to coated pits and subsequently to coated vesicles and tubulovesicular structures before associating with multivesicular bodies. To directly compare the intracellular pathway followed by CT-gold to that followed by alpha 2m-gold, CT-gold (7 nm) was coincubated with alpha 2m-gold (15 nm). By 10 min of incubation at 22 degrees C, up to 66% of tubulovesicular units contained both ligands when analyzed in serial sections. Subsequently, both ligands were colocalized in multivesicular bodies. We conclude that CT-gold endocytosed via noncoated vesicles and alpha 2m-gold endocytosed through coated vesicles subsequently associate with the same tubulovesicular units, multivesicular bodies, and lysosomes.