Monoclonal antibodies against a glycoprotein localized in coated pits and endocytic vesicles inhibit α₂-macroglobulin binding and uptake

Eight monoclonal antibodies, all IgG(2a), which recognize a 180/90-kDa glycoprotein similar in properties to the receptor for α₂-macroglobulin of mouse embryo 3T3 cell plasma membranes, have been tested for their effect on the binding and uptake of α₂-macroglobulin by live cells. One antibody directly inhibited binding of ¹²⁵I-α₂-macroglobulin under conditions in which ¹²⁵I-transferrin binding to the transferrin receptor was unaffected. Another monoclonal antibody decreased α₂-macroglobulin binding when preincubated with cells at 37°C. This antibody was also capable of specifically binding to ligand-receptor complexes formed by preincubating ¹²⁵I-α₂-macroglobulin with detergent extracts of Swiss 3T3 cells. Immunoelectron microscopy showed that the 180/90-kDa glycoprotein was localized in coated pits of the cell surface and in intracellular endocytic vesicles (receptosomes/endosomes). The data suggest that the 180/90-kDa glycoprotein is a component of the receptor for α₂-macroglobulin.