Identification of rat yolk sac target protein of teratogenic antibodies, gp280, as intrinsic factor cobalamin receptor

Previous studies in the rat have shown that antibodies to gp280, a protein > 200 kD and closely associated with the early endocytic system can induce fetal malformations. Although gp280 is thought to act as a receptor, its ligand(s) is not known. In the current study, we report that purified gp280 from rat kidney, like the intrinsic factor-Cobalamin receptor (IFCR), binds to the intrinsic factor-cobalamin (IF-Cbl) complex with an association constant of 0.3 x 10(9) M-1 and mediates its internalization. Furthermore, antibodies raised to purified gp280 and IFCR inhibited the binding of IF-[Co-57]Cbl complex to intestinal, renal, and yolk sac apical membranes and revealed a single identically sized protein on immunoblotting of the renal membranes. Both antibodies precipitated a single radiolabeled protein > 200 kD from cellular extract from [S-35]methionine-labeled yolk sac epithelial cells, and antibody to gp280 inhibited the uptake and internalization of (IF)-I-125-Cbl. Immunoelectron microscopy using the two antibodies revealed that in the kidney, both proteins were colocalized. These observations suggest that IF-Cbl complex is a ligand for gp280 and that gp280 and IFCR are identical proteins.