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 pro tein > 200 kD and closely associated with the early endocytic system c an induce fetal malformations. Although gp280 is thought to act as a r eceptor, its ligand(s) is not known. In the current study, we report t hat purified gp280 from rat kidney, like the intrinsic factor-Cobalami n receptor (IFCR), binds to the intrinsic factor-cobalamin (IF-Cbl) co mplex 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, re nal, and yolk sac apical membranes and revealed a single identically s ized protein on immunoblotting of the renal membranes. Both antibodies precipitated a single radiolabeled protein > 200 kD from cellular ext ract from [S-35]methionine-labeled yolk sac epithelial cells, and anti body to gp280 inhibited the uptake and internalization of (IF)-I-125-C bl. Immunoelectron microscopy using the two antibodies revealed that i n the kidney, both proteins were colocalized. These observations sugge st that IF-Cbl complex is a ligand for gp280 and that gp280 and IFCR a re identical proteins.