Identification and characterization of two distinct ligand binding regionsof cubilin

Using polymerase chain reaction-amplified fragments of cubilin, an endocyti c receptor of molecular mass 460 kDa, we have identified two distinct ligan d binding regions. Region 1 of molecular mass 71 kDa, which included the 11 3-residue N terminus along with the eight epidermal growth factor (EGF)-lik e repeats and CUB domains 1 and 2, and region 2 of molecular mass 37 kDa co nsisting of CUB domains 6-8 bound both intrinsic factor-cobalamin (vitamin B-12; Cbl) (IF-Cbl) and albumin. Within these two regions, the binding of b oth ligands was confined to a 110-115-residue stretch that encompassed eith er the 113-residue N terminus or CUB domain 7 and 8. Ca2+ dependence of lig and binding or the ability of cubilin antiserum to inhibit ligand binding t o the 113-residue N terminus was 60-65%. However, a combination of CUB doma ins 7 and 8 or 6-8 was needed to demonstrate significant Ca2+ dependence or inhibition of ligand binding by cubilin antiserum. Antiserum to EGF inhibi ted albumin but not IF-Cbl binding to the N-terminal cubilin fragment that included the eight EGF-like repeats. While the presence of excess albumin h ad no effect on binding to IF-Cbl, IF-Cbl in excess was able to inhibit alb umin binding to both regions of cubilin. Reductive alkylation of the 113-re sidue N terminus or CUB 6-8, CUB 7, or CUB 8 domain resulted in the abolish ment of ligand binding. These results indicate that (a) cubilin contains tw o distinct regions that bind both IF-Cbl and albumin and that (b) binding o f both IF-Cbl and albumin to each of these regions can be distinguished and is regulated by the nonassisted formation of local disulfide bonds.