Structure of Intrinsic Factor-Cobalamin bound to its receptor Cubilin
[IF_HUMAN] Defects in GIF are the cause of hereditary intrinsic factor deficiency (IFD) [MIM:261000]; also known as congenital pernicious anemia. IFD is an autosomal recessive disorder characterized by megaloblastic anemia. [CUBN_HUMAN] Grasbeck-Imerslund disease. Defects in CUBN are a cause of recessive hereditary megaloblastic anemia 1 (RH-MGA1) [MIM:261100]; also known as MGA1 Norwegian type or Imerslund-Grasbeck syndrome (I-GS). RH-MGA1 is due to selective malabsorption of vitamin B12. Defects in vitamin B12 absorption lead to impaired function of thymidine synthase. As a consequence DNA synthesis is interrupted. Rapidly dividing cells involved in erythropoiesis are particularly affected. 
[IF_HUMAN] Promotes absorption of the essential vitamin cobalamin (Cbl) in the ileum. After interaction with CUBN, the GIF-cobalamin complex is internalized via receptor-mediated endocytosis. [CUBN_HUMAN] Cotransporter which plays a role in lipoprotein, vitamin and iron metabolism, by facilitating their uptake. Binds to ALB, MB, Kappa and lambda-light chains, TF, hemoglobin, GC, SCGB1A1, APOA1, high density lipoprotein, and the GIF-cobalamin complex. The binding of all ligands requires calcium. Serves as important transporter in several absorptive epithelia, including intestine, renal proximal tubules and embryonic yolk sac. Interaction with LRP2 mediates its trafficking throughout vesicles and facilitates the uptake of specific ligands like GC, hemoglobin, ALB, TF and SCGB1A1. Interaction with AMN controls its trafficking to the plasma membrane and facilitates endocytosis of ligands. May play an important role in the development of the peri-implantation embryo through internalization of APOA1 and cholesterol. Binds to LGALS3 at the maternal-fetal interface.    
Publication Abstract from PubMed
Cobalamin (Cbl, vitamin B(12)) is a bacterial organic compound and an essential coenzyme in mammals, which take it up from the diet. This occurs by the combined action of the gastric intrinsic factor (IF) and the ileal endocytic cubam receptor formed by the 460-kilodalton (kDa) protein cubilin and the 45-kDa transmembrane protein amnionless. Loss of function of any of these proteins ultimately leads to Cbl deficiency in man. Here we present the crystal structure of the complex between IF-Cbl and the cubilin IF-Cbl-binding-region (CUB(5-8)) determined at 3.3 A resolution. The structure provides insight into how several CUB (for 'complement C1r/C1s, Uegf, Bmp1') domains collectively function as modular ligand-binding regions, and how two distant CUB domains embrace the Cbl molecule by binding the two IF domains in a Ca(2+)-dependent manner. This dual-point model provides a probable explanation of how Cbl indirectly induces ligand-receptor coupling. Finally, the comparison of Ca(2+)-binding CUB domains and the low-density lipoprotein (LDL) receptor-type A modules suggests that the electrostatic pairing of a basic ligand arginine/lysine residue with Ca(2+)-coordinating acidic aspartates/glutamates is a common theme of Ca(2+)-dependent ligand-receptor interactions.
Structural basis for receptor recognition of vitamin-B(12)-intrinsic factor complexes.,Andersen CB, Madsen M, Storm T, Moestrup SK, Andersen GR Nature. 2010 Mar 18;464(7287):445-8. PMID:20237569
From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.