The functional cobalamin (vitamin B12)-intrinsic factor receptor is a novel complex of cubilin and amnionless.
Imerslund-Gräsbeck syndrome (I-GS, megaloblastic anemia 1) is an autosomal recessive disorder characterized by intestinal cobalamin (vitamin B(12)) malabsorption and proteinuria. I-GS-causing mutations are found in either of 2 genes encoding the epithelial proteins: cubilin and amnionless (AMN). Cubilin recognizes intrinsic factor (IF)-cobalamin and various other proteins to be endocytosed in the intestine and kidney, respectively, whereas the function of AMN is unknown. Here we show that cubilin and AMN colocalize in the endocytic apparatus of polarized epithelial cells and copurify as a tight complex during IF-cobalamin affinity and nondenaturing gel filtration chromatography. In transfected cells expressing either AMN or a truncated IF-cobalamin-binding cubilin construct, neither protein alone conferred ligand endocytosis. In cubilin transfectants, cubilin accumulated in early biosynthetic compartments. However, in cells cotransfected with AMN and the cubilin construct, cubilin trafficked to the cell surface and endosomes, and the cells exhibited IF-cobalamin endocytosis and lysosomal degradation of IF. These data indicate that cubilin and AMN are subunits of a novel cubilin/AMN (cubam) complex, where AMN binds to the amino-terminal third of cubilin and directs subcellular localization and endocytosis of cubilin with its ligand. Therefore, mutations affecting either of the 2 proteins may abrogate function of the cubam complex and cause IG-S.
Fyfe
John C
JC
Department of Microbiology and Molecular Genetics, Michigan State University, East Lansing, USA.
Madsen
Mette
M
Højrup
Peter
P
Christensen
Erik I
EI
Tanner
Stephan M
SM
de la Chapelle
Albert
A
He
Qianchuan
Q
Moestrup
Søren K
SK
D000595:Amino Acid Sequence; D000818:Animals; D016466:CHO Cells; D002850:Chromatography, Gel; D006224:Cricetinae; D018076:DNA, Complementary / Q000378:metabolism; D004847:Epithelial Cells / Q000378:metabolism; D006801:Humans; D007668:Kidney / Q000378:metabolism; D007672:Kidney Cortex / Q000378:metabolism; D008024:Ligands; D008565:Membrane Proteins / Q000737:chemistry*; D018613:Microscopy, Confocal; D016253:Microscopy, Immunoelectron; D008954:Models, Biological; D008969:Molecular Sequence Data; D009154:Mutation; D011485:Protein Binding; D011956:Receptors, Cell Surface / Q000737:chemistry* / Q000502:physiology; D017386:Sequence Homology, Amino Acid; D014162:Transfection
C424167:Amn protein, mouse; D018076:DNA, Complementary; D008024:Ligands; D008565:Membrane Proteins; D011956:Receptors, Cell Surface; C029486:intrinsic factor-cobalamin receptor