title: Characterization of an epithelial approximately 460-kDa protein that facilitates endocytosis of intrinsic factor-vitamin B12 and binds receptor-associated protein.
abstract: By using receptor-associated protein (RAP) as an affinity target, an intrinsic factor-vitamin B12 (IF-B12)-binding renal epithelial protein of approximately 460 kDa was copurified together with the transcobalamin-B12-binding 600-kDa receptor, megalin. IF-B12 affinity chromatography of renal cortex membrane from rabbit and man yielded the same approximately 460-kDa protein. Binding studies including surface plasmon resonance analyses of the protein demonstrated a calcium-dependent and high affinity binding of IF-B12 to a site distinct from the RAP binding site. The high affinity binding of IF-B12 was dependent on complex formation with vitamin B12. Light and electron microscope autoradiography of rat renal cortex cryosections incubated directly with IF-57Co-B12 and rat proximal tubules microinjected in vivo with the radioligand demonstrated binding of the ligand to endocytic invaginations of proximal tubule membranes followed by endocytosis and targeting of vitamin B12 to lysosomes. Polyclonal antibodies recognizing the approximately 460-kDa receptor inhibited the uptake. Immunohistochemistry of kidney and intestine showed colocalization of the IF-B12 receptor and megalin in both tissues. In conclusion, we have identified the epithelial IF-B12-binding receptor as a approximately 460-kDa RAP-binding protein facilitating endocytosis.
authors.affiliation: Department of Cell Biology, 8000 Aarhus C, Denmark.
authors.forename: H
authors.initials: H
authors.lastname: Birn
authors.forename: P J
authors.initials: PJ
authors.lastname: Verroust
authors.forename: E
authors.initials: E
authors.lastname: Nexo
authors.forename: H
authors.initials: H
authors.lastname: Hager
authors.forename: C
authors.initials: C
authors.lastname: Jacobsen
authors.forename: E I
authors.initials: EI
authors.lastname: Christensen
authors.forename: S K
authors.initials: SK
authors.lastname: Moestrup
mesh_terms: D000818:Animals; D001425:Bacterial Outer Membrane Proteins; D004705:Endocytosis; D029968:Escherichia coli Proteins; D006801:Humans; D007437:Intrinsic Factor / Q000378:metabolism*; D007687:Kidney Tubules, Proximal / Q000378:metabolism / Q000648:ultrastructure; D008297:Male; D026901:Membrane Transport Proteins; D008854:Microscopy, Electron; D051381:Rats; D017208:Rats, Wistar; D018000:Receptors, Peptide / Q000378:metabolism*; D014805:Vitamin B 12 / Q000378:metabolism*
chemical_list: D001425:Bacterial Outer Membrane Proteins; C044592:BtuB protein, E coli; D029968:Escherichia coli Proteins; D026901:Membrane Transport Proteins; D018000:Receptors, Peptide; C036746:intrinsic factor-vitamin B 12 complex; D007437:Intrinsic Factor; D014805:Vitamin B 12