title: Structure of Human B12 Trafficking Protein CblD Reveals Molecular Mimicry and Identifies a New Subfamily of Nitro-FMN Reductases.
abstract: In mammals, B12 (or cobalamin) is an essential cofactor required by methionine synthase and methylmalonyl-CoA mutase. A complex intracellular pathway supports the assimilation of cobalamin into its active cofactor forms and delivery to its target enzymes. MMADHC (the methylmalonic aciduria and homocystinuria type D protein), commonly referred to as CblD, is a key chaperone involved in intracellular cobalamin trafficking, and mutations in CblD cause methylmalonic aciduria and/or homocystinuria. Herein, we report the first crystal structure of the globular C-terminal domain of human CblD, which is sufficient for its interaction with MMADHC (the methylmalonic aciduria and homocystinuria type C protein), or CblC, and for supporting the cytoplasmic cobalamin trafficking pathway. CblD contains an α+β fold that is structurally reminiscent of the nitro-FMN reductase superfamily. Two of the closest structural relatives of CblD are CblC, a multifunctional enzyme important for cobalamin trafficking, and the activation domain of methionine synthase. CblD, CblC, and the activation domain of methionine synthase share several distinguishing features and, together with two recently described corrinoid-dependent reductive dehalogenases, constitute a new subclass within the nitro-FMN reductase superfamily. We demonstrate that CblD enhances oxidation of cob(II)alamin bound to CblC and that disease-causing mutations in CblD impair the kinetics of this reaction. The striking structural similarity of CblD to CblC, believed to be contiguous in the cobalamin trafficking pathway, suggests the co-option of molecular mimicry as a strategy for achieving its function.
authors.affiliation: From the Department of Biochemistry, Uniformed Services University of the Health Sciences, Bethesda, Maryland 20814 and.
authors.forename: Kazuhiro
authors.initials: K
authors.lastname: Yamada
authors.affiliation: the Department of Biological Chemistry, University of Michigan Medical School, Ann Arbor, Michigan 48109-0600.
authors.forename: Carmen
authors.initials: C
authors.lastname: Gherasim
authors.affiliation: the Department of Biological Chemistry, University of Michigan Medical School, Ann Arbor, Michigan 48109-0600 rbanerje@umich.edu.
authors.forename: Ruma
authors.initials: R
authors.lastname: Banerjee
authors.affiliation: From the Department of Biochemistry, Uniformed Services University of the Health Sciences, Bethesda, Maryland 20814 and markos.koutmos@usuhs.edu.
authors.forename: Markos
authors.initials: M
authors.lastname: Koutmos
mesh_terms: D013766:5-Methyltetrahydrofolate-Homocysteine S-Methyltransferase / Q000378:metabolism; D020816:Amino Acid Motifs; D000595:Amino Acid Sequence; D018360:Crystallography, X-Ray; D003593:Cytoplasm / Q000378:metabolism; D038181:FMN Reductase / Q000235:genetics*; D006712:Homocystinuria / Q000378:metabolism; D006801:Humans; D047908:Intracellular Signaling Peptides and Proteins; D008024:Ligands; D008247:Lysosomes / Q000378:metabolism; D033681:Mitochondrial Membrane Transport Proteins / Q000737:chemistry* / Q000235:genetics*; D018716:Molecular Mimicry; D008969:Molecular Sequence Data; D009154:Mutation; D020125:Mutation, Missense; D010100:Oxygen / Q000378:metabolism; D011487:Protein Conformation; D017510:Protein Folding; D017433:Protein Structure, Secondary; D021381:Protein Transport; D017386:Sequence Homology, Amino Acid; D014805:Vitamin B 12 / Q000737:chemistry*; D014961:X-Ray Diffraction
chemical_list: D047908:Intracellular Signaling Peptides and Proteins; D008024:Ligands; C527938:MMADHC protein, human; D033681:Mitochondrial Membrane Transport Proteins; D038181:FMN Reductase; D013766:5-Methyltetrahydrofolate-Homocysteine S-Methyltransferase; D014805:Vitamin B 12; D010100:Oxygen
keywords: B12 metabolism; chaperone; crystal structure; intracellular trafficking; metabolic disease; protein motif