Cookies on this website
We use cookies to ensure that we give you the best experience on our website. If you click 'Continue' we'll assume that you are happy to receive all cookies and you won't see this message again. Click 'Find out more' for information on how to change your cookie settings.

Conversion of vitamin B12 (cobalamin, Cbl) into the cofactor forms methyl-Cbl (MeCbl) and adenosyl-Cbl (AdoCbl) is required for the function of two crucial enzymes, mitochondrial methylmalonyl-CoA mutase and cytosolic methionine synthase, respectively. The intracellular proteins MMACHC and MMADHC play important roles in processing and targeting the Cbl cofactor to its destination enzymes, and recent evidence suggests that they may interact while performing these essential trafficking functions. To better understand the molecular basis of this interaction, we have mapped the crucial protein regions required, indicate that Cbl is likely processed by MMACHC prior to interaction with MMADHC, and identify patient mutations on both proteins that interfere with complex formation, via different mechanisms. We further report the crystal structure of the MMADHC C-terminal region at 2.2 Å resolution, revealing a modified nitroreductase fold with surprising homology to MMACHC despite their poor sequence conservation. Because MMADHC demonstrates no known enzymatic activity, we propose it as the first protein known to repurpose the nitroreductase fold solely for protein-protein interaction. Using small angle x-ray scattering, we reveal the MMACHC-MMADHC complex as a 1:1 heterodimer and provide a structural model of this interaction, where the interaction region overlaps with the MMACHC-Cbl binding site. Together, our findings provide novel structural evidence and mechanistic insight into an essential biological process, whereby an intracellular "trafficking chaperone" highly specific for a trace element cofactor functions via protein-protein interaction, which is disrupted by inherited disease mutations.

Original publication

DOI

10.1074/jbc.M115.683268

Type

Journal article

Journal

J Biol Chem

Publication Date

04/12/2015

Volume

290

Pages

29167 - 29177

Keywords

crystal structure, metabolic disease, nitroreductase fold, protein-protein interaction, site-directed mutagenesis, small-angle x-ray scattering (SAXS), vitamin B12, Amino Acid Sequence, Animals, Binding Sites, Carrier Proteins, Crystallography, X-Ray, Humans, Metabolic Diseases, Mice, Mitochondrial Membrane Transport Proteins, Molecular Chaperones, Molecular Sequence Data, Mutagenesis, Site-Directed, Mutation, Nitroreductases, Phenotype, Protein Binding, Protein Interaction Mapping, Protein Multimerization, Protein Structure, Secondary, Protein Transport, Recombinant Proteins, Sequence Homology, Amino Acid, Vitamin B 12