Visualization of a radical B12 enzyme with its G-protein chaperone

Proc Natl Acad Sci U S A. 2015 Feb 24;112(8):2419-24. doi: 10.1073/pnas.1419582112. Epub 2015 Feb 9.


G-protein metallochaperones ensure fidelity during cofactor assembly for a variety of metalloproteins, including adenosylcobalamin (AdoCbl)-dependent methylmalonyl-CoA mutase and hydrogenase, and thus have both medical and biofuel development applications. Here, we present crystal structures of IcmF, a natural fusion protein of AdoCbl-dependent isobutyryl-CoA mutase and its corresponding G-protein chaperone, which reveal the molecular architecture of a G-protein metallochaperone in complex with its target protein. These structures show that conserved G-protein elements become ordered upon target protein association, creating the molecular pathways that both sense and report on the cofactor loading state. Structures determined of both apo- and holo-forms of IcmF depict both open and closed enzyme states, in which the cofactor-binding domain is alternatively positioned for cofactor loading and for catalysis. Notably, the G protein moves as a unit with the cofactor-binding domain, providing a visualization of how a chaperone assists in the sequestering of a precious cofactor inside an enzyme active site.

Keywords: G protein; crystallography; metallochaperone; metallocofactor delivery; vitamin B12.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Apoproteins / chemistry
  • Apoproteins / metabolism
  • Catalytic Domain
  • Coenzymes / metabolism
  • Conserved Sequence
  • Crystallography, X-Ray
  • Cupriavidus / enzymology*
  • GTP Phosphohydrolases / chemistry
  • GTP-Binding Proteins / chemistry*
  • GTP-Binding Proteins / metabolism*
  • Guanosine Diphosphate / chemistry
  • Guanosine Diphosphate / metabolism
  • Methylmalonyl-CoA Mutase / chemistry*
  • Methylmalonyl-CoA Mutase / metabolism*
  • Mitochondrial Membrane Transport Proteins / chemistry
  • Models, Molecular
  • Molecular Chaperones / chemistry*
  • Molecular Chaperones / metabolism*
  • Protein Binding
  • Protein Structure, Tertiary
  • Protein Subunits / chemistry
  • Protein Subunits / metabolism


  • Apoproteins
  • Coenzymes
  • Mitochondrial Membrane Transport Proteins
  • Molecular Chaperones
  • Protein Subunits
  • Guanosine Diphosphate
  • GTP Phosphohydrolases
  • GTP-Binding Proteins
  • Methylmalonyl-CoA Mutase

Associated data

  • PDB/4XC6
  • PDB/4XC7
  • PDB/4XC8