The CO dehydrogenase accessory protein CooT is a novel nickel-binding protein

In Rhodospirillum rubrum, maturation of Carbon Monoxide Dehydrogenase (CODH) requires three accessory proteins, CooC, CooT and CooJ, dedicated to nickel insertion into the active site, which is constituted by a distorted [NiFe₃S₄] cubane coordinated with a mononuclear Fe site. CooC is an ATPase proposed to provide the energy required for the maturation process, while CooJ is described as a metallochaperone with 16 histidines and 2 cysteines at the C-terminus, likely involved in metal binding and/or storage. Prior to the present study, no information was available on CooT at the molecular level. Here, the X-ray structure of RrCooT was obtained, which revealed that this protein is a homodimer featuring a fold that resembles an Sm-like domain, suggesting a role in RNA metabolism that was however not supported by experimental observations. Biochemical and biophysical evidence based on circular dichroism spectroscopy, light scattering, isothermal titration calorimetry and site-directed mutagenesis showed that RrCooT specifically binds a single Ni(ii) per dimer, with a dissociation constant of 9 nM, through the pair of Cys2, highly conserved residues, located at the dimer interface. Despite its role in the activation of RrCODH in vivo, CooT was thought to be a unique protein, found only in R. rubrum, with an unclear function. In this study, we extended the biological impact of CooT, establishing that this protein is a member of a novel Ni(ii)-binding protein family with 111 homologues, linked to anaerobic metabolism in bacteria and archaea, and in most cases to the presence of CODH.

SufA/IscA: reactivity studies of a class of scaffold proteins involved in [Fe-S] cluster assembly

IscA/SufA proteins belong to complex protein machineries which are involved in iron-sulfur cluster biosynthesis. They are defined as scaffold proteins from which preassembled clusters are transferred to target apoproteins. The experiments described here demonstrate that the transfer reaction proceeds in two observable steps: a first fast one leading to a protein-protein complex between the cluster donor (SufA/IscA) and the acceptor (biotin synthase), and a slow one consisting of cluster transfer leading to the apoform of the scaffold protein and the holoform of the target protein. Mutation of cysteines in the acceptor protein specifically inhibits the second step of the reaction, showing that these cysteines are involved in the cluster transfer mechanism but not in complex formation. No cluster transfer from IscA to IscU, another scaffold of the isc operon, could be observed, whereas IscU was shown to be an efficient cluster source for cluster assembly in IscA. Implications of these results are discussed.

Adenosylmethionine as a source of 5'-deoxyadenosyl radicals

The combination of an iron-sulfur cluster and S-adenosylmethionine provides a novel mechanism for the initiation of radical catalysis in an unanticipated variety of metabolic processes. Molecular details of the cluster-mediated reductive cleavage of S-adenosylmethionine to methionine and, presumably, a 5'-deoxyadenosyl radical are the targets of recent studies.

Iron-sulfur cluster assembly: characterization of IscA and evidence for a specific and functional complex with ferredoxin

The synthesis of iron-sulfur clusters in Escherichia coli is believed to require a complex protein machinery encoded by the isc (iron-sulfur cluster) operon. The product of one member of this operon, IscA, has been overexpressed, purified, and characterized. It can assemble an air-sensitive [2Fe-2S] cluster as shown by UV-visible and resonance Raman spectroscopy. The metal form but not the apoform of IscA binds ferredoxin, another member of the isc operon, selectively, allowing transfer of iron and sulfide from IscA to ferredoxin and formation of the [2Fe-2S] holoferredoxin. These results thus suggest that IscA is involved in ferredoxin cluster assembly and activation. This is an important function because a functional ferredoxin is required for maturation of other cellular Fe-S proteins.