Crystal structure of sulerythrin, a rubrerythrin-like protein from a strictly aerobic archaeon, Sulfolobus tokodaii strain 7, shows unexpected domain swapping

Sulerythrin is the first rubrerythrin-like protein to be isolated from an aerobic organism, Sulfolobus tokodaii strain 7, and it lacks a C-terminal rubredoxin-like FeS4 domain. The protein purified from Sulfolobus cells was crystallized, and the crystal structure was determined at 1.7 Angstrom resolution. The dimer of sulerythrin exhibited "domain-swapping" at the loop connecting alphaB and alphaC, hybrid four-helix bundles consisting of alphaA/B and alphaC/D being formed. The structure and atomic identity of the binuclear metal center were determined by means of anomalous scattering analysis. The site contained 1.0 mol of hexacoordinate Fe, 0.80-0.87 mol of tetracoordinate Zn, and 0.73-0.88 mol of putative O-2 per monomer. The metal ions were found at exchanged positions compared to those in the Fe/Zn-containing rubrerythrin from Desulfovibrio vulgaris. The results demonstrate that the binuclear metal center of rubrerythrin-like proteins is plastic in its ability to bind metal ions.