The Structure of the Soluble Domain of an Archaeal Rieske Iron–Sulfur Protein at 1.1 Å Resolution

The first crystal structure of an archaeal Rieske iron–sulfur protein, the soluble domain of Rieske iron–sulfur protein II (soxF) from the hyperthermo-acidophile Sulfolobus acidocaldarius, has been solved by multiple wavelength anomalous dispersion (MAD) and has been refined to 1.1 Å resolution. SoxF is a subunit of the terminal oxidase supercomplex SoxM in the plasma membrane of S. acidocaldarius that combines features of a cytochrome bc1 complex and a cytochrome c oxidase. The [2Fe–2S] cluster of soxF is most likely the primary electron acceptor during the oxidation of caldariella quinone by the cytochrome a587/Rieske subcomplex. The geometry of the [2Fe–2S] cluster and the structure of the cluster-binding site are almost identical in soxF and the Rieske proteins from eucaryal cytochrome bc1 and b6f complexes, suggesting a strict conservation of the catalytic mechanism. The main domain of soxF and part of the cluster-binding domain, though structurally related, show a significantly divergent structure with respect to topology, non-covalent interactions and surface charges. The divergent structure of soxF reflects a different topology of the soxM complex compared to eucaryal bc complexes and the adaptation of the protein to the extreme ambient conditions on the outer membrane surface of a hyperthermo-acidophilic organism.