X‐ray crystallographic analysis of the sulfur carrier protein SoxY from Chlorobium limicola f. thiosulfatophilum reveals a tetrameric structure

• Published in: Protein science 2007-04, Vol.16 (4), p.589-601
• Main Authors: Stout, Jan, Van Driessche, Gonzalez, Savvides, Savvas N., Van Beeumen, Jozef
• Format: Article
• Language: English
• Subjects: Amino Acid Sequence; Bacterial Proteins - chemistry; Chlorobium - chemistry; Chlorobium limicola; Chlorobium limicola f. thiosulfatophilum; Chromatography, Gel; crystal structure; Crystallography, X-Ray; Dimerization; Disulfides - chemistry; Molecular Sequence Data; Protein Conformation; Sequence Homology, Amino Acid; SoxY; sulfur binding; thiosulfate oxidation
• ISSN: 0961-8368; 1469-896X
• DOI: 10.1110/ps.062633607

Dissimilatory oxidation of thiosulfate in the green sulfur bacterium Chlorobium limicola f. thiosulfatophilum is carried out by the ubiquitous sulfur‐oxidizing (Sox) multi‐enzyme system. In this system, SoxY plays a key role, functioning as the sulfur substrate‐binding protein that offers its sulfur substrate, which is covalently bound to a conserved C‐terminal cysteine, to another oxidizing Sox enzyme. Here, we report the crystal structures of a stand‐alone SoxY protein of C. limicola f. thiosulfatophilum, solved at 2.15 Å and 2.40 Å resolution using X‐ray diffraction data collected at 100 K and room temperature, respectively. The structure reveals a monomeric Ig‐like protein, with an N‐terminal α‐helix, that oligomerizes into a tetramer via conserved contact regions between the monomers. The tetramer can be described as a dimer of dimers that exhibits one large hydrophobic contact region in each dimer and two small hydrophilic interface patches in the tetramer. At the tetramer interface patch, two conserved redox‐active C‐terminal cysteines form an intersubunit disulfide bridge. Intriguingly, SoxY exhibits a dimer/tetramer equilibrium that is dependent on the redox state of the cysteines and on the type of sulfur substrate component bound to them. Taken together, the dimer/tetramer equilibrium, the specific interactions between the subunits in the tetramer, and the significant conservation level of the interfaces strongly indicate that these SoxY oligomers are biologically relevant.