Crystallization, preliminary crystallographic analysis and phasing of the thiosulfate-binding protein SoxY from Chlorobium limicola f. thiosulfatophilum

The 22 kDa SoxYZ protein complex from the green sulfur bacterium Chlorobium limicola f. thiosulfatophilum is a central player in the sulfur-oxidizing (Sox) enzyme system of the organism by activating thiosulfate for oxidation by SoxXA and SoxB. It has been proposed that SoxYZ exists as a heterodimer or heterotetramer, but the properties and role of the individual components of the complex thus far remain unknown. Here, the heterologous expression, purification, and the crystallization of stable tetrameric SoxY are reported. Crystals of SoxY diffract to 2.15 Å resolution and belong to space group C222₁, with unit-cell parameters a = 41.22, b = 120.11, c = 95.30 Å. MIRAS data from Pt²⁺- and Hg²⁺-derivatized SoxY crystals resulted in an interpretable electron-density map at 3 Å resolution after density modification.