Sulfocyanin and subunit II, two copper proteins with novel features, provide new insight into the archaeal SoxM oxidase supercomplex.

The isolation of a fully functional SoxM terminal oxidase supercomplex from the archaeon Sulfolobus acidocaldarius has failed thus far and several of its constituents have only been predicted genetically, such as the small Cu protein sulfocyanin and the subunit II bearing a Cu(A) center. Here we report the recombinant expression of sulfocyanin and prove its transcription in Sulfolobus as well as its presence in the enriched complex. It reveals a redox potential of +300 mV and spectroscopic features that are characteristic of type I copper centers. It is highly thermostable and firmly attached to the complex by one putative transmembrane anchor. Surprisingly, subunit II is completely missing from the isolated complex and behaves as an easily dissociable constituent which is a unique case within the terminal oxidase family. Its loss into the soluble phase upon cell disruption can be considered the reason for the inactivity of the isolated membrane complex.