Phylogenetic and functional analysis of cyanobacterial Cytochrome c (6)-like proteins

All known photosynthetic cyanobacteria carry a cytochrome c (6) protein that acts transferring electrons from cytochrome b (6) f complex to photosystem I, in photosynthesis, or cytochrome c oxidase, in respiration. In most of the cyanobacteria, at least one homologue to cytochrome c (6) is found, the so-called cytochrome c (6B) or cytochrome c (6C). However, the function of these cytochrome c (6)-like proteins is still unknown. Recently, it has been proposed a common origin of these proteins as well as the reclassification of the cytochrome c (6C) group as c (6B), renaming the new joint group as cytochrome c (6BC). Another homologue to cytochrome c (6) has not been classified yet, the formerly called cytochrome c (6-3), which is present in the heterocyst-forming filamentous cyanobacteria Nostoc sp. PCC 7119. In this work, we propose the inclusion of this group as an independent group in the genealogy of cytochrome c (6)-like proteins with significant differences from cytochrome c (6) and cytochrome c (6BC), with the proposed name cytochrome c (6D). To support this proposal, new data about phylogeny, genome localisation and functional properties of cytochrome c (6)-like proteins is provided. Also, we have analysed the interaction of cytochrome c (6)-like proteins with cytochrome f by isothermal titration calorimetry and by molecular docking, concluding that c (6)-like proteins could interact with cytochrome b (6) f complex in a similar fashion as cytochrome c (6). Finally, we have analysed the reactivity of cytochrome c (6)-like proteins with membranes enriched in terminal oxidases of cyanobacteria by oxygen uptake experiments, concluding that cytochrome c (6D) is able to react with the specific copper-oxidase of the heterocysts, the cytochrome c oxidase 2.