Structure of cytochrome c₆ from Arthrospira maxima: an assembly of 24 subunits in a nearly symmetric shell

Cytochrome c₆ from the cyanobacterium Arthrospira maxima is present in isoforms that can be resolved by size-exclusion chromatography. One isoform crystallized in space group I4₁32 with eight protein molecules in the asymmetric unit and a total of 384 molecules in the unit cell. Within the crystal, the molecules are arranged as clusters of 24 cytochrome c₆ molecules. Each cluster is a hollow shell with approximate octahedral (432) symmetry. Structural and biochemical studies of cytochrome c₆ isolated from other cyanobacteria and algae have led to the suggestion that cytochrome c₆ forms oligomers. The cytochrome c₆ complex described here is the largest assembly of cytochrome c₆ molecules observed thus far.