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

Cytochrome c(6) from the cyanobacterium Arthrospira maxima is present in isoforms that can be resolved by size-exclusion chromatography. One isoform crystallized in space group I4(1)32 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 c(6) molecules. Each cluster is a hollow shell with approximate octahedral (432) symmetry. Structural and biochemical studies of cytochrome c(6) isolated from other cyanobacteria and algae have led to the suggestion that cytochrome c(6) forms oligomers. The cytochrome c(6) complex described here is the largest assembly of cytochrome c(6) molecules observed thus far.