Cross-reconstitution of extrinsic cytochrome c-550 and 12 kDa protein between cyanobacterial and red algal PSII

Abstract

Red algal and cyanobacterial PSII contain two similar extrinsic proteins, namely, cytochrome c-550 and a 12 kDa protein. Their amino acid sequences showed a relatively high homology of about 40% between red algae and cyanobacteria. In this study, cross-reconstitution experiments of the extrinsic proteins with red algal (Cyanidium caldarium) or cyanobacterial (Synechococcus vulcanus) PSII were performed in order to compare the binding and functional properties of the extrinsic proteins between red algae and cyanobacteria. Red algal cytochrome c-550, as well as cyanobacterial cytochrome c-550, bound directly to cyanobacterial PSII. However, none of them bound directly to red algal PSII. This indicates that direct binding of cytochrome c-550 to PSII depends upon the structure of intrinsic proteins of PSII but not cytochrome c-550 itself. Red algal 12 kDa protein functionally bound to cyanobacterial PSII, while cyanobacterial 12 kDa protein did not bind to red algal PSII. The antibodies raised against cyanobacterial or red alga 12 kDa protein did not cross-react with red algal or cyanobacterial 12 kDa protein, respectively. These results imply that, while the structure and function of cytochrome c-550 were rather conserved, those of the 12 kDa protein have been changed during evolution from cyanobacteria to red algae.