Compositional and structural analyses of the photosystem II isolated from the red alga cyanidioschyzon merolae

Abstract

Members of the rhodophytan order Cyanidiales are unique among phototrophs in their ability to live in extremely low pH levels and moderately high temperatures. The photosynthetic apparatus of the red alga Cyanidioschyzon merolae represents an intermediate type between cyanobacteria and higher plants, suggesting that this alga may provide the evolutionary link between prokaryotic and eukaryotic phototrophs. While red algal PSI resembles that of the higher plants, the PSII complex is more reminiscent of the cyanobacterial ancestor in that it contains phycobilisomes as the light-harvesting system instead of Chla/b binding proteins of green algae and higher plants, as well as the PsbU and PsbV subunits stabilising the oxygen evolving complex (OEC). The most remarkable feature of the red algal PSII is the presence of the fourth extrinsic protein of 20 kDa (PsbQ’) which is not found in the cyanobacterial OEC and which is distantly related with the green algal PsbQ. This feature together with some differences in the structural cooperation between the OEC subunits suggests that the lumenal side of red algal PSII may vary from the prokaryotic ancestor. In order to elucidate the structural differences between cyanobacterial and eukaryotic PSII, we have isolated highly active and stable dimeric complexes of the C. merolae PSII and subjected them to high throughput crystallization and mass spectrometry analyses. Here we report the full subunit composition and preliminary results of 3D crystallization of the dimeric C. merolae PSII.