Purification And Crystallization Of Photosystem Ii Dimer Complex From A Red Alga Cyanidium Caldarium

Abstract

The central part of photosystem II (PSII) is highly conserved from prokaryotic cyanobacteria to eukaryotes; however, there are some apparent differences in the extrinsic proteins involved in oxygen evolution among different organisms. So far, the crystal structure of PSII from cyanobacteria has been reported, whereas no reports have been published on the structure of any eukaryotic PSII. Red alga is one of the eukaryotic algae closely related to cyanobacteria, but its PSII differs from that of cyanobacteria in that the former contains a 20 kDa protein, a unique, fourth extrinsic protein. In order to elucidate the structure of red algal PSII and its differences with cyanobacterial PSII, we purified and crystallized PSII from an acidophilic, thermophilic red alga Cyanidium caldarium. In this study, the previously published procedure for purification of PSII from the red alga was improved, yielding a highly purified PSII dimer preparation with high oxygen-evolving activities comparable with that of thermophilic cyanobacterial PSII. We obtained three-dimensional crystals of the red algal PS II under several conditions, and characterized the crystals by X-ray diffraction. The results showed that the red algal PSII crystals had a space group of P2221, which is different from that of cyanobacterial crystals. The unit cell parameters of red algal PSII were also different from those of cyanobacterial PSII, with a and c axes longer and b axis shorter than those of cyanobacterial PSII.