Abstract
Photosystem II supercomplex of plants and algae consists of dimeric core complex and the surrounding light-harvesting complexes (PSII-LHCII), whose structure via single particle analysis using electron microscope has been reported. In higher plants, including spinach and Arabidopsis, PSII particles associated with four LHCII trimers have been observed. In a green alga Chlamydomonas reinhardtii, however, PSII particles with only two LHCII trimers have been observed. This difference has been accounted for by the lack of the minor monomeric LHCII CP24 in this alga. In this study, we thoroughly examined conditions for PSII-LHCII solubilizing, and revisited the structure of PSII-LHCII in C. reinhardtii. Solubilized thylakoid membranes were separated by sucrose density gradient ultracentrifugation, and the obtained PSII-LHCII fraction was negatively stained. The results of single particle analysis indicated a similar PSII-LHCII structure, with four LHCII trimers associated as in higher plants. This larger PSII-LHCII was formed in a green alga lacking a minor monomeric LHCII CP24. We will also report the results using LHCII mutants.
