Functional Analysis of PsbZ in Photosystem II from the Thermophilic Cyanobacterium Thermosynechococcus elongatus BP-1

Abstract

PsbZ is a membrane protein of 6.5 kDa found in photosystem II (PSII) complexes from cyanobacteria to land plants. The two helices of PsbZ are located on the perimeter of the dimer near CP43 and PsbK in the cyanobacterial PSII structure. In tobacco and Chlamydomonas reinhardtii, psbZ knockout leads to reduce amounts of the minor antenna protein CP26. However, CP26 is not found in the cyanobacterial thylakoids. The function of PsbZ in cyanobacteria is still not clear. In this work, we deleted psbZ gene in the thermophilic cyanobacterium, Thermosynechococcus elongatus BP-1 to analyze the function of PsbZ. No difference was observed in the growth under various light conditions, and in the oxygen-evolving activities between the wild type and the mutant. However, the oxygen-evolving activity in the mutant PSII complexes purified by Ni-affinity chromatography was lower than those in the purified wild-type PSII complexes. Electrophoretic profile of the mutant PSII complexes revealed that most of PsbK band had disappeared but no significant release of the extrinsic proteins was observed. N-terminal amino acid sequences of PsbK and PsbM/Ycf12 bands showed that most of Ycf12 as well as PsbK was lost in the mutant PSII complexes. These indicate that PsbZ is required for the stable binding of PsbK and Ycf12 to the PSII core. We suggest that Ycf12 is the unidentified X1 polypeptide (Loll et al. 2005) that is observed near PsbZ and PsbK in the crystal structure of the PSII complex.