LPA2 protein is involved in Photosystem II assembly in Chlamydomonas reinhardtii

Photosynthetic eukaryotes require the proper assembly of photosystem II (PSII) to strip electrons from water and fuel the carbon fixation reactions. In Arabidopsis thaliana, one of the PSII subunits (CP43/PsbC) was suggested to be assembled into the PSII complex via its interaction with an auxiliary protein called Low PSII Accumulation 2 (LPA2). However, the original articles describing the role of LPA2 in PSII assembly have been retracted. To investigate the function of LPA2, here we generated in the model organism for green algae, Chlamydomonas reinhardtii, knockout lpa2 mutants by using the CRISPR-Cas9 target-specific genome-editing system. Biochemical analyses revealed the thylakoidal localization of LPA2 protein in the WT while lpa2 mutants were characterized by a drastic reduction in the level of D1, D2, CP47 and CP43 proteins. Consequently, reduced PSII supercomplex accumulation, chlorophyll content per cell, PSII quantum yield and photosynthetic oxygen evolution were measured in the lpa2 mutants, leading to an almost complete impairment of photoautotrophic growth. Pulse-chase experiments demonstrated that the absence of LPA2 protein caused a reduced PSII assembly and reduced PSII turnover. Taken together, our data indicate that, in C. reinhardtii, LPA2 is required for PSII assembly and its proper function.