GreenCut protein LPB1 is required for SQDG accumulation and optimal photosynthetic electron transfer from QA− to QB in Chlamydomonas reinhardtii

The thylakoid membrane lipid sulfoquinovosyldiacylglycerol (SQDG) is an anionic molecule that functions in stabilizing the thylakoid membranes and photosystem (PS) supercomplexes. In this study, we characterized the function of GreenCut protein CrLPB1, which encodes UDP-glucose pyrophosphorylase (UGP3), an enzyme involved in SQDG biosynthesis. The lpb1 mutants had reduced levels of SQDG, grew more slowly than wild type cells or the complemented strain under photoautotrophic conditions and were impacted in its rate of oxygen evolution and photosystem II (PSII) activity, especially electron transfer from QA− to QB. Furthermore, the structure of the PSII supercomplex and morphology of the thylakoid membranes were also both altered in lpb1. In conclusion, LPB1 is involved in SQDG biosynthesis, which in turn appears to be critical in maintaining normal thylakoid membrane structure and PSII activity/stability.