Regulation of light harvesting in Chlamydomonas: two protein phosphatases are involved in state transitions.

Protein phosphorylation plays important roles in short-term regulation of photosynthetic electron transfer, and during state transitions, the kinase STATE TRANSITION 7 (STT7) of Chlamydomonas (Chlamydomonas reinhardtii) phosphorylates components of light-harvesting antenna complex II (LHCII). This reversible phosphorylation governs the dynamic allocation of a part of LHCII to photosystem I or photosystem II, depending on light conditions and metabolic demands, but counteracting phosphatase(s) remain unknown in Chlamydomonas. Here we analyzed state transitions in Chlamydomonas mutants of two phosphatases, PROTEIN PHOSPHATASE 1 (CrPPH1) and PHOTOSYSTEM II PHOSPHATASE (CrPBCP), which are homologous to proteins that antagonize the state transition kinases (STN7 and STN8) in Arabidopsis (Arabidopsis thaliana). The transition from state 2 to state 1 was retarded in pph1, and surprisingly also in pbcp. However both mutants eventually returned to state 1. In contrast, the double mutant pph1;pbcp appeared strongly locked in state 2. The complex phosphorylation patterns of the LHCII trimers and of the monomeric subunits were affected in the phosphatase mutants. Their analysis indicated that the two phosphatases have different yet overlapping sets of protein targets. The dual control of thylakoid protein de-phosphorylation and the more complex antenna phosphorylation patterns in Chlamydomonas compared to Arabidopsis are discussed in the context of the stronger amplitude of state transitions and the more diverse LHCII isoforms in the alga.