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 TRANSITION7 (STT7) ofChlamydomonas reinhardtiiphosphorylates components of light-harvesting antenna complex II (LHCII). This reversible phosphorylation governs the dynamic allocation of a part of LHCII to PSI or PSII, depending on light conditions and metabolic demands, but counteracting phosphatase(s) remain unknown inC. reinhardtii. Here we analyzed state transitions inC. reinhardtiimutants of two phosphatases, PROTEIN PHOSPHATASE1 and PHOTOSYSTEM II PHOSPHATASE, 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 inpph1, and surprisingly also inpbcp. However, both mutants eventually returned to state 1. In contrast, the double mutantpph1;pbcpappeared 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 dephosphorylation and the more complex antenna phosphorylation patterns inC. reinhardtiicompared to Arabidopsis are discussed in the context of the stronger amplitude of state transitions and the more diverse LHCII isoforms in the alga. In Chlamydomonas, two protein phosphatases control state transitions and regulate the complex phosphorylation patterns of the light-harvesting antenna complex LHCII.