Structural basis of LhcbM5-mediated state transitions in green algae

In green algae and plants, state transitions serve as a short-term light acclimation process to regulate light harvesting capacity of photosystems I and II (PSI and PSII). During the process, a portion of the light-harvesting complexes II (LHCII) are phosphorylated, dissociate from PSII and bind PSI to form PSI-LHCI-LHCII supercomplex. Here we report high-resolution structures of PSI-LHCI-LHCII supercomplex from Chlamydomonas reinhardtii, revealing the mechanism of assembly between PSI-LHCI complex and two phosphorylated LHCII trimers containing all four types of LhcbM proteins. Two specific LhcbM isoforms, namely LhcbM1 and LhcbM5, directly interact with the PSI core through their phosphorylated amino-terminal regions. Furthermore, biochemical and functional studies on mutant strains lacking either LhcbM1 or LhcbM5 indicate that only LhcbM5 is indispensable in the supercomplex formation. The results unraveled the specific interactions and potential excitation energy transfer routes between green algal PSI and two phosphorylated LHCIIs.