Dissociation of Light-Harvesting Complex II from Photosystem II During State Transitions in Chlamydomonas reinhardtii

Abstract

State transition is a plant photoacclimation mechanism to regulate the equilibrium of the light-driven excitation at photosystem I (PSI) and photosystem II (PSII). It has been considered that redistribution of light-harvesting complex (LHC) II adjusts the absorption-cross section of each photosystem. Several lines of biochemical evidence for the association of LHCII to PSI were recently provided by isolating a protein supercomplex composed of PSI, LHCI, and LHCII. However, the detachment of LHCIIs from PSII, which should occur simultaneously during state transitions, still remains unclear. In this study, we established a streamlined protocol to isolate a protein supercomplex comprised of PSII and LHCII, a so-called PSII-LHCII supercomplex, to explore the changes in its structure during state transitions in Chlamydomonas reinhardtii. The results suggested that the PSII-LHCII supercomplex in state 1 formed a dimer with the molecular weight of around 1,500 kDa. We speculate that, in the course of a transition to State 2, monomerization of the PSII-LHCII supercomplex occurs first, and subsequently the detachment of LHCII from PSII takes\place, which is accompanied by the phosphorylation of LHCIIs.