Identification of the mobile light-harvesting complex Ⅱ polypeptides for state transitions in Chlamydomonas reinhardtii

摘要

State transition in photosynthesis is a short-term balancing mechanism of energy distribution between photosystem Ⅰ (PSⅠ) and photosystem Ⅱ (PSⅡ). When PSⅡ is preferentially excited (state 2), a pool of mobile light-harvesting complex Ⅱ (LHCⅡ) antenna proteins is thought to migrate from PSII to PSI, but biochemical evidence for a physical association between LHCⅡ proteins and PSⅠ in state 2 is weak. Here, using the green alga Chlamydomonas reinhardtii, which has a high capacity for state transitions, we report the isolation of PSⅠ-light-harvesting complex Ⅰ (LHCⅠ) super-complexes from cells locked into state 1 and state 2. We solubilized the thylakoid membranes with a mild detergent, separated the proteins by sucrose density gradient centrifugation, and subjected gradient fractions to gel-filtration chromatography. Three LHCⅡ polypeptides were associated with a PSⅠ-LHCⅠ supercomplex only in state 2; we identified them as two minor monomeric LHCⅡ proteins (CP26 and CP29) and one previously unreported major LHCⅡ protein type Ⅱ, or LhcbM5. These three LHCⅡ proteins, in addition to the major trimeric LHCⅡ proteins, were phosphorylated upon transition to state 2. The corresponding phylogenetic tree indicates that among the LHCⅡ proteins associated with PSⅡ, these three LHCⅡ proteins are the most similar to the LHC proteins for PSⅠ (LHCⅠ). Our results are important because CP26, CP29, and LhcbM5, which have been viewed as belonging solely to the PSⅡ complex, are now postulated to shuttle between PSⅠ and PSⅡ during state transitions, thereby acting as docking sites for the trimeric LHCⅡ proteins in both PSⅠ and PSⅡ.