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

Abstract

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) supercomplexes 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.photoacclimation ͉ photosynthesis ͉ photosystem I n oxygen-evolving photosynthetic organisms, two types of photosystems, photosystem I (PSI) and photosystem II (PSII), operate in series in the transfer of electrons from water to NADP ϩ . The two photosystems contain their own reaction centers and light-harvesting antenna systems. The PSI lightharvesting antenna system contains light-harvesting complex I (LHCI) proteins as peripheral monomeric antennas, whereas the PSII light-harvesting antenna system contains major and minor light-harvesting complex II (LHCII) proteins as peripheral trimeric and inner monomeric antennas, respectively (see review in ref. 1). The distribution of absorbed light energy between the two photosystems is dynamically balanced, ensuring maximum efficiency for photosynthetic electron transport in changing light environments (2, 3). That balance is regulated by a process termed ''state transitions'': state 1 is induced by preferential excitation of PSI, and state 2 is induced by preferential excitation of PSII (see recent reviews in refs. 4-6). This short-term adaptation process involves thylakoid-bound protein kinase(s) that is responsible for the phosphorylation of LHCII (7,8). The activation of the kinase(s) is regulated by the redox state of the intersystem pool of plastoquinone (9) through cytochrome b 6 f complexes (10). The phospho-LHCII proteins in the PSIIenri...

PHOTOSYSTEM II SUBUNIT R Is Required for Efficient Binding of LIGHT-HARVESTING COMPLEX STRESS-RELATED PROTEIN3 to Photosystem II-Light-Harvesting Supercomplexes in Chlamydomonas reinhardtii

“…Notably, LHCBM1, LHCBM3, LHCBM4/6, and LHCBM2/7, which are part of trimeric LHCII complexes (Stauber et al, 2003), were found to be decreased between 3.5-and 2-fold, where LHCBM1 was most affected, with 3.5-fold reduction in the ami-RNApsbr-2 strain. In contrast, LHCBM5, involved in state transitions in C. reinhardtii (Takahashi et al, 2006), appeared to be unchanged.…”

Inactivation of genes coding for mitochondrial Nd7 and Nd9 complex I subunits in Chlamydomonas reinhardtii. Impact of complex I loss on respiration and energetic metabolism

“…In complex I-deficient cells, we found that two protein spots corresponding to LHCII proteins Lhcb5/CP26 and Lhcbm2 were less abundant than in wild type but this was not paralleled with a change in other LHCII protein amounts, PSII-LHCII amount, or PSII absorption cross section. Both proteins play a critical role in state transition process and Lhcb5/CP26 is phosphorylated upon state transition (Ferrante et al, 2012;Takahashi et al, 2006). This might indicate that the decrease in Lhcb5 protein observed in complex I mutant is due a slight difference in the ratio between non-phosphorylated/phosphorylated Lhcb5/CP26 protein.…”

Mechanisms of Acido-Tolerance and Characteristics of Photosystems in an Acidophilic and Thermophilic Red Alga, Cyanidium Caldarium

“…This gives rise to an asymmetric conformation of the complex, as all 4 LHCI subunits are located in one side of the complex. The opposite side is occupied by PsaL and PsaH, of which, PsaH has been suggested to serve as a docking site for mobile LHCII possibly migrated from PSII upon state-transitions (Takahashi et al, 2006;Amunts et al, 2007). This subunit is not present in cyanobacterial as well as red algal PSI, since the antennae of PSII in both organisms are membrane-peripheral phycobilisomes instead of LHCII.…”