Light‐harvesting complex II protein CP29 binds to photosystem I of Chlamydomonas reinhardtii under State 2 conditions

Abstract

Excitation of the membrane-bound protein complexes photosystem I (PSI) and II (PSII) by light must be optimized to ensure the highest efficiency of photosynthetic electron transport. Redistribution of excitation energy between both photosystems as an immediate and dynamic response to changing illumination conditions occurs during the process termed 'State transitions', where State 1 is induced by excess PSI light and State 2 by excess PSII light [1]. State 1 to State 2 transition occurs in response to the reduction of the plastoquinone pool, triggering the activation of thylakoid-bound kinases which in turn phosphorylate the mobile light-harvesting complex II (LHCII) antenna [2][3][4][5]. The phosphorylated LHCII is proposed to transfer physically from PSII to PSI to balance energy distribution between, and optimize the rate of electron transfer through, the two photosystems or induce cyclic electron flow around PSI [6][7][8][9]. Conversely, in PSI-favouring light, oxidation of plastoquinone occurs, leading to deactivation of LHCII-specific kinases and dephosphorylation of mobile LHCII by redox-independent phosphatases. As a consequence, LHCII detaches from PSI and functionally couples to PSII (State 2 to State 1 transition). Recent studies of the mutants that were blocked in State 1 revealed that thylakoid protein kinase Stt7 from green alga Chlamydomonas reinhardtii and its higher plant orthologue STN7 are required for phosphorylation of several LHCII polypeptides [4,5], thus providing further evidence that protein phosphorylation is essential for State transitions. The State 1 to State 2 transition in the photosynthetic membranes of plants and green algae involves the functional coupling of phosphorylated lightharvesting complexes of photosystem II (LHCII) to photosystem I (PSI). We present evidence suggesting that in Chlamydomonas reinhardtii this coupling may be aided by a hyper-phosphorylated form of the LHCII-like CP29 protein (Lhcbm4). MS analysis of CP29 showed that Thr6, Thr16 and Thr32, and Ser102 are phosphorylated in State 2, whereas in State 1-exposed cells only phosphorylation of Thr6 and Thr32 could be detected. The LHCI-PSI supercomplex isolated from the alga in State 2 was found to contain strongly associated CP29 in phosphorylated form. Electron microscopy suggests that the binding site for this highly phosphorylated CP29 is close to the PsaH protein. It is therefore postulated that redoxdependent multiple phosphorylation of CP29 in green algae is an integral part of the State transition process in which the structural changes of CP29, induced by reversible phosphorylation, determine the affinity of LHCII for either of the two photosystems.Abbreviations Chl, chlorophyll; DDM, b-dodecyl maltoside; EM, electron microscopy; IMAC, immobilized metal affinity chromatography; LHCII, lightharvesting complex II; PSI, photosystem I; PSII, photosystem II; S1 and S2, State 1 and State 2.

The structure of a plant photosystem I supercomplex at 3.4 Å resolution

“…Genetic studies suggest that PsaH, PsaL, and PsaK play important roles in this process (Scheller et al, 2001;Zhang and Scheller, 2004). Electron microscopy studies have identified the binding site of the additional antennae complexes along the PsaL/PsaH-PsaK side (Kargul et al, 2005;Kouril et al, 2005). A new chlorophyll bound by PsaH was identified at the current resolution.…”

“…Under state II conditions, PSI associates with a mobile pool of the LHCII antennae, which increases its absorbance cross-section (Kargul et al, 2005;de Bianchi et al, 2010). Genetic studies suggest that PsaH, PsaL, and PsaK play important roles in this process (Scheller et al, 2001;Zhang and Scheller, 2004).…”

Structural Organization of Photosynthetic Apparatus in Agranal Chloroplasts of Maize

“…However, we did not detect any projections over and above the classic LHCI⅐PSI supercomplex, suggesting that in maize, LHCII remains associated with the PSII core. It cannot be excluded, however, that a subfraction of mobile LHCII may transiently associate with PSI but it is displaced following the detergent treatment, as previously reported (60,61). To explore this possibility, further investigation is required using digitonin solubilization, an approach that has successfully been used to identify a labile LHCII⅐LHCI⅐PSI supercomplex in the membranes of a C 3 plant, Arabidopsis thaliana (60).…”

“…Thylakoid membranes from MS and BS chloroplasts (0.8 mg/ml Chl) were solubilized with 0.9% DDM and fractionated by centrifugation on continuous sucrose density gradients, as detailed previously (61). Protein analyses were conducted using an SDS-PAGE Tris-Tricine system described by Schägger and von Jagow (32).…”

Live-cell imaging of photosystem II antenna dissociation during state transitions

“…Subsequently, to increase the excitation level in PSI, a certain number of LHCII (including minor LHCII) need to migrate from the aggregated structure (Fig. 5C) to the stroma lamellae (non-appressed region) and reassociate with PSI (12,34,35), thereby completing the state 2 transition (Fig. 5D).…”