The Qo site of cytochrome b6f complexes controls the activation of the LHCII kinase

Abstract

We created a Qo pocket mutant by site-directed mutagenesis of the chloroplast petD gene in Chlamydomonas reinhardtii. We mutated the conserved PEWY sequence in the EF loop of subunit IV into PWYE. The pwye mutant did not grow in phototrophic conditions although it assembled wild-type levels of cytochrome b 6 f complexes. We demonstrated a complete block in electron transfer through the cytochrome b 6 f complex and a loss of plastoquinol binding at Qo. The accumulation of cytochrome b 6 f complexes lacking affinity for plastoquinol enabled us to investigate the role of plastoquinol binding at Qo in the activation of the light-harvesting complex II (LHCII) kinase during state transitions. We detected no fluorescence quenching at room temperature in state II conditions relative to that in state I. The quantum yield spectrum of photosystem I charge separation in the two state conditions displayed a trough in the absorption region of the major chlorophyll a/b proteins, demonstrating that the cells remained locked in state I. 33 P i labeling of the phosphoproteins in vivo demonstrated that the antenna proteins remained poorly phosphorylated in both state conditions. Thus, the absence of state transitions in the pwye mutant demonstrates directly that plastoquinol binding in the Qo pocket is required for LHCII kinase activation.

Activation of the Stt7/STN7 Kinase through Dynamic Interactions with the Cytochrome b6f Complex

“…Interaction between Stt7/STN7 and the Cytb 6 f Complex It is well established that the Stt7/STN7 kinase is closely associated with the Cytb 6 f complex (Lemeille et al, 2009) and that its activation is critically dependent on this association (Vener et al, 1997;Zito et al, 1999). Additional investigations using pull-down assays revealed that the interaction between this kinase and Cytb 6 f occurs through the Rieske protein (Lemeille et al, 2009).…”

“…Excess excitation of PSII relative to PSI leads to reduction of the PQ pool and thus favors the docking of PQH 2 to the Qo site of the Cyt b 6 f complex. This process activates the Stt7/STN7 protein kinase (Vener et al, 1997;Zito et al, 1999), which is closely associated with this complex and leads to the phosphorylation of some LHCII proteins and to their detachment from PSII and binding to PSI (Depège et al, 2003;Lemeille et al, 2009). Although both Lhcb1 and Lhcb2 are phosphorylated, only the phosphorylated form of Lhcb2 is associated with PSI whereas phosphorylated Lhcb1 is excluded from this complex (Longoni et al, 2015).…”

The Phosphorylation Status of the Chloroplast Protein Kinase STN7 of Arabidopsis Affects Its Turnover

“…Excess stimulation of PSII relative to PSI leads to a more reduced redox state of the plastoquinone pool that favors docking of plastoquinol to the Qo site of the Cyt b 6 f complex. This activates the thylakoid protein kinase Stt7/STN7 that is required for the phosphorylation of the light-harvesting system of PSII (LHCII; Depège et al, 2003;Bellafiore et al, 2005), an event that leads to the displacement of LHCII from PSII to PSI (state 2; Vener et al, 1997;Zito et al, 1999). This process is reversible as excess stimulation of PSI oxidizes the plastoquinone pool and leads to the inactivation of the Stt7/STN7 kinase and to the dephosphorylation of LHCII by the PPH1/TAP38 phosphatase (Pribil et al, 2010;Shapiguzov et al, 2010) and its return to PSII (state 1).…”

A novel plant protein undergoing light-induced phosphorylation and release from the photosynthetic thylakoid membranes

“…A unique light-and redox-controlled protein phosphorylation system has evolved in plant thylakoid membranes for regulation of the photosynthetic process (1,2). Intrinsic protein kinases in chloroplast thylakoid membranes (3)(4)(5) are activated by light or reducing conditions and controlled by the reduction of plastoquinone and its binding to the reduced cytochrome bf complex (6,7). Additional modulation of protein phosphorylation in thylakoid membranes involves the thiol redox state (8,9) as well as light-modulated conformational changes of substrate proteins (10).…”