Environmental and metaboliccontrol of LHCII protein phosphorylation: revealing the mechanismsfor dual regulation of the LHCII kinase

Light‐harvesting complex II (LHCII) protein phosphorylation inplant chloroplasts is under complex regulation. Combination of the invivo monitoring of LHCII protein phosphorylation (by immunoblotting)with the in vitroγ‐³²P]ATPphosphorylation assays revealed that the basic activation/deactivationmodel of the LHCII kinase, regulated by reversible occupation/releaseof plastoquinol at the plastoquinol oxidation (Q_o) siteof the cytochrome b₆f (cyt b₆f) complex, isconsistent with, but not sufficient to explain the data obtainedwith isolated chloroplasts, leaf discs or intact leaves. Not onlythe light conditions but also the metabolic state of the entireplant, particularly the sugar metabolism, exerted a control overLHCII protein phosphorylation. Feeding of leaves with glucose (alsowith glutathione) activated the LHCII kinase in darkness. On the otherhand, independently of the basic activation/deactivationmechanism of the kinase, a strong inhibition of LHCII protein phosphorylationoccurred in vivo at increasing irradiances and even at lowlight conditions, depending on the metabolic state of the plant.Both the experiments with intact chloroplasts and the reconstitutionexperiments with isolated thylakoids to mimic LHCII kinase inhibition,disclosed that the kinase in its activated state (plastoquinol at theQ_o site of cyt b₆f complex) is protected againstinhibition by thiol reductants. However, directly upon deactivationof the kinase (release of plastoquinol from the Q_o site) itbecomes a target for inhibition by thiol reductants. Thus the twointerdependent regulatory systems of the LHCII kinase, the constantlyoccurring activation and deactivation on the one hand and the inhibitionby thiol reductants on the other, are strongly dependent on theconcentration of reducing equivalents in the chloroplast stroma.A scheme demonstrating the interconversion of activated, deactivated andinhibited states of the LHCII kinase in the chloroplast environmentof intact leaves is presented.