Regulation of thylakoid protein phosphorylation at the substrate level: Reversible light-induced conformational changes expose the phosphorylation site of the light-harvesting complex II

Light-dependent activation of thylakoid protein phosphorylation regulates t he energy distribution between photosystems I and II of oxygen-evolving pho tosynthetic eukaryotes as well as the turnover of photosystem II proteins. So far the only known effect of light on the phosphorylation process is the redox-dependent regulation of the membrane-bound protein kinase(s) activit y via plastoquinol bound to the cytochrome bf complex and the redox state o f thylakoid dithiols. By using a partially purified thylakoid protein kinas e and isolated native chlorophyll (chl) a/b light-harvesting complex II (LH CII), as well as recombinant LHCII, we find that illumination of the chi-pr otein substrate exposes the phosphorylation site to the kinase. Light does not activate the phosphorylation of the LHCII apoprotein nor the recombinan t pigment-reconstituted complex lacking the N-terminal domain that contains the phosphothreonine site. The suggested light-induced conformational chan ge exposing the N-terminal domain of LHCII to the kinase is evidenced also by an increase in its accessibility to tryptic cleavage after light exposur e. Light activates preferentially the trimeric form of LHCII, and the proce ss is paralleled by chi fluorescence quenching. Both phenomena are slowly r eversible in darkness. Light-induced exposure of the LHCII N-terminal domai n to the endogenous protein kinase(s) and tryptic cleavage occurs also in t hylakoid membranes. These results demonstrate that light may regulate thyla koid protein phosphorylation not only via the signal transduction chain con necting redox reactions to the protein kinase activation, but also by affec ting the conformation of the chl-protein substrate.