Characterization of the chloroplast protein kinase Stt7 involved in state transitions

Sylvain Lemeille, Adrian Willig, Nathalie Depège*, Christian Delessert and Jean-David Rochaix

Departments of Molecular Biology and Plant Biology, University of Geneva, 30, Quai Ernest Ansermet 1211 Geneva * Present address: Reproduction et Développement des Plantes- Institut Fédératif de Recherche 128, Allée d'Italie, 69364 Lyon Cedex 07, France

The chloroplast protein kinase Stt7/STN7 is required for LHCII phosphorylation and state transitions in Chlamydomonas reinhardtii and Arabidopsis thaliana. This light acclimation process involves a reversible displacement of the mobile antenna from photosystem II to photosystem I. It is mediated by the Stt7/STN7 - dependent phosphorylation of LHCII and leads to a rebalancing of the light excitation energy between photosystem II and photosystem I. Here we have examined the interacting partners of this kinase. Fractionation of solubilized thylakoid membranes of C. reinhardtii by sucrose density gradient centrifugation and immunoblotting reveals that Stt7 is associated with a large molecular weight complex. It accumulates to higher levels under state 2 conditions where it is active than under state 1 conditions where it is inactive, a process which appears to be regulated at the post-translational level. Co-immunoprecipitations, pull-down and co-fractionation experiments indicate that the Stt7 protein kinase is associated with the cytochrome b6f complex and that it interacts directly with the Rieske protein. Stt7 also interacts with the light-harvesting complex and photosystem I, but not with photosystem II suggesting that it may either escort the mobile LHCII to PSI or be part of a cytochrome b6f - photosystem I supercomplex.

e-mail address of presenting author: Sylvain.Lemeille@molbio.unige.ch