The Enigmatic Chloroplast Stt7 Kinase: Trans-Membrane Function With Cytochrome B6f Complex In Situ; Kinase Activity In Vitro

The distribution of light energy between the two photosystems in oxygenic photosynthesis is regulated in the alga, C. reinhardtii, by a 754 residue State Transition Kinase, Stt7, which is unique in using a trans-membrane topology to carry out its signaling function (1). It is activated by oxidation of plastoquinol on the electrochemically positive, lumen side of the cytochrome b6f complex (2), and phosphorylates the major light-harvesting chlorophyll protein II (3) on the opposite side of the membrane (1). Stt7, which binds to cyt b6f (1), requires a disulfide bond on the p-side proximal to the quinol oxidation site for its activation (1, 3). In the present study, Stt7 has been cloned in E. coli and purified as a soluble protein whose monomer mass, determined by mass spectrometry is 79,501. On Clear Native PAGE, Stt7 runs close to a position expected for a heptamer. Purified Stt7 has significant redox-dependent kinase activity in vitro. The demonstration of in vitro activity, the absence of a documented membrane bound state, and the small stoichiometry (circa 1:20) of interaction with the b6f complex (1), suggest that reaction of the kinase with the LHCII occurs through a membrane-peripheral domain of the b6f complex. (1) Lemeille, S. et al. (2009) PLoS Biology 7, e1000045; (2) Vener, A. V. et al. (1997) PNAS, 94, 1585-1590. (3) Millner, P. et al., J. Biol. Chem., 257, 1736-1742, 1982. Support from NIHGMS-038323.