Role of Chloroplast Protein Kinase Stt7 in LHCII Phosphorylation and State Transition in Chlamydomonas

Abstract

Photosynthetic organisms adapt to changes in light quality by redistributing light excitation energy between two photosystems through state transition. This reorganization of antenna systems leads to an enhanced photosynthetic yield. Using a genetic approach in Chlamydomonas reinhardtii to dissect the signal transduction pathway of state transition, we identified a chloroplast thylakoid-associated serine-threonine protein kinase, Stt7, that has homologs in land plants. Stt7 is required for the phosphorylation of the major light-harvesting protein (LHCII) and for state transition.

Protein kinases and phosphatases involved in the acclimation of the photosynthetic apparatus to a changing light environment

“…Among all Cys residues, these are the only two that are conserved between Stt7 and STN7 [27]. Site-directed mutagenesis of either of these two residues abolished kinase activity, indicating that they are essential for state transitions although they are not part of the catalytic kinase domain [32].…”

“…Several mutants were isolated in this screen and are listed in table 1. One mutant of particular interest, stt7-7, was deficient in a protein kinase called Stt7 [27]. Besides being unable to perform state transitions, this mutant was deficient in LHCII phosphorylation under conditions favouring state 2 and blocked in state 1.…”

“…Together, they form a duo of kinases that is conserved in land plants where the orthologues of Stt7 and Stl1 are called STN7 and STN8, respectively [29]. Although these two kinases appear to have distinct substrates and functions with Stt7/STN7 required for LHCII phosphorylation and state transitions [27,29] and Stl1/STN8 for PSII core protein phosphorylation [30,31], there is some overlap between these two kinases as seen by the comparison of the protein phosphorylation patterns of stn7 and stn8 with that of the stn7-stn8 double mutant. While several proteins are still phosphorylated both in the stn7 or stn8 mutant, most of these phosphorylations are lost in the double mutant.…”

“…In this way, two allelic mutants were identified which were unable to dephosphorylate LHCII upon transfer from state 2 to state 1. The gene inactivated in these mutants encodes a PP2C phosphatase [27,[29][30][31]. STN7 is also involved in retrograde signalling (LTR) [30].…”

Molecular Architecture of Plant Thylakoids under Physiological and Light Stress Conditions: A Study of Lipid-Light-Harvesting Complex II Model Membranes

“…Other protein components that undergo reversible phosphorylation are D1, D2, CP43, and CP29 (Bennett, 1984;Bergantino et al, 1995). The LHCII complex is phosphorylated on the N-terminal Thr by STN7 kinase (Depège et al, 2003), which causes the complex to detach from photosystem II (PSII) in grana regions and its longrange diffusion toward the stroma lamellae. The protein phosphorylation therefore results in a dynamic, lateral reorganization of the thylakoid membranes and, consequently, in the redistribution of excitation energy between the regions enriched in the reaction centers of PSI and PSII (Fristedt et al, 2009;Tikkanen and Aro, 2012).…”