Analysis of the Chloroplast Protein Kinase Stt7 during State Transitions

Abstract

State transitions allow for the balancing of the light excitation energy between photosystem I and photosystem II and for optimal photosynthetic activity when photosynthetic organisms are subjected to changing light conditions. This process is regulated by the redox state of the plastoquinone pool through the Stt7/STN7 protein kinase required for phosphorylation of the light-harvesting complex LHCII and for the reversible displacement of the mobile LHCII between the photosystems. We show that Stt7 is associated with photosynthetic complexes including LHCII, photosystem I, and the cytochrome b 6 f complex. Our data reveal that Stt7 acts in catalytic amounts. We also provide evidence that Stt7 contains a transmembrane region that separates its catalytic kinase domain on the stromal side from its N-terminal end in the thylakoid lumen with two conserved Cys that are critical for its activity and state transitions. On the basis of these data, we propose that the activity of Stt7 is regulated through its transmembrane domain and that a disulfide bond between the two lumen Cys is essential for its activity. The high-light–induced reduction of this bond may occur through a transthylakoid thiol–reducing pathway driven by the ferredoxin-thioredoxin system which is also required for cytochrome b 6 f assembly and heme biogenesis.

Activation of the Stt7/STN7 Kinase through Dynamic Interactions with the Cytochrome b6f Complex

“…Excess excitation of PSII relative to PSI leads to reduction of the PQ pool and thus favors the docking of PQH 2 to the Qo site of the Cyt b 6 f complex. This process activates the Stt7/STN7 protein kinase (Vener et al, 1997;Zito et al, 1999), which is closely associated with this complex and leads to the phosphorylation of some LHCII proteins and to their detachment from PSII and binding to PSI (Depège et al, 2003;Lemeille et al, 2009). Although both Lhcb1 and Lhcb2 are phosphorylated, only the phosphorylated form of Lhcb2 is associated with PSI whereas phosphorylated Lhcb1 is excluded from this complex (Longoni et al, 2015).…”

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

“…In fact, this kinase could never be detected in free form upon mild solubilization of thylakoid membranes. Immunoprecipitations revealed that this kinase is also associated with LHCII and PSI but no stable association with PSII could be detected [32]. An intriguing structural feature of Stt7/STN7 is the Model of Stt7/STN7 kinase in the thylakoid membrane.…”

“…The Stt7/STN7 kinase contains a transmembrane domain that separates its N-terminal part in the lumen from the catalytic kinase domain on the stromal side of the thylakoid membrane (figure 2) [32]. This kinase is firmly associated with the cyt b 6 f complex.…”

“…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]. The LHCII kinase has been shown to be inactivated under high light through the ferredoxin-thioredoxin system (figure 2) [7].…”

Short-term acclimation of the photosynthetic electron transfer chain to changing light: a mathematical model

“…Whereas land plants mobilize only 15-20% of the LHCII pool, state transitions in the alga Chlamydomonas reinhardtii have been reported to involve up to 80% of the LHCII antennae [35]. Some regulatory features of the LHCII kinase (Stt7/STN7) are shared between algae and land plants, such as its decreased activity under high light stress [36][37][38] and its activation by the binding of plastoquinol to Cytb6f [39][40][41]. It is currently unknown whether the LHCII phosphatase PPH1/TAP38 [42,43] is regulated, and specific orthologues have not been characterized outside the land plant lineage.…”