Serine and threonine residues of plant STN 7 kinase are differentially phosphorylated upon changing light conditions and specifically influence the activity and stability of the kinase

Summary STN 7 kinase catalyzes the phosphorylation of the globally most common membrane proteins, the light‐harvesting complex II ( LHCII ) in plant chloroplasts. STN 7 itself possesses one serine (Ser) and two threonine (Thr) phosphosites. We show that phosphorylation of the Thr residues protects STN 7 against degradation in darkness, low light and red light, whereas increasing light intensity and far red illumination decrease phosphorylation and induce STN 7 degradation. Ser phosphorylation, in turn, occurs under red and low intensity white light, coinciding with the client protein ( LHCII ) phosphorylation. Through analysis of the counteracting LHCII phosphatase mutant tap38 / pph1 , we show that Ser phosphorylation and activation of the STN 7 kinase for subsequent LHCII phosphorylation are heavily affected by pre‐illumination conditions. Transitions between the three activity states of the STN 7 kinase (deactivated in darkness and far red light, activated in low and red light, inhibited in high light) are shown to modulate the phosphorylation of the STN 7 Ser and Thr residues independently of each other. Such dynamic regulation of STN 7 kinase phosphorylation is crucial for plant growth and environmental acclimation.