Inhibition of protein or glutamine biosynthesis affect the activation of the light stimulated SBiP1 chaperone by dephosphorylation in Symbiodiniaceae

Phosphorylation/dephosphorylation is fundamental for transduction of external stimuli into physiological responses. In photosynthetic dinoflagellates Symbiodinium microadriaticum CassKB8, Thr-phosphorylated SBiP1 under dark conditions, is activated through dephosphorylation upon light stimuli. We evaluated the effect of protein synthesis inhibitors on light modulated Thr phosphorylation of SBiP1. Inhibition of cytoplasmic protein synthesis by cycloheximide but not of chloroplastic protein synthesis by chloramphenicol, promoted inactivation via Thr re-phosphorylation of the protein under the light. Additionally, inhibition of glutamine synthetase by glufosinate produced a delay in the light induced activation by dephosphorylation of the chaperone. Heat shock reverted the effect in cycloheximide-treated cells suggesting that heat stress overrides the cycloheximide-induced inactivation to restore chaperone activity. These results suggest that light and stress are critical switches of SBiP1 chaperone activity that function along with common pathways of protein synthesis and ammonia assimilation, and further confirm that the light induced SBiP1 Thr dephosphorylation is independent of photosynthesis.