Crosstalk of protein phosphorylation and lysine acetylation on 14-3-3 proteins and their applications in brassinosteroid signaling and photosynthesis

Protein phosphorylation and lysine acetylation are two important post-translational modifications (PTMs) in eukaryotes. However, the crosstalk between these two modifications is largely unknown. We demonstrated that both phosphorylation and lysine acetylation occurred to an Arabidopsis protein, 14-3-3??. 14-3-3?? is a member of the 14-3-3 family of proteins that have been widely found in plants. 14-3-3s function mainly as phosphoserine/phosphothreonine-binding proteins that can affect the enzymatic activity, subcellular localization or protein turnover of their binding partners. Phosphorylation of 14-3-3 proteins is one of the mechanisms regulating the binding of 14-3-3s to their target proteins. In animals, it was shown that the phosphorylation of 14-3-3s forced the release of their binding partners and resulted in signaling cascades. In contrast, little is known about the roles of 14-3-3 phosphorylation in signal transduction in plants. Thus, in Chapter 2 we characterized the phosphorylation of 14-3-3?? by the brassinosteroid receptor kinase BRI1 and its co-receptor BAK1. We identified eight novel Ser/Thr residues that were phosphorylated by BRI1 and/or BAK1 in vitro. Four of these phosphorylation sites (Thr-41, Thr-83, Thr94 and Thr-140) were confirmed to be present in planta in Arabidopsis and phosphorylation was greater in the 14-3-3?? that was associated with microsomal membranes compared to the soluble phase. 14-3-3?? was co-immunoprecipitated with the BRI1-BAK1 complex in vivo, and BRI1-bound 14-3-3?? was phosphorylated at Thr-94 and Thr-140 sites. Moreover, the phosphorylation of both sites responded to brassinolide (BL) with increased phosphorylation, suggesting the phosphorylation of 14-3-3?? is related to brassinosteroid (BR) signaling in Arabidopsis. 14-3-3?? expressed in E. coli was acetylated on Lys-53, a conserved residue that is known to be important for binding of target proteins. We demonstrated that directed mutant of 14-3-3?? (K53E) was unable to interact with neither BRI1 nor BAK1 in vitro and the phosphorylation of 14-3-3?? (K53E) by either kinase was dramatically decreased. On the other hand, the binding of the wildtype 14-3-3?? was shown to enhance its phosphorylation by BRI1 and BAK1, and phosphorylation of wildtype 14-3-3?? by BRI1 appeared to also enhance its binding to BRI1. The distinct responses of the directed mutant K53E suggests that acetylation of Lys-53 could be a negative regulator for the phosphorylation of 14-3-3?? by BRI1 and BAK1 kinases. The acetylation of lysine was further studied in other proteins of Arabidopsis (Charter 3). We discovered that lysine acetylation is a widespread protein modification of diverse proteins in Arabidopsis. In particular, a group of photosynthesis-related proteins was newly identified to be acetylated on lysine residues in Arabidopsis, including light harvest proteins (LHCb1), Rubisco large and small subunits (RbCL and RbCS), subunits of PSII and PSI photoreaction centers, the ATP synthase beta subunit, and glyceraldehyde 3-phosphate dehydrogenase. We demonstrated that lysine acetylation of LHCb1 was correlated with its localization in relation to PSII photoreaction center; acetylation of RbCL may affect enzyme stability and the activity of Rubisco holoenzyme. Our data indicated that lysine acetylation is functionally important in photosynthesis. In summary, we identified new roles of 14-3-3 protein phosphorylation in BR signaling and provided the first systematic study of the lysine acetylome in Arabidopsis. Protein phosphorylation and lysine acetylation appeared to be two inter-related modifications for 14-3-3?? proteins