Reversible modifications of chloroplast proteins and assessment of their functions

Abstract: Oxygenic photosynthesis is the process of solar energy conversion into chemical energy in the form of carbohydrates. This event is carried out by plants, algae and cyanobacteria and represents the starting point of the food chain in which most organisms are fed. Due to never-ending changes in the surrounding environment, these photoautotrophic organisms have evolved different acclimatizing strategies to optimize photosynthesis. Many of these fine-tuning mechanisms are dependent on reversible modifications of proteins on a post-translational level. In my research I have been focused on such reversible modifications of proteins in the organelle where photosynthesis takes place – the chloroplast – using the model plant Arabidopsis thaliana.Within chloroplasts, light-driven reactions of photosynthesis are catalyzed by several multi-subunit protein complexes in the thylakoid membrane. Proteins need to be folded properly in order to function correctly. A rate-limiting step of protein folding is the isomerization of the peptide bond around proline, a step that is catalyzed by enzymes possessing peptidyl-prolyl cis-trans isomerase (PPIase) activity. Within the thylakoid lumen, only two proteins have been found to possess PPIase activity, FKBP13 and CYP20-2. Both these enzymes belong to a protein superfamily called immunophilins - ubiquitous proteins attributed with several different functions. By characterization of Arabidopsis mutants lacking FKBP13 and CYP20-2 I found that PPIase activity is a dispensable function of immunophilins in the thylakoid lumen.A common post-translational modification of chloroplast proteins is phosphorylation. Protein phosphorylation alters protein functions and is a reversible mechanism utilized by plants for rapid acclimation to changes in the incident light. These events require the action of kinases and phosphatases that either add or remove phosphate groups on proteins, respectively. I have characterized mutants deficient in protein phosphatases responsible for dephosphorylation of thylakoid proteins. These phosphatases, PPH1 and PBCP, represent key players in acclimation of the photosynthetic machinery to changes in light quality/quantity. In addition, I discovered that phosphorylation of pTAC16, a protein associated with the chloroplast gene-expression machinery, depends on the presence of STN7; a light-regulated protein kinase located in the thylakoid membrane. This finding could provide a link between the redox state of the photosynthetic apparatus and chloroplast gene expression.