Abstract 1107 Chloroplast acetyltransferases

Acetylation is one of the most common chemical modifications affecting a variety of molecules ranging from metabolites to proteins. Recent development of enrichment techniques and mass spectrometry has revealed that acetylation is a prevalent modification also in plants, and that in addition to cytosolic and nuclear proteins also numerous chloroplast proteins are targets of N-terminal and/or lysine acetylation. We have recently characterized a chloroplasts-localized family of GNAT acetyltransferases in Arabidopsis thaliana. The GNAT enzymes are unique among the acetyltransferase enzymes as they possess dual protein acetylation activity, i.e. they catalyze acetylation of the N-terminal amino acid as well as acetylation of the integral lysine residues of the protein. Intriguingly, the GNAT enzymes are also involved in the acetylation of serotonin and thus in the biosynthesis of melatonin, a recently identified master regulator of plant growth and stress responses. Our results show that each GNAT enzyme has distinct specificity in terms of favored substrates, and that they play unique roles in the accumulation of specific plant metabolites, e.g. ascorbate and oxylipins. Depletion of GNAT2 has marked effects on the acetylation level of several chloroplast proteins, organization of thylakoid protein complexes and photosynthetic properties of plants. Specifically, formation of the Photosystem I-LHCII complex is prevented in the gnat2 knock-out plants, which results in impaired balancing of the light energy between the photosystems (state transitions). Moreover, loss of GNAT2 severely disturbs light-dependent dynamics of thylakoid stacking. Altogether, our results indicate that chloroplast acetyltransferases are new and important regulators of photosynthetic light harvesting with a marked impact on the growth and metabolism of plants. This work was conducted in the framework of EraCaps "KatNat" consortium and funded by the Academy of Finland.