Challenging the role of plastid cotranslational N-terminal modifications upon stress response

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Chloroplasts primarily function as bioreactors converting light into chemical energy. In addition, they also act as regulatory hub for intracellular communication and mediation of environmental impacts to regulate nuclear gene expression. Chloroplast proteins are encoded either by the plastid or nuclear genome. Several chloroplast multi-protein complexes, such as RuBisCO (D-ribulose 1,5-bisphosphate carboxylase/oxygenase), which is the major enzyme of the Calvin-Benson cycle and the most abundant protein on earth, are assembled from plastid- as well as nuclear-encoded protein subunits. All nuclear and plastid-encoded chloroplast-localized proteins undergo many co- and post-translational modifications (CTMs and PTMs), which have important roles in controlling stability, accumulation, activity, assembly, and compartmentalisation of these proteins. However, CTMs and PTMs of plastid proteins and their catalytic modifiers have not intensively been explored up to date and the number of modifications continuously increases. Over the last years, the collaborative work among the three teams (Gif, Münster and Berlin) have brought multiple preliminary results highlighting unique properties of plastid N-terminal modifiers together with specific and essential modulated functions of the catalysed NPMs, which now build the research hypothesis and objectives of the CANMORE project. The overarching aim of CANMORE is to elucidate the role(s) of specific plastid N-terminal modifications (NPMs) and their regulatory interdependency with other PTMs. Via three interconnecting objectives, we will perform a structural and mechanistic characterization of the plastid modifiers involved in NPMs and related PTMs. Strong emphasis will be given to the unconventional and enigmatic N-terminal maturation of the RuBisCO large subunit. Hence, a detailed understanding of the protein modifications occurring on RuBisCO, especially under adverse environmental conditions, will be necessary for our understanding and possible future improvement of RuBisCO activity and photosynthesis in general. In this context, we will provide a thorough investigation of the plastid N-terminome and acetylome in response to temperature stress, which is particularly important for RuBisCO activity. Collectively, the CANMORE project will not only provide a comprehensive analysis of the plastid NPM machinery, but will also significantly enhance our understanding of the consequences of NPMs on specific plastid activities and during changing environmental conditions. For this project, a synergy of complementary approaches involving proteomics, biochemistry, structural biology, genetics and cell biology will be proposed. The CANMORE project will bring together three highly complementary groups [Carmela Giglione, Team1 (France); Bernhard Grimm, Team2 (Germany); and Iris Finkemeier, Team3, (Germany)] with interests and world lead in both CTMs/PTMs and chloroplast physiology.