Chloroplasts are essential organelles, which harbor the photosynthetic protein complexes in their internal membrane systems, the thylakoids. They develop in meristematic tissues from proplastids, which harbor an outer and an inner envelope membrane, but rarely any internal membrane structures and are therefore photosynthetically inactive. Therefore, not only the photosynthetic complexes must be assembled during the transition from proplastids to chloroplasts, but also the thylakoid membrane must be formed. To date, only little is known about the protein factors that drive these processes during the early stages of chloroplast biogenesis. Using biochemical as well as genetics approaches, we have identified a number of potential candidates, which we are currently investigating.
Although a number of proteins of the thylakoid membrane complexes are encoded on the chloroplast genome, a number of essential gees were transferred to the nuclear genome during evolution. This holds also true for a number of assembly factors, which are not only essential for the initial biogenesis of the complexes, but also for their maintenance under changing environmental conditions. Since nuclear encoded proteins are synthesized in the cytosol, an efficient targeting system towards the organelles need to be ensured. This a highly dynamic and tightly regulated process, which is essential for the integration of all organelles and cellular compartments into the cellular network. Molecular crowding can easily lead to misfolding and aggregation of precursor proteins, which in turn inhibits the import into the respective organelles. We are therefore investigating the role of cytosolic chaperones, as well as post-translational modifications during targeting and transport of precursor proteins into the organelles. Moreover, we are investigating the function of regulatory factors, which influence protein import under changing environmental conditions and thereby contribute to optimization of the photosynthetic performance.