The secret life of chloroplast precursor proteins in the cytosol: Modifications, turnover and their role in the assembly of the photosynthetic machinery

• Applicant: Professor Dr. Sacha Baginsky
• Subject Area: Plant Biochemistry and Biophysics ; Plant Cell and Developmental Biology
• Term: since 2012
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 220285883

Chloroplast protein import is essential for thylakoid membrane biogenesis. Several thousand nuclear-encoded proteins are transported into chloroplasts as precursor proteins, thus the bulk of the chloroplast proteome is shuttled through the cytosol. During this transition, chloroplast precursor proteins are modified, degraded and in case they accumulate initiate a signal to adjust nuclear gene expression for photosynthetic proteins. With this project, we will scrutinize the contribution of the cytosolic precursor transition on the assembly of the chloroplast proteome. We have shown that mild genetic impairment of the proteasome results in improved photosynthetic performance in import-impaired and also in wildtype plastids. We have interpreted this finding as mild proteasome impairment affecting the turnover of precursor proteins in the cytosol leading to elevated protein import from a larger cytosolic pool. The data suggest that under wildtype conditions, synthesis of the photosynthetic apparatus is constrained by proteasomal activity. We propose to investigate the mechanistic and quantitative details of this previously unrecognized cellular process. The project can be divided into two different parts that address the following questions: i. Is the effect of proteasome impairment on photosynthesis in wildtype and in plastid import-deficient plants generalizable and what is the exact molecular contribution of the different proteasome components and ii. What is the impact of precursor modification and turnover in the cytosol on thylakoid membrane biogenesis and what are the factors involved in its regulation? Together both parts will feed into the overall understanding of proteasomal control of chloroplast proteome assembly and thylakoid membrane biogenesis that occur independent of transcriptional regulation. Both parts converge when attempts to modify precursor stability will be made and their suitability for biotechnological application be assessed.

• DFG Programme: Research Grants