TOC-maker · The assembly and structure of the chloroplast protein import machinery in plants

The assembly and structure of the chloroplast protein import machinery in plants

1.1 What is the problem/issue being addressed? Plants capture solar energy and carbon dioxide to produce oxygen and chemical energy in the form of sugars by a mechanism called photosynthesis, which provides fuel for life on planet earth. Photosynthesis is driven by tiny compartments within plant and algal cells called chloroplasts, which have the status of partial independence owing to their endosymbiotic origins as photosynthetic bacteria. Chloroplasts are majorly controlled by the cell’s nucleus. Chloroplasts are protein-rich organelles, containing ~3000 different proteins which are busily engaged in conducting photosynthesis and other reactions. Such photosynthetic proteins are synthesized outside of the chloroplast by the cell’s protein manufacturing apparatus. To import these proteins from outside to the inside of the chloroplasts, the outer and inner membranes of chloroplasts are equipped with multi-protein nanomachines (complexes) called TOC and TIC, respectively. TOC is composed of a central channel (core) subunit, and two peripheral proteins (receptors) that are exposed to the outside of the chloroplast and act to trap and direct chloroplast proteins toward the channel component. Although the composition of TOC is revealed, the biogenesis, assembly, and structure of the plant TOC remains elusive. I addressed these fundamental questions in plant cell biology. The major challenges in this investigation included the rapid nature of the assembly process and the fragile nature of TOC. I overcame these challenges by generating plant TOC transgenic lines with tags (tags are utilized for capturing proteins of interest) and optimising protocols to rapidly pull-down fragile TOC complexes during chloroplast biogenesis, when photosynthesis is actively established. These biological materials and technical optimizations supported thorough biochemical and structural investigation. 1.2 Why is it important for society? The human population is growing rapidly and is predicted to reach approximately 10 billion by 2050 due to advancements in medical and economic developments as well as inadequate control measures on population growth. In addition, climate change due to anthropogenic activities exacerbates pressure on food security and natural resources. It is clear that a major challenging task of our era is to deliver increased agricultural production with resilience to environmental stresses and disease. To meet this challenge, we must develop improved crops, by delivering and then exploiting advances in our understanding of key areas in plant sciences. 1.3 What are the overall objectives? My research aim in this project was to elucidate the biogenesis, assembly, and structure of TOC complexes in plant chloroplasts, in fine molecular detail. My specific objectives were as follows: (i) Identification of assembly factors for, and elucidation of the assembly process of, TOC complexes; (ii) Structural determination of the TOC-P complex at high resolution by cryo-electron microscopy. In addition, as an MSCA-IF project, another aim was to foster my development as an independent researcher.

Data: CORDIS, © European Union