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

Project Information

TOC-maker

Grant agreement ID: 894893

DOI

10.3030/894893

Project closed

EC signature date 12 March 2020

Start date 1 April 2021

End date 29 June 2023

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 224 933,76

EU contribution

€ 224 933,76

Coordinated by

THE CHANCELLOR, MASTERS AND SCHOLARS OF THE UNIVERSITY OF OXFORD
United Kingdom

Periodic Reporting for period 1 - TOC-maker (The assembly and structure of the chloroplast protein import machinery in plants)

Reporting period: 2021-04-01 to 2023-03-31

Summary of the context and overall objectives of the project

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.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

2.1 Work performed. In carrying out this project, (i) I have successfully generated transgenic lines of the model plant Arabidopsis thaliana (in total, three lines) with an HA-tag inserted at multiple positions of the TOC channel component, Toc75, by agrobacterium-mediated transformation. Such HA-tags facilitate the purification of TOC complexes. (ii) As noted earlier, the TOC complex is fragile, and thus, it is important to design a purification protocol to protect the fragile nature of the protein complex. We have developed a methodology to rapidly purify the complex under mild solubilization conditions from chloroplast envelope membranes. This methodology includes a short solubilization time and a mild detergent. (iii) To reveal the biogenesis and assembly of the TOC complex, it is necessary to understand a specific stage where this process operates at its maximum. We have found that early plant greening is the suitable stage for this investigation. (iv) To detect the newly synthesized components in the assembly of TOC, I have labelled young plant seedlings with 35S radio-isotopes. Then, pull-down experiments were conducted to isolate and study radio-labelled (newly formed) TOC complexes from young plant seedlings. (v) To reveal the structure of TOC complex, I have successfully purified the intact and pure TOC complex, and this complex is under investigation by cryo-electron microscope.

2.2 Results achieved so far: This project provided novel insights and mechanistic details concerning the biogenesis and assembly of the TOC complex. We revealed that the central core component of the TOC complex is the master player in the biogenesis of the TOC complex, controlling the assembly and stability of all the peripheral receptor proteins; it forms a photosynthetic-type TOC configuration (TOC-P) in green chloroplasts, and a non-photosynthetic-type TOC configuration (TOC-NP) that is abundant in non-green plastids. Functionally, the preference for different substrates for protein import is the key difference between TOC-P and TOC-NP. We proposed the step-wise assembly of the TOC complex in chloroplasts, in which the central core component is the initial component in the assembly of the TOC complex, followed by its stabilization with a small peripheral subunit. In the final step, a large peripheral protein is then integrated to form a fully mature TOC complex. Our structural investigation of the plant TOC complex by cryo-electron microscopy is extended beyond the timeline of this fellowship; however, I have developed an advanced pipeline.

2.3 Exploitation and dissemination. The results of this project were thoroughly discussed within the group and departmental seminars organized monthly or annually at the Department of Biology, Oxford. Prof Paul Jarvis (PI) and I have presented the results of this project at national (Plastid Preview – 2023, Oxford) and international (EMBO meeting on Protein translocation – 2022, Spain) conferences. A further major route of dissemination is via publication; I have already published a review article in J. Cell Sci. (2023), and I am preparing a manuscript to submit to Nature Commun. or PNAS. I will update my discoveries on the lab’s webpages and/or social media, which are well viewed; and I will communicate the work to the general public by interacting with the mainstream media when opportunities arise (e.g. via the University’s News and Information Office). All research publications arising from the project will be deposited in the Oxford Research Archive, which is freely accessible. With regard to exploitation, I will take care to protect (e.g. by patent applications) new intellectual property coming from the project using procedures already established in the host laboratory. Because of the vital role that chloroplasts (and by extension chloroplast protein import) play in plant growth, productivity, and stress tolerance, the new information my work generates may enable novel crop improvement strategies.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

This project generated new knowledge on principles of assembly of protein complexes and chloroplast biogenesis. Thus, the work has relevance extending far beyond plants to essentially all areas of cell biology including the assembly of many understudied but key protein complexes.

Nellaepalli Sreedhar