Resolving the structure and function of the plant chloroplast protein import machinery
Lead Research Organisation:
University of Oxford
Department Name: Biology
Abstract
Context
Rapid human population growth (set to reach 9.8 billion by 2050) and anthropogenic climate change together place ever increasing pressure on food security and natural resources. Thus, a key challenge of our era is to deliver increased agricultural yields with resilience to stress and disease. To meet this challenge, we must develop improved crop varieties, by delivering and then exploiting advances in our understanding in key areas of plant biology.
Chloroplasts
Chloroplasts are tiny, subcellular structures ("organelles") that define plants. They contain the green pigment chlorophyll that absorbs light in photosynthesis - the process whereby sunlight energy is used to power cellular activities and plant growth. Because photosynthesis is the only significant mechanism of energy-input into the living world, chloroplasts are vitally importance, not only to plants but to all life on Earth - including our staple crops. Owing to their role in photosynthesis, and in plant responses to environmental challenges (or stresses) such as drought and salinization, chloroplasts are prime targets for crop improvement strategies.
Chloroplasts are built from thousands of different proteins. Most of these proteins are encoded by genes in the cell nucleus, and so are made outside of the organelle in the cellular matrix called the cytosol. Since chloroplasts are surrounded by a double-membrane "envelope", sophisticated machinery is needed to import these proteins into the organelle. This machinery has two parts, called TOC ("Translocon at the Outer envelope membrane of Chloroplasts") and TIC. Each part is composed of several subunits that cooperate to drive protein import.
The project
Our research group has a long track-record of studying chloroplast protein import mechanisms. In recent years, we uncovered how activity of the TOC translocon is regulated by a novel mechanism called CHLORAD (chloroplast-associated protein degradation)1; and we showed how such regulation is vital for diverse aspects of plant growth, including resilience to adverse environments2 and fruit ripening3. However, while the regulation of protein import is well studied, our understanding of the molecular structure of the translocons is rudimentary. Moreover, a well-supported model for the protein import mechanism (i.e., how the machinery imports proteins) is lacking. This project will directly address these issues.
As already noted, the translocons are assemblies of several protein subunits - i.e., they are multiprotein complexes. We aim to elucidate how the individual subunits are arranged (with atomic resolution) within the TOC machine, and how TOC assembles with TIC. Having such a detailed understanding of the translocons is extremely important, because this is the only way to finally resolve the chloroplast protein import mechanism. The main challenges in this project are that the translocons exist only in very small quantities in plants, and are rather unstable, making it difficult to purify them for analysis. Fortunately, we have now substantially overcome these hurdles as our preliminary results show. Thus, we are in an exciting and unprecedented position to deliver a step-change in our understanding of translocon function, and of chloroplast protein import and plant biology in general.
This new information on chloroplast development and functions may offer applications in the future development of crops with improved photosynthetic performance, stress resilience, and yields. Thus, the project has strong relevance to BBSRC's long-term research and innovation priorities in the area of plant health and agri-food.
Rapid human population growth (set to reach 9.8 billion by 2050) and anthropogenic climate change together place ever increasing pressure on food security and natural resources. Thus, a key challenge of our era is to deliver increased agricultural yields with resilience to stress and disease. To meet this challenge, we must develop improved crop varieties, by delivering and then exploiting advances in our understanding in key areas of plant biology.
Chloroplasts
Chloroplasts are tiny, subcellular structures ("organelles") that define plants. They contain the green pigment chlorophyll that absorbs light in photosynthesis - the process whereby sunlight energy is used to power cellular activities and plant growth. Because photosynthesis is the only significant mechanism of energy-input into the living world, chloroplasts are vitally importance, not only to plants but to all life on Earth - including our staple crops. Owing to their role in photosynthesis, and in plant responses to environmental challenges (or stresses) such as drought and salinization, chloroplasts are prime targets for crop improvement strategies.
Chloroplasts are built from thousands of different proteins. Most of these proteins are encoded by genes in the cell nucleus, and so are made outside of the organelle in the cellular matrix called the cytosol. Since chloroplasts are surrounded by a double-membrane "envelope", sophisticated machinery is needed to import these proteins into the organelle. This machinery has two parts, called TOC ("Translocon at the Outer envelope membrane of Chloroplasts") and TIC. Each part is composed of several subunits that cooperate to drive protein import.
The project
Our research group has a long track-record of studying chloroplast protein import mechanisms. In recent years, we uncovered how activity of the TOC translocon is regulated by a novel mechanism called CHLORAD (chloroplast-associated protein degradation)1; and we showed how such regulation is vital for diverse aspects of plant growth, including resilience to adverse environments2 and fruit ripening3. However, while the regulation of protein import is well studied, our understanding of the molecular structure of the translocons is rudimentary. Moreover, a well-supported model for the protein import mechanism (i.e., how the machinery imports proteins) is lacking. This project will directly address these issues.
As already noted, the translocons are assemblies of several protein subunits - i.e., they are multiprotein complexes. We aim to elucidate how the individual subunits are arranged (with atomic resolution) within the TOC machine, and how TOC assembles with TIC. Having such a detailed understanding of the translocons is extremely important, because this is the only way to finally resolve the chloroplast protein import mechanism. The main challenges in this project are that the translocons exist only in very small quantities in plants, and are rather unstable, making it difficult to purify them for analysis. Fortunately, we have now substantially overcome these hurdles as our preliminary results show. Thus, we are in an exciting and unprecedented position to deliver a step-change in our understanding of translocon function, and of chloroplast protein import and plant biology in general.
This new information on chloroplast development and functions may offer applications in the future development of crops with improved photosynthetic performance, stress resilience, and yields. Thus, the project has strong relevance to BBSRC's long-term research and innovation priorities in the area of plant health and agri-food.
