Photosynthesis: How this vital process is maintained by the UPS system

Paulmann, CC BY-SA 3.0, via Wikimedia Commons

Researchers at the University of Oxford, in collaboration with the Ling Lab at the Chinese Academy of Sciences Center for Excellence in Molecular Plant Science, have identified a new role for the ubiquitin-proteasome system in the maintenance of photosynthetic machinery in plants.

This work was led by the Jarvis group in the Department of Biology at Oxford and identifies a new target for engineering crops with higher yields.

At the bottom of the food chain, the carbohydrates made in photosynthesis—generated from carbon dioxide using energy from sunlight—fuel all life on earth. For this system to remain efficient, the range of proteins that make up the photosynthetic machinery must be kept in good condition. This relies on the regular removal of any defective or unwanted proteins.

Previously, it was thought this upkeep was entirely carried out by internal proteases (enzymes that degrade proteins) in the chloroplasts, but new evidence from this study indicates an unexpected role for the ubiquitin-proteasome system (UPS).

Ubiquitin, a small protein itself, can be added to existing proteins in the cell by enzymes to create a unique signal that is recognised by other proteins in the cell.

In the UPS, ubiquitin acts as a ‘death signal’ when added to proteins, targeting them for destruction by the proteasome.

Scientists had not previously considered a role for ubiquitin-mediated proteolysis in the maintenance of photosynthesis as the double membrane surrounding chloroplasts acts as a barrier between the CHLORAD (chloroplast-associated protein degradation system—the UPS specific to chloroplasts in plants) and photosynthetic proteins located inside.

A recent paper co-authored by the two research groups provides the first concrete evidence for the ubiquitin-mediated destruction of proteins localised inside the chloroplast and could aid efforts to improve crop productivity in the future.

The researchers used a combination of immunoblot analysis, ubiquitinomics, and proteomics to identify whether proteins in purified chloroplast mixtures were modified by ubiquitin and subsequently degraded by the proteasome. Using purified chloroplasts eliminated the possibility of contamination by modified cytosolic proteins, an error identified in earlier studies that had come to the similar conclusion of the UPS acting on proteins from the chloroplast.

These methods identified a large number of substrates for the CHLORAD system, with a particular emphasis on proteins with a role in photosynthesis. This indicates its importance for sustaining the health of the whole chloroplast proteome.

A growing global population in combination with threats such as global warming and reduced land for agriculture increases the importance of optimising crop yields.

Conclusive evidence that the ubiquitin-proteasome system contributes to the upkeep of photosynthetic machinery provides a novel area of research for improving efficiency.

Further investigations on the role of the UPS system is needed. For example, it is still unknown how much internal proteases and the CHLORAD system each contribute to overall chloroplast protein degradation. The researchers speculate in this study that the CHLORAD system—situated outside of the chloroplast—may act on specific stable substrates in response to changing conditions, whereas regular cycling of proteins is carried out by the more readily available proteases.

Further characterisation of this process and understanding its exact significance in chloroplast function will be key to implementing this new discovery agriculturally.