"Science" Study: Mechanism for the Breakdown of Chloroplast Proteins Clarified

Chloroplasts are the site of photosynthesis and belong to the plastids, of which there are many types with different functions. Plastids of one type can differentiate into other types. With Kaiserslautern biologist Dr. Raphael Trösch, researchers from Oxford have for the first time shown how membrane-bound receptors responsible for the uptake of photosynthesis-relevant proteins are broken down in chloroplasts. The amount of receptors could play a role in plastid differentiation. The results are particularly interesting for biotechnological production of pigments in plastids. The study has been published in the journal Science.

Plastids are small cellular organelles that possess their own genome and are distinguished from the rest of the cell by a surrounding membrane. In plants, they perform various tasks: chloroplasts carry out photosynthesis, and amyloplasts store starch. The special thing is: plastids can differentiate. They have a high degree of plastidicity. Hence their name. "A tomato is initially green to carry out photosynthesis," explains biologist Dr. Raphael Trösch from the Technical University of Kaiserslautern (TUK). "Over time, it turns red and accumulates the pigment lycopene. The chloroplasts transform into chromoplasts and take on a new function."

For chloroplasts to carry out photosynthesis, they need many different proteins. Most are not produced within the organelle itself but in the cytoplasm. "To transport them into the chloroplasts, there are transport proteins in the chloroplast membrane," explains Trösch, who researches in the field of eukaryotic genetics. "These bind to receptors that either specifically recognize photosynthesis proteins or recognize other proteins."

Together with the team around the two first authors of the study, Qihua Ling and William Broad from the University of Oxford, Trösch examined how the degradation process of the receptors that recognize the photosynthesis proteins looks like in cells of the Arabidopsis thaliana. "In other cellular membranes, there are already well-studied mechanisms for the degradation of membrane proteins," the researcher continues.

For the first time, the scientists have now also studied a similar mechanism in detail in chloroplasts and identified the three molecules involved. The degradation process works as follows: "To remove the receptor, it must first be marked. Subsequently, specific proteins, using energy, pull the marked receptor through a channel, after which it can be degraded in the cytoplasm. These proteins are, in a way, part of the cell’s garbage disposal system," explains Trösch. He compares the cellular degradation system to forestry work in the forest, where a forester first marks sick or damaged trees with an "X". Then, the forest workers fell the marked trees without knowing why they need to be removed.

The researchers named the mechanism "Chloroplast-associated protein degradation" (CHLORAD). With it, a plant cell can regulate the amount of specific receptors, and thus possibly how many photosynthesis proteins a chloroplast takes up. The scientists also suspect that it could play a key role in differentiation. However, its exact function would need to be clarified in further studies.

The results are particularly interesting for biotechnology. Plant pigments from chloroplasts are used in cosmetics, as sunscreens, food, or pharmaceuticals. The findings help to better understand the fundamentals of plastid differentiation. Natural pigments could potentially be produced in larger quantities and more easily in differentiated plastids in the future.

The study has been published in the renowned journal Science: "Ubiquitin-dependent chloroplast-associated protein degradation in plants." Qihua Ling, William Broad, Raphael Trösch, Mats Töpel, Tijen Demiral Sert, Panagiotis Lymperopoulos, Amy Baldwin, R. Paul Jarvis.

http://science.sciencemag.org/content/363/6429/eaav4467.long

DOI: 10.1126/science.aav4467