Scientists solve the mystery of chloroplast “gatekeepers”

TOC-TIC transports proteins encoded by nuclear genes into chloroplasts Courtesy of Westlake University

Working model of the TOC-TIC super complex Courtesy of Westlake University

On November 21, Yan Zhen’s laboratory, a distinguished researcher at Westlake University, published a research paper in the journal “Cell”, uncovering the mystery of chloroplast protein transport – proteins entering chloroplasts need to pass through the TOC-TIC complex, as if passing through the “factory gate”, and they analyzed the complete and clear structure of the TOC-TIC complex for the first time, allowing people to see the appearance of the “gatekeeper” for the first time.

Scan the code to enter the station, scan the code to enter the company, scan the code to enter the school… For the past three years, scanning codes into public areas has been part of our daily lives. Like parallel universes, there are strikingly similar scenes on chloroplasts. As an important site for photosynthesis, chloroplasts are a “light energy factory” with 2,000 to 3,000 proteins that need to be identified and then enter the chloroplasts to “work.” So, what does this “gate” look like? And how does it work? The industry has been arguing for nearly four decades.

Previously, the scientific community had known that chloroplasts are double-membrane structures, with transport factors TIC on the inner membrane and transport factor TOC on the outer membrane, which combine to form a super complex TOC-TIC, which acts as the “gate” for proteins to enter chloroplasts. Although this area has been studied for nearly four decades, there are still many core problems that have not been solved. For example, what exactly is TIC made of? And how do TOC and TIC form super complexes to exercise the function of “gate”? Since the full picture of the TOC-TIC complex is not known, everyone’s research is like “blind people touching elephants”, each holding on to a word.

The membrane protein structure and function laboratory led by Yan Jun uses biochemical and structural biology methods to reveal the composition, assembly, and transport mechanism of TOC-TIC complexes. Through the analysis and induction of the previous research results, the laboratory selected the TOC-TIC super complex of Chlamydomonas Klein as the research object, and added affinity tags to two TOC (Toc34) and TIC (Tic20) components that have been identified and highly conserved in different species for purification.

It can be popularly understood that the laboratory has set up two sets of experiments, one group to pull out a complete elephant through the “elephant ears” that have been confirmed by previous people, and one to pull out the elephant through the “elephant tail”. That is, in a specific chloroplast sample, the TOC-TIC complex is precisely found by affinity tags and purified. The idea seems simple, but the experimental process needs to overcome a lot of difficulties, it is necessary to exclude various interference factors, and at the same time not to destroy the structure of the TOC-TIC complex. Interestingly, the protein components purified by these two different strategies were exactly the same, and the electron microscopy structures resolved were also highly consistent. Since then, multiple experiments have also verified that the TOC-TIC super complex has finally been found and accurately identified.

In the three-dimensional software of Yan Zhen’s laboratory, the three-dimensional structure of TOC-TIC is presented as the chloroplast “gate”. This TOC-TIC super complex on Chlamydomonas klein chloroplasts with a resolution of 2.5 ° clearly demonstrates the high-resolution structure and assembly patterns of each component of TOC-TIC. It contains a total of 14 components, of which 8 are previously reported components and 6 are new components whose function is unknown. Combined with the theory of predecessors, on the double-layer chloroplast membrane, this important channel is like a gatekeeper who checks the health code, allowing the protein to enter the store one by one with the transport signal peptide. For example, can the “gate” be made more efficient and expedite the release? Can the “gatekeeper” be allowed to let the “special workers” go? Can the “gatekeeper” be used as a template to create all kinds of “gatekeepers”?

A reviewer commented that the study solved a core problem in the chloroplast biology of photosynthetic species by purifying and analyzing the structure, and took a big step forward in improving the understanding and understanding of how algae and plant chloroplasts develop and evolve. Because of the potential impact of chloroplast biology on food safety and climate change, and the fundamental question of how proteins are transported, the study also brings valuable information to scientists in other non-specific fields. (Source: China Science News, Wen Caifei, Zhang Chi)

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