Almost all human food comes from photosynthesis carried out by chloroplasts, and the chloroplast protein-import system is the foundation enabling photosynthesis to occur.

Based on an innovative approach, a team of researchers led by Dr. Hsou-min Li at Academia Sinica’s Institute of Molecular Biology has found the bridge linking, and allowing proteins to cross, the two surrounding membranes of chloroplasts. They have named the bridge TIC236. Their study sheds new light on how chloroplasts work and how plants evolve. The team’s results were published in the renowned journal Nature, and highlighted by a News and Views article recommending their findings.

Dr. Li said that if we view a plant cell as a city, chloroplasts are the farms providing food for the city, with photosynthetic proteins being the farmhands recruited from outside the farms. The two membrane layers surrounding chloroplasts are like the walls surrounding the farms, and there is even a moat between the walls. This study found that TIC236 serves as a bridge between the walls, allowing proteins to cross the walls and moat to go into chloroplasts to “work”. Without TIC236, Li said, chloroplasts would “lack labor”, which would cause a plant to perish in its nascent stage as an embryo.

Apart from discovering how chloroplasts work, the team’s study also found that TIC236 has evolved from a cyanobacterial ancestor, which is a significant discovery of the chloroplast evolutionary process. In the evolutionary history of life, an unknown ancient cyanobacterium was engulfed by another unicellular organism, evolved into chloroplasts and created all the plants on Earth. Identification of TIC236 will facilitate identifying what kind of cyanobacterium was the true ancestor of the chloroplast.

In addition, this research project discovered how organelles use different ways to import proteins. A plant’s cells are full of organelles importing proteins in different ways, one instance being mitochondria making a path for proteins by shortening the distance between the two surrounding membranes at a few places. The team’s research found that chloroplasts import proteins by building a long bridge between the two membranes. This difference in the ways of importing proteins suggests that the chloroplast and mitochondrion have different bacterial ancestors.

Dr. Li has devoted many years to studying how proteins get imported into chloroplasts, and has made a number of major discoveries. One of the reasons of the significant contributions this time lies in her project’s innovative methodology. Dr. Li remarked that previous studies were unable to provide an accurate analysis of the surrounding membranes because the massive internal thylakoid membrane system of the chloroplast severely interfere with composition analyses by mass spectrometry. This project’s breakthrough came from the researcher team’s success in surmounting the difficulty of getting research material from leucoplasts, which are similar to chloroplasts but lack the thylakoid system.

The team’s paper was published in the November 21, 2018 issue of Nature, under the title “TIC236 links the outer and inner membrane translocons of the chloroplast”.

Preproteins are transported into chloroplasts by the TOC and TIC (translocons at the outer- and inner-envelope membranes of chloroplasts, respectively) machineries, but how TOC and TIC are assembled together is unknown.

Dr. Hsou-min Li’s research team reports that TIC236 links the outer and inner membrane translocons of the chloroplast.

The result also suggests that the backbone of the chloroplast protein-import machinery evolved from the bacterial BamA –TamB protein-secretion system.