Continuous repair cycle
The research addressed the biogenesis of protein complexes in chloroplasts. It focused primarily on the formation of the protein D1, an essential component of photosystem II. The protein is integrated into the thylakoid membrane and is constantly damaged to such an extent, particularly in strong light, that it has to be degraded and rebuilt in a continuous repair cycle. "This degradation and reconstruction is an incredibly complicated process, just like the de novo synthesis of the entire photosystem," stresses Danja Schünemann. "The more than 20 subunits of this system all have to be produced in the cell, transported to their final destination and eventually assembled there."
Watching ribosomes at work
In order to gain a better understanding of these mechanisms, her team developed a novel in-vitro method that for the first time makes it possible to purify the ribosomes that are currently producing the D1 protein while they are doing their work. "Until now, it was only possible to purify ribosomes in general," explains Danja Schünemann. "Now we can look over their shoulders as they work, so to speak." The researchers also determined which additional factors - apart from the ribosomes - are involved in the assembly of D1. They identified 140 proteins - some that were already known from other processes, others that had not yet been described.
For a more in-depth analysis, the team picked out a particularly prominent protein: STIC2. "We knew that it fulfills an important function in the construction of the thylakoid membrane, but we didn't know exactly what that function was," explain the researchers. They demonstrated that this protein works closely with another protein, SRP54, in the formation and repair of D1. "STIC2 interacts with certain structures in the thylakoid membranes, which is crucial for the correct incorporation of D1 and presumably other central proteins of the photosystems into the membrane," says Danja Schünemann.
Cooperation partners
Danja Schünemann's research group cooperated with researchers from the Faculties of Biology and Biotechnology and Chemistry and Biochemistry at Ruhr University Bochum as well as with groups from the Max Planck Institute of Molecular Plant Physiology in Potsdam.
