by Staff Writers
Bochum, Germany (SPX) Oct 25, 2011
How the light-harvesting complexes required for photosynthesis get to their site of action in the plant cell is reported by RUB biologists in the Journal of Biological Chemistry.
The team led by Prof. Dr. Danja Schunemann (RUB working group on the molecular biology of plant organelles) has demonstrated for the first time that a membrane protein interacts with a single soluble protein to anchor the subunits of the light-harvesting complexes in the membrane. The researchers propose a new model that explains the integration into the membrane through the formation of a pore.
"The proteins of the light-harvesting complexes are the most abundant membrane proteins on Earth" says Dr. Beatrix Dunschede of the RUB.
"There is a special transport mechanism that conveys them into the chloroplasts and incorporates them into the photosynthetic membrane". Exactly how the various transport proteins interact with each other had, up to now, been unclear.
Interaction between only two proteins
Bochum's biologists examined intact, isolated plant cells and found that, for this purpose, Alb3 interacts with only a single soluble transport protein (cpSRP43). They confirmed this result in a second experiment with artificial membrane systems.
"In a further experiment, we identified the region in Alb3 to which the soluble protein cpSRP43 binds" explains the RUB biologist Dr. Thomas Bals. "It turned out that the binding site is partly within the membrane and thus cannot be freely accessible for cpSRP43."
Through the pore into the membrane
There, the soluble transport protein cpSRP43 interacts with the membrane protein Alb3, which then forms a pore. The proteins of the light-harvesting complexes get into the pore, and from there they are released laterally into the membrane.
"There are proteins in other organisms which are very similar to Alb3 and apparently also form pores" says Dunschede. "This supports our model. We are now planning new experiments in order to recreate the entire transport path in an artificial system."
B. Dunschede, T. Bals, S. Funke, D. Schunemann (2011) Interaction studies between the chloroplast signal recognition particle subunit cpSRP43 and the full-length translocase Alb3 reveal a membrane-embedded binding region in Alb3, Journal of Biological Chemistry, 286, 35187-35195, doi: 10.1074/jbc.M111.250746
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