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Photosynthesis

Surprising new role for well-known actor

Munich, 02/19/2013

Photosynthesis takes place on specialized membranes, made up of proteins and lipids, in the chloroplasts of plant cells. LMU researchers have now discovered an unexpected link between the processes of lipid and protein biosynthesis.

The process of photosynthesis is a fundamental part of Earth’s biosphere. In plants and algae, the photosynthetic machinery is located in chloroplasts, intracellular compartments that have their own genetic machinery. The protein complexes required for photosynthesis are embedded in a lipid bilayer, forming specialized membranes called thylakoids. Thylakoid proteins and lipids are synthesized in the chloroplast but, as Professor Jörg Nickelsen of LMU‘s Biozentrum points out: “So far, we know very little about how the two biosynthetic pathways are linked to allow for the coordinated assembly of thylakoids.”

This is because the two pathways have traditionally been studied independently of each other. However, Nickelsen and his team have now identified a protein that is involved in both processes in the chloroplasts of the green alga Chlamydomonas. This protein is the so-called E2 subunit of the pyruvate dehydrogenase enzyme, and appears to provide the first functional link between the two pathways. Pyruvate dehydrogenase is known to supply fatty-acid precursor molecules required for lipid synthesis. “Our findings now reveal that this enzyme has an unexpected second function in the spatial organization of gene expression in the chloroplast,” says Nickelsen.

Growth-based control of enzyme function
In this context, the enzyme subunit acts as an RNA-binding factor to regulate synthesis of the photosynthetic protein complexes that are integrated into the thylakoids, allowing the levels of these proteins to be adjusted to accommodate the cell’s needs. “Which function is active at any given time appears to depend primarily on the prevailing growth conditions via the metabolic signals that they generate,” Nickelsen explains. “At all events, the E2 subunit participates in lipid synthesis and protein synthesis, the two basic processes necessary for photosynthetic membrane biogenesis.

The researchers assume that the E2 subunit serves as an important control point in membrane biogenesis, which provides a way to adjust the relative rates of protein and lipid synthesis in accordance with demand. Moreover, there are indications that this phenomenon is not restricted to green algae, but is also present in distantly related organisms – such as cyanobacteria and even humans. The new results could therefore throw new light on the mechanisms that regulate membrane biogenesis more generally.

In addition, these findings relating to the regulation of membrane biogenesis could open up new opportunities for targeted manipulations designed to enhance the efficiency of photosynthesis and increase the productivity of staple crops. (PLoS Biology, 2013)

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