Theg lab pinpoints energetic motor for import of proteins into chloroplasts

Steve Theg

Steve Theg, professor of Plant Biology

07/08/14

Plant Biology Professor Steve Theg's laboratory has answered a key question about the energy source for transporting proteins into plant chloroplasts, identifying the heat-shock protein Hsp70 as the primary driver of the process.

Publishing their results in The Plant Cell, Theg, Li Liu, Robert McNeilage and Lan-Xin Shi conducted research in the moss Physcomitrella patens to pinpoint which of two so-called heat-shock proteins – Hsp70 or Hsp93 – is responsible for the energy that powers protein import.

Their work revealed that Hsp70 is by far the primary driver, providing new insight into the working of plant chloroplasts, which is essential knowledge for crop improvement.

"If you want to be able to exploit genetic engineering for crop improvement to feed the world's nine billion people, you have to be able to manipulate what goes on in chloroplasts because they are responsible for so much of plant metabolism," Theg said.

The Theg team studies how proteins get into chloroplasts, which must import most of their resident proteins. In previous research, they and other research teams have identified Hsp70 and Hsp93 as two potential motor proteins present in the area where protein import occurs.

Heat shock proteins serve as molecular chaperones, aiding the actions of other proteins. Their overall function is to help other proteins to work properly at the right places. They are involved in many activities in addition to transport: the folding of new proteins, refolding and disaggregation of misfolded and aggregated ones, and turnover of damaged and old proteins.

To study Hsp70's role in chloroplast protein import, Theg's group designed an experiment in P. patens in which they impaired the protein's ability to hydrolyze adenosine triphosphate (ATP), the metabolic energy source for protein import, thereby rendering Hsp70 less efficient.

"The beauty of this moss is that we can readily engineer our genes of interest and create mutant plants in which to study their functions," Shi said. "We mutated a critical ATP-binding amino acid, and the consequence of the mutation is similar to reducing the fuel efficiency of a car engine," Shi said.

They then put the mutated Hsp70 back into the chloroplast and analyzed whether protein import became less efficient.

"It really matched perfectly," Theg said. "We made it about three times less efficient as an individual protein, we put it back into the chloroplast and the import process became about three times less efficient."

Shi added that future work will focus on identifying the exact roles of chloroplast Hsp70 and Hsp93 together, as both play important roles in the organelle. "This is the first experiment, to our knowledge, to show that Hsp70 is the dominant ATPase contributing to chloroplast protein import," Shi said. "Next we will focus on finding specific stages where chloroplast Hsp70 and Hsp93 act during the course of protein import."