The first complete measurement of how much energy it takes to move a protein across a cellular membrane has been carried out by researchers at the University of California, Davis.

The amount of energy required is surprisingly high, according to Steven Theg, professor of plant biology at UC Davis and one of the authors of the paper. If the result turns out to be generally applicable, it would have implications for understanding how cells generate and spend energy, he said.

Theg and graduate student Nathan Alder studied transport of proteins in chloroplasts, the structures in a plant cell that carry out photosynthesis. The Tat pathway, which moves proteins across membranes inside the chloroplast, is driven by hydrogen ions moving from one side of the membrane to the other. By measuring the gradient of hydrogen ions across the membrane, Alder and Theg could work out the energy lost as the protein is driven across the membrane.

The Tat pathway is unusual in that it relies only on the hydrogen ion gradient for power, while most protein transport pathways use other power sources as well.

Moving one protein molecule through the Tat pathway cost about 80,000 hydrogen ions, which, when summed over all the protein flow on this pathway, accounts for approximately 3 percent of the energy output of the chloroplast. As transporting proteins across membranes inside and outside cells is a crucial part of cell physiology, this could turn out to be a significant energy drain for the cell.

The paper is published in the Jan. 24 issue of the journal Cell.