Protein transport: At what cost?

03/11/13

UC Davis plant biologists Steven Theg and Lan-Xin Shi have puzzled out the amount of energy a plant cell expends in transporting a protein from the cytoplasm into the chloroplast, a fundamental cellular activity.

Their findings have been published in the January 15 issue of the journal Proceedings of the National Academy of Sciences, in the article “Energetic cost of protein import across the envelope membranes of chloroplasts.”

“There has been a lot of research into how proteins cross membranes and a lot of talk about the energy put into this action, but few labs have measured how much,” Theg says. “The thing that sets our study apart is that it’s a complete accounting of the energy required for a protein to cross a biological membrane.”

A central question in cell biology, applying to all animals and plants, is how a cell expends its energy to complete functions such as protein and lipid synthesis. Transport of biomolecules to different locations is part of that picture.

This paper is just the second complete accounting of the energy cost of protein transport across a biological membrane. The first was also performed in Theg’s lab and was published ten years ago.

“Half the proteins made in a eukaryotic cell travel across at least one membrane. That’s a lot of metabolic activity,” Theg says. “We set out many years ago to discover how much it costs a cell for all this protein transport activity.”

Chloroplasts derive evolutionarily from bacteria, and some of the transporters in the chloroplasts are remnants of those that bacteria use. In addition, some of the transport systems in chloroplasts are similar to those found in mitochondria and the endoplasmic reticulum. As a result, this new work will have direct corollaries for both eukaryotic bacteria and for animals.

In chloroplasts there are four protein transporters—known by their acronyms TAT, TOC/TIC, SEC and SRP—that have analogs in other places and organisms. Shi and Theg have now measured the energy utilized by the first two transporters, with the second pair yet to be measured before they will know the actual total percentage of its energy a chloroplast devotes to protein transport.

For their latest study, Shi and Theg determined about 650 ATP are hydrolyzed per protein imported into chloroplasts. Although that is far less than the 11,500 ATP equivalents measured earlier per protein transported across the thylakoid membrane on the TAT pathway, it is still a significant energy requirement. Shi and Theg estimate that it would consume about 0.6 percent of the total energy output of a chloroplast.

The team still needs to measure the energy consumed by the SEC and SRP pathways before they can extrapolate too much from these two measurements, “But if we do make such a back-of-the-envelope extrapolation, we can estimate that the total energy cost to a cell for all its protein transport activity may be as high as 15 percent of the total energy expenditure,” Theg says.

“It’s still speculation, but if that’s right, I think it becomes a significant, previously unaccounted for portion of a cell’s energy budget.”

Theg credits Shi with conducting the experiments meticulously, isolating chloroplasts from garden peas and measuring the activity within them.

“It was a herculean effort on Lan-Xin’s part to make these measurements, because the experiments were very difficult,” Theg says. “The results were only obtained because of her skill and tenacity.”