January 20, 2016
Eggs start out as slimy and awful, but can end up warm, firm and wonderful. All it takes is some heat to denature the egg proteins and voilà, a tasty breakfast.
Not that anyone would want to do it, but of course it is impossible to do the reverse. You can’t take a fried egg and turn it back into a raw one. The denaturation is pretty much permanent.
When a cell is hit with high temperatures, its proteins start to denature as well. And scientists thought that most of the denaturation of many of these proteins was as irreversible as the eggs. The thought was that many or most of these denatured proteins were “eaten” through proteolytic degradation. Although cellular chaperones are capable of disaggregating and refolding some heat-denatured proteins, it wasn’t known which aggregated proteins met which fate in a living cell.
A new study out in Cell by Wallace and colleagues shows that at least in yeast, most eggs get unfried. After a heat shock, aggregated proteins in the cell return to their unaggregated form and get back to work.
Now those earlier scientists weren’t crazy or anything. The proteins they looked at did indeed clump up and get broken down by the cell after a heat shock. But these were proteins introduced to the cell.
In the current study, Wallace and colleagues looked at normal yeast proteins being made at their normal levels. And now what happens after a brief heat shock is an entirely different story.
The first experiment they did looked at which endogenous yeast proteins aggregated after they were shifted from their normal 30 to 46 degrees Celsius for 2, 4, or 8 minutes. The researchers detected aggregation using ultracentrifugation—those proteins that shifted from the supernatant to the pellet after a spin in the centrifuge were said to have aggregated.
Using stable isotope labeling and liquid chromatography coupled to tandem mass spectroscopy (LC-MS/MS), they were able to detect 982 yeast proteins easily. Of these, 177 went from the supernatant to the pellet after the temperature shift. (And 4 did the reverse and went from the pellet to the supernatant!)
After doing some important work investigating these aggregated proteins, the researchers next set out to see what happened to them when the cells are returned to 30 degrees Celsius. Are they chewed up and recycled, or nursed back to health and returned to the wild?
To figure this out they did an experiment where proteins are labeled at two different times using two different labels. The researchers first grew the yeast cells at 30 degrees Celsius in the presence of arginine and lysine with a “light” label. This labels all of the proteins in the cell that have an arginine and/or lysine.
Then the cells are washed and a new media is added that contains “heavy” labeled arginine and lysine. The cells are shifted to 42 degrees Celsius for 10 minutes and then allowed to recover for 0, 20, or 60 minutes.
After 60 minutes of recovery, the ratio of light to heavy aggregated proteins looked the same as proteins that hadn’t aggregated. In other words, aggregation did not cause proteins to turn over more quickly.
It looks as if aggregated proteins are untangled and allowed to go about their business. So after a heat shock the cell doesn’t throw its hands in the air and simply start things over.
Other experiments done by Wallace and coworkers in this study, that we do not have the space to tackle here, suggest that the cell has an orderly process for dealing with heat stress. After a heat shock, certain proteins aggregate with chaperones in specific areas of the cell. Once the temperature returns to normal, these stress granules disassemble and the aggregated proteins are released intact.
None of this will help us unfry an egg — a denatured egg protein is obviously significantly different than an aggregated protein protected by chaperones in a stress granule. But this study does help us better understand how our cells work. And that’s a good thing.
Unlike Mr. Bill’s dog, most aggregated yeast proteins can return from a heat shock.
by Barry Starr, Ph.D., Director of Outreach Activities, Stanford Genetics
Categories: Research Spotlight