Metamorphic proteins can be thought of as the "shapeshifters" of human, animal and bacterial cells. Their ability to drastically switch between two different shapes enables them to adapt to changing environments and carry out diverse functions.
Little is known about how metamorphic proteins transform despite their usefulness in living organisms. To help tackle this mystery, a new paper in the "Perspectives" section of the journal Proceedings of the National Academy of Sciences (PNAS) offers a "bold theory," said co-author John Orban , a professor in the University of Maryland's Department of Chemistry and Biochemistry and Institute for Bioscience and Biotechnology Research (IBBR).
Orban and his co-author Andy LiWang, a professor of chemistry and biochemistry at the University of California, Merced, suggest that many metamorphic proteins have an "underlying temperature dependence." If confirmed, this would mean that temperature—and cold temperature in particular—plays a fundamental role in setting off shapeshifting in metamorphic proteins.
Ultimately, a better understanding of metamorphic proteins could advance biomedical research and the development of lifesaving drugs.
"It may be possible to design proteins that are switchable and have more than one function," Orban said. "They could potentially be stealth proteins that go into a cancer cell and pretend to be one state, but under certain environmental conditions switch to a state that could kill the cell, for example."
Metamorphic proteins are known to change shape in response to various environmental "triggers"—such as changes in acidity or oxidation—but Orban's and LiWang's theory takes this one step further. Their research seeks to explain why an equilibrium, or balance, exists between the various shapes that metamorphic proteins can take.
"Metamorphic proteins can't shapeshift unless there's an equilibrium between the two states and our hypothesis is that the underlying reason for that equilibrium is based on temperature," Orban said. "We think this may be some sort of universal mechanism."
Orban said this hypothesis was inspired by an earlier study he co-authored in 2023. That paper revealed that an engineered metamorphic protein switched back and forth between folded states when researchers adjusted the temperature over a "relatively narrow" range between 5 and 30 degrees Celsius.
"There are now some other examples out there of naturally occurring metamorphic proteins that do this, but this was the first example of a designed protein that switches reversibly using only temperature," Orban said. "Andy and I started talking more and wondered whether other metamorphic proteins followed the same pattern."
In their new PNAS paper, Orban and LiWang surveyed 26 pairs of metamorphic proteins that have been studied before, though never with this temperature dependence theory in mind. Specifically, the researchers analyzed differences in hydrophobic contacts—water-repelling zones that help keep structures together—from one protein state to the next.
Where experimental data was available, the researchers found that nearly every protein pair had significant differences in hydrophobic contacts and that these differences were closely linked to temperature-dependent changes. Low temperature states were associated with fewer hydrophobic contacts, resulting in a more flexible state that can be conducive to shapeshifting.
The evidence uncovered so far seems to back up their theory on the role of temperature in shapeshifting proteins.
"It's a working hypothesis, but so far it's been supported," Orban said. "We were surprised because we thought this was a pretty bold idea."
Going forward, this research could be applied to the search for more metamorphic proteins, which are difficult to identify. The global Protein Data Bank contains about 200,000 known monomorphic proteins—those with a single, stable structure—but fewer than 100 metamorphic proteins. By using temperature as a trigger, Orban believes that some proteins believed to be monomorphic might transform, revealing their true nature as metamorphic proteins.
While Orban's main motivation is to answer questions about the underlying mechanisms that trigger shapeshifting proteins, he's also optimistic about the future applications.
"Our interest so far has been mostly fundamental, but we talk about possible biotechnology applications and I don't think it's pie in the sky," Orban said. "I think it's entirely possible that in the not-too-distant future we will be predicting metamorphic proteins more reliably, designing them and putting them to work for us."
