Protein molecules are the workhorses of the human body, performing an enormous variety of biological functions. For proteins to work, they need to be able to fold properly, transforming from a wiggly chain of amino acids into their final three-dimensional structure.
But many proteins can’t fold on their own and require helper molecules called chaperones, which interact with the newly formed protein to encourage it toward a properly folded shape. If left unsupervised, many proteins fold into the wrong shape—or worse, clump together with anything nearby, forming aggregates that can damage cells.
In a new study published in Molecular Cell, researchers at University of Utah Health and Brigham Young University have tracked how a chaperone guides protein folding, challenging a prevailing model of this process and paving the way for interventions to fix it when it goes wrong.
Zooming in on an essential biological pathway
The researchers had set their sights on understanding the folding of a protein called Gβ5, which is involved with a pervasive process that underlies a huge number of essential cell functions. The study’s co-lead author, Barry Willardson, Ph.D., professor of biochemistry at Brigham Young University, says that this process—G protein signaling—is so fundamental to biology that a third of all known pharmaceuticals target it. “This is the number one system for drug targeting,” Willardson adds.
Like many proteins, Gβ5 requires a chaperone to fold properly, but capturing how the two molecules interact over time was no easy feat. To observe this intricate process, the researchers used an advanced imaging technique called cryo-electron microscopy (cryo-EM). After isolating samples of Gβ5 and its chaperone, CCT, in the middle of folding, the researchers flash-froze the proteins, locking them in place. They then used an electron microscope to take pictures of the proteins at near-atomic resolution.
Each protein molecule was frozen at a slightly different stage along the folding process, which meant that if the researchers got enough pictures of enough proteins, they could order each individual snapshot into its place along a continuum—like piecing together a movie out of thousands of still frames.
Challenging an established model
The results were surprising. CCT is shaped a bit like a cage, with an empty space in the middle that Gβ5 sits in while it’s folding. For decades, the prevailing theory held that the chaperone acts like a cage, too: providing a safe, enclosed space for new proteins to fold themselves and preventing interactions with other molecules that could make the process go awry.
But what the researchers saw is that CCT plays a much more active role in guiding the folding process, interacting with specific parts of Gβ5 to kickstart folding. Shuxin Wang, Ph.D., at the time a graduate student in the lab of co-lead author Peter Shen, Ph.D., was the first to discover these results. That day, the whole research team was “euphoric”, Willardson says. “As soon as we saw that, we knew we had something that had never been seen before.”
The researchers add that while some chaperone molecules do appear to act like simple cages, CCT seems to actively guide folding for more proteins than just Gβ5, hinting that such specific interactions could be common.
Shen, an assistant professor of biochemistry at the University of Utah, adds that mutations in Gβ5 that interfere with its folding cause serious diseases involving seizures and cognitive disability. This study’s findings could help lead to treatments for those diseases. “Now that we understand how the normal protein folds, we can apply the same tools and strategies to understand why the mutant protein does not fold properly,” he says. “This gives us a much more informed way of trying to fix this problem at a molecular level.”