Mutants aren't always bad. Mutant forms of prion proteins — found naturally in some species, including sheep, goats, and humans — can spur the aggregate of prion proteins to disassociate. Brown University's Tricia Serio and her team have been studying just how that works using two mutant prion proteins, Q24R and G58D. Once the cause of how the aggregate is destabilized is known, therapies could be developed that target protein misfolding.
"We liked the idea of trying to compare the action of mutants in different regions of the protein to see if they inhibited through different mechanisms," Serio says. The results from the team, led by graduate student Susanne DiSalvo, were published in Nature Structural & Molecular Biology in April.
There are four basic steps in the life of a prion. First, the protein is synthesized. Then, it folds into prion form by interacting with an aggregate of prion proteins already present in the cell that serves as a template. As those aggregates grow, cellular chaperones break them down into smaller complexes that continue to convert proteins. The aggregates can then also be transmitted to daughter cells during cell division. Serio's lab has developed assays to study each of those stages separately. "What we did was we took yeast strains that had wild-type or normal copy of the protein and we expressed these mutants along side of it. And we monitored each of those individual steps," she says. From this, they found that the Q24R mutation affects the conversion step and G58D affects the fragmentation step.
Just what it is about Q24R and G58D that lead to the breakup of the aggregates is not yet fully understood. The region where Q24R is located is required for interactions between molecules, likely a binding event, Serio says. But G58D is found in a region of the protein that is buried within the aggregate, suggesting that the mutation works to destabilize it, though "the exact mechanism of -occurrence is not clear," Serio says.
These results, she adds, parallel other findings in mammalian systems. "It at least opens the possibility that our work could provide a framework for people to look in a mammalian system," she says.