Brown University investigators explain how two different beneficial mutant prions foil the amplification of harmful clumps of misfolded proteins in yeast. They observed that the mutants act at distinct stages to tip the balance back in favor of the cells.
The researchers thus suggest that there are more steps than can be targeted by a therapeutic than was previously thought. Their study was published in advance online March 20 in Nature Structural and Molecular Biology. The paper is titled “Dominant prion mutants induce curing through pathways that promote chaperone-mediated disaggregation.”
Although cells typically recognize and process misfolded proteins, prion proteins evade protective measures by forming stable, self-replicating aggregates. However, co-expression of dominant-negative prion mutants can overcome aggregate accumulation and disease progression through currently unknown pathways.
The Brown University scientists were able to determine the mechanisms by which two mutants of the Saccharomyces cerevisiae Sup35 protein cure the prion. They show that both mutants incorporate into wild-type aggregates and alter their physical properties in different ways, diminishing either their assembly rate or their thermodynamic stability.
One mutant prion, Q24R, hinders the ability of misfolded proteins to aggregate into harmful clumps. Another helpful mutant prion known as G58D assists the cell by speeding up its ability to unfold and refold misfolded proteins.
Susanne DiSalvo, a Brown biology graduate student and lead author of the paper, conducted experiments that showed that the cells would only be cured when she both added a mutant and allowed the cells’ own quality assurance system to work. Adding the mutant G58D, for example, could cure a cell of infection by the Sup35 prion, but if she perturbed the cell’s quality assurance system then G58D would not work.
Whereas wild-type aggregates are recalcitrant to cellular intervention, mixed aggregates are disassembled by the molecular chaperone Hsp104. Thus, rather than simply blocking misfolding, dominant-negative prion mutants target multiple events in aggregate biogenesis to enhance their susceptibility to endogenous quality-control pathways.
"Scientists have typically assumed that the only way to stop the runaway misfolding of some proteins was right at the beginning. DiSalvo’s work suggests that there are many opportunities throughout the process where even a mild intervention could give cells what they need to gain the upper hand,” says Tricia Serio, M.Phil, Ph.D., associate professor of medical science and leader of the research group. “That’s one of the biggest outcomes of Susanne’s work—that if you just even slightly interfere with this process, the cell can deal with it and get rid of it.
“The dogma in the field is that these conformations were so abnormal the cell couldn’t resolve them. But what we’ve found is that this process of misfolding is so efficient the cells can’t keep up with it. If you make it even just a little bit less efficient, the cell can get rid of the pathological state.”