Heat shock proteins perform a range of functions related to protein quality control. Among them: breaking down dangerous protein aggregates called amyloid fibers. The buildup of certain amyloids has been linked to neurodegenerative disease.
But the exact mechanism by which these amyloids are broken down wasn’t known. Two papers published recently in Nature revealed key details of how the heat shock protein machinery disassembles the α-synuclein amyloids linked to Parkinson’s disease.
One paper revealed a key activation step required for the heat shock protein machinery to work; the other demonstrates how that machinery exerts a high enough force to shred the amyloids. “It’s a breakthrough in understanding the mechanism,” says molecular biologist and biochemist Bernd Bukau, who coauthored both papers. A mechanistic understanding could ultimately lead to new targets for neurodegenerative drug screening, adds Bukau, a professor at Heidelberg University and the German Cancer Research Center.
Prior research established that the heat shock protein 70 (HSP70) family can be associated with any of some 40 subtypes of chaperones called J-domain proteins. The HSP70 and J-protein, together with a cofactor, can dock onto misfolded proteins to fix them—or, apparently, break them apart. Mysteriously, previous work had only found that one of these J-domain proteins, a member of class B, works with HSP70 to dock onto α-synuclein amyloids and actually break them apart. Structural biologist Rina Rosenzweig, senior author of the activation step paper, wondered why.
Rosenzweig, who’s based at the Weizmann Institute of Science in Rehovot, Israel used nuclear magnetic resonance (NMR) to solve the structures of several J-domain proteins. The NMR analyses revealed that the class B J-protein has two binding sites for HSP70, instead of the usual one. The first binding site is naturally blocked by a helical structure. However, according to the study, the blockage is released by an allosteric mechanism when the second binding site contacts HSP70. Rosenzweig and her colleagues think that the two-step mechanism ensures the HSP70 binds tight and close to the amyloid surface, and that such binding is necessary in order to break the fiber apart.
Biochemical experiments also tagged the amyloids with a fluorescent dye, which dimmed over time as the HSP70 machinery broke the fibers down. Notably, when the researchers used mutant J-proteins that lacked a functional 2-step binding mechanism, the amyloids did not break apart and the fluorescence did not dim. Hence, successful binding of the J-domain protein to HSP70 is a “key step” to form the molecular machinery necessary for amyloid breakdown, Bukau says, adding that this “has significant implications for understanding neurodegenerative diseases.”
The second of the two Nature papers also used NMR, this time to show the protein machinery as it docks on the fibers. The data revealed multiple HSP70s accumulating around the middle of each amyloid fiber, thereby exerting enough force to break it.
Together, the papers could inform drug design, notes cell biologist Harm Kampinga, who was not involved in the new work but does collaborate with Bukau’s lab. A pharmaceutical tailored to the class B J-protein could target the pathway that breaks down α-synuclein amyloids, notes Kampinga, who’s based at the University Medical Center Groningen, in the Netherlands. “There’s just one big ‘but’,” he adds. It’s unclear whether breaking down these amyloids prevents disease or exacerbates it by generating fragments that seed worsening illness. A clear next step, he says, is to clarify whether a drug should stimulate the two-step binding or inhibit it.
Several other classes of J-proteins also have the helix, Rosenzweig notes. Other proteins from this family are likely to have very different release mechanisms for the inhibition “just waiting to be discovered,” she says. If future research confirms that’s the case, then one possible next step for the field would be to develop drugs that are tailored to different J-proteins. Such drugs could in principle hold promise for a new wave of highly-specific medications, Kampinga says, acting on discrete cellular pathways with potentially minimal side effects.