ALS Study Reveals a Domain Crucial for Degradation of Misfolded Proteins

A recent study published in the journal Molecular Cell, revealed that a specific domain in a particular heat-shock protein plays a major role in the degradation of misfolded proteins. The study is entitled “The Hsp104 N-Terminal Domain Enables Disaggregase Plasticity and Potentiation”.

Disorders such as Parkinson’s disease, Alzheimer’s disease and amyotrophic lateral sclerosis (ALS) are caused by proteins that are misfolded and clumped. Heat-shock proteins (HSPs), highly conserved proteins expressed in all organisms, can refold some of these proteins avoiding complications to the organism. HSPs are known chaperones as they play an essential role in aiding the folding/unfolding of proteins, assembly of multi-protein complexes, protein transport into correct subcellular locations and cell protection against stress. The yeast Hsp104 is one of these chaperones, although the mechanism by which it aids clumped and misshapened proteins is poorly understood.

Dr. James Shorter, of the Perelman School of Medicine at the University of Pennsylvania, has been studying how the protein Hsp104 can be reprogrammed to improve its therapeutic features. He and colleagues have now discovered that the N-terminal domain (NTD) of Hsp104 plays a major role in the breakdown of proteins.

“We’ve defined in unprecedented detail the mechanism by which Hsp104 dissolves its natural substrate, Sup35 prions,” explained Dr. Shorter. “We found that the N-terminal domain of Hsp104 allows the enzyme to function in a way that enables the disintegration of the prion.” The team had established previously a panel of human HSPs that are capable of dissolving prions. Prions are abnormal, pathogenic agents that are able to induce anomalous folding of specific cellular proteins. Interestingly, while prions are causative agents of diseases in humans, in yeast they can be advantageous.

Hsp104 in particular is present in most of the less complex organisms, while it is absent in complex animals and humans. “We don’t understand quite why Hsp104 was lost. But it could be useful in a therapeutic setting because we could add back an activity that humans don’t really have: the ability to rapidly dissolve and refold prions.” said Dr. Shorter.

The researchers showed that deletion of Hsp104 NTD resulted in a promotion of prion formation instead of their disintegration as the protein can no longer break down prions due to their increased stability. Hsp104 (which is shaped like a hexagonal short tube) shuttles molecules through its central channel; however, when the NTD is deleted, the structure is altered and this mechanism is no longer functional. “Hsp104 extracts individual proteins from the prion fibril by pumping them through its central channel and that’s how it dissolves them. The N-terminal domain of Hsp104 allows the enzyme to function in a more powerful way that enables dissolution of the very stable Sup35 yeast prion,” explained the lead author, Dr. Elizabeth Sweeny.

“We’ve advanced to a new level of understanding that will help us design and engineer the enzyme to work better against the human proteins that are causing issues in disease,” explained Dr. Shorter. “Our next step will be to achieve even greater insight into Hsp104 structure. Another goal is to engineer the N-terminal domain to work better against clumped human disease proteins. Prior to this paper, we wouldn’t even have thought about doing that because the domain was not considered important. That’s one of the key findings from this study.” The researchers believe that their work can provide insight and can be applied to other human diseases caused by misfolded proteins.