Amyloid oligomers prevent protein folding
Authors: Mukherjee Abhisek , Chacon Marcelo , Mendez Nicolas , Shahnawaz Mohammad , Soto Claudio , John P , Kathrine G 
Published: June 2021

Background

Protein Misfolding Disorders (PMDs) are a group of diseases where at least one protein or peptide misfold, aggregates into amyloid‐like structures and accumulates in tissues where the disease‐specific damage occurs. There are at least 30 different diseases in the PMDs group, including several neurodegenerative disorders such as Alzheimer disease (AD), Parkinson disease (PD), as well as diverse systemic disorders, such as Type 2 Diabetes (T2D). A general collapse of protein homeostasis in cells harboring amyloids has been extensively reported. However, the mechanism is not completely understood. Nascent polypeptide chains and non‐native proteins, such as proteins undergoing translocation across membranes or unfolding before degradation, typically expose hydrophobic regions, which are prone to aggregate by non‐native interactions. Molecular chaperones transiently bind these exposed hydrophobic motifs, in an organized manner to prevent aggregation, followed by the release of the client when folding progresses and the hydrophobic segments become buried inside the protein. Interestingly, disease associated amyloids including, amyloid β (Aβ) and tau in AD, α‐synuclein in PD or islet amyloid polypeptide (IAPP) in T2D also expose hydrophobic surfaces in the cellular environment and have a high propensity to bind hydrophobic sequences.

Hypothesis

We propose that amyloid oligomers disrupt protein folding inside cells by irreversibly binding nascent polypeptide chains or non‐native proteins displaying exposed hydrophobic surfaces, sequestering them, preventing their interaction with molecular chaperones and ultimately arresting their proper folding.

Methods

We prepared amyloid aggregates of Aβ, tau and α‐synuclein and characterized them by CD, FTIR and electron microscopy. Next, we tested refolding of chemically denatured luciferase protein in presence of these amyloids. As controls, we used globular proteins such as bovine serum albumin (BSA) and non‐disease polymeric proteins, such as actin polymers. We also compared the binding affinity of the amyloids for unfolded and natively folded luciferase.

Results

Our results indicate that aggregates of Aβ, tau and α‐synuclein significantly reduced spontaneous refolding efficiency of denatured luciferase in a dose dependent manner. Importantly, neither BSA nor actin polymer produced any significant reduction in refolding efficiency. We also demonstrated that oligomeric species are more potent inhibitors of protein refolding compared to fibrillar aggregates.

Conclusion

Failure of the proteostasis system in the presence of disease associated amyloids leads to further protein misfolding. Hijacking of chaperone system and the protein degradation machinery by protein aggregates has been postulated as the underlining cause. Our data suggest that disease associated amyloids may directly interact with nascent polypeptide chains or non‐native proteins, preventing protein folding in general, which may lead to a collapse of protein homeostasis.

Support or Funding Information

Mitchell Foundation Center Grant to CS.

This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.