Before Aggregation: How a Disease-Associated Protein Slowly Unravels.

PLASTIMORF researchers directly observe how disease-associated polyalanine expansions drive the progressive loss of protein structure, revealing an early event that precedes aggregation.

We have identified, in real time, how a disease-associated protein progressively loses its structural integrity. The study, published in ACS Bio & Med Chem Au, reveals a slow process of structural unraveling in PABPN1, a protein involved in oculopharyngeal muscular dystrophy (OPMD). PABPN1 normally contains a stretch of ten consecutive alanine residues. Expansions of this polyalanine tract are the main genetic cause of OPMD, a rare and currently incurable neuromuscular disease. Although these expansions are known to promote protein aggregation, the molecular events that precede aggregation have remained poorly understood.

Using high-resolution nuclear magnetic resonance (NMR) spectroscopy, the PLASTIMORF team studied PABPN1 variants carrying progressively longer polyalanine tracts and found that the expansions do not simply destabilize the protein. Instead, they initially favor more structured and rigid α-helical conformations.

Surprisingly, however, these apparently stable structures are not permanent. By following the protein over time, we directly observed a slow transition from the α-helical state towards more disordered conformations. This structural unraveling begins preferentially at the C-terminal end of the polyalanine helix, where increased molecular dynamics emerge as one of the earliest detectable changes.

The results suggest that the expanded polyalanine tract behaves as a metastable structure: increasingly helical and rigid, but ultimately prone to collapse into a disordered state, following a “tensioned helix” model. This progressive loss of structure may represent an early step towards the irreversible aggregation associated with disease.

From structural plasticity to therapeutic opportunities

The findings also provide a broader perspective on polyalanine expansion disorders. Previous work from PLASTIMORF showed that disease-associated expansions in PHOX2B, the protein responsible for congenital central hypoventilation syndrome (CCHS, or Ondine's syndrome), generate alternative and partially disordered structural states that can lead to irreversible aggregation.

The observation of a similar structural mechanism in PABPN1 suggests that progressive unraveling may be a common feature of proteins carrying disease-associated polyalanine expansions.

Importantly, identifying the earliest structural changes opens a potential route towards therapeutic intervention. Rather than targeting the final aggregates, future strategies could aim to stabilize the functional protein before structural unraveling takes place. The newly identified structural motifs may therefore provide starting points for the development of structure-based binders capable of preventing harmful conformational changes and preserving protein function.

Our results show that aggregation may be the consequence of an earlier structural event: the progressive loss of a metastable protein conformation. Understanding these transient states gives us an opportunity to intervene before irreversible aggregation occurs.

Reference:Antón R, Oroz J. Direct Observation of Structural Unraveling in PABPN1 Polyalanine Expansions. ACS Bio & Med Chem Au. 2026. DOI: 10.1021/acsbiomedchemau.6c00065.