The structure of pathogenic huntingtin exon-1 defines the bases of its aggregation propensity

Huntingtons Disease is a neurodegenerative disorder caused by a CAG expansion of the first exon of the HTT gene, resulting in an extended poly-glutamine (poly-Q) tract in the N-terminus of the protein huntingtin (httex1). The structural changes occurring to the poly-Q when increasing its length remain poorly understood mainly due to its intrinsic flexibility and the strong compositional bias of the protein. The systematic application of site-specific isotopic labeling has enabled residue-specific NMR investigations of the poly-Q tract of pathogenic httex1 variants with 46 and 66 consecutive glutamines. The integrative analysis of the data reveals that the poly-Q tract adopts long -helical conformations stabilized by glutamine side-chain to backbone hydrogen bonds. 19F-NMR of site-specifically incorporated fluoro-glutamines and molecular dynamics simulations demonstrate that the mechanism propagating -helical conformations towards the poly-Q from the upstream N17 domain is independent of the poly-Q track length. Aggregation and atomic force microscopy experiments show that the presence of long and persistent -helices in the poly-Q tract is a stronger signature in defining the aggregation kinetics and the structure of the resulting fibrils than the number of glutamines. The ensemble of our observations provides a structural perspective of the pathogenicity of expanded httex1 and paves the way to a deeper understanding of poly-Q related diseases.