Polyglutamine disruption of the huntingtin exon 1 N terminus triggers a complex aggregation mechanism

Abstract

Simple polyglutamine (polyQ) peptides aggregate in vitro via a nucleated growth pathway directly yielding amyloid-like aggregates. We show here that the 17 amino acid flanking sequence (httNT) N-terminal to the polyQ in the toxic huntingtin exon1 fragment imparts onto this peptide a complex alternative aggregation mechanism. In isolation the httNT peptide is a compact coil that resists aggregation. When polyQ is fused to this sequence, it induces in httNT, in a repeat-length dependent fashion, a more extended conformation that greatly enhances its aggregation into globular oligomers with httNT cores and exposed polyQ. In a second step, a new, amyloid-like aggregate is formed with a core composed of both httNT and polyQ. The results indicate unprecedented complexity in how primary sequence controls aggregation within a substantially disordered peptide, and have implications for the molecular mechanism of Huntington's disease. There are nine known expanded CAG repeat diseases, in which expansion of a disease protein's polyglutamine (polyQ) sequence beyond a threshold repeat length causes progressive neurodegeneration through a predominantly gain-of-function mechanism 1. In Huntington's disease (HD) the repeat length threshold is about 37 glutamines 2. A majo