Polyglutamine disruption of the huntingtin exon1 N-terminus triggers a complex aggregation mechanism

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 (htt(NT)) N-terminal to the polyQ in the toxic huntingtin exon1 fragment imparts onto this peptide a complex alternative aggregation mechanism. In isolation the htt(NT) peptide is a compact coil that resists aggregation. When polyQ is fused to this sequence, it induces in htt(NT), in a repeat-length dependent fashion, a more extended conformation that greatly enhances its aggregation into globular oligomers with htt(NT) cores and exposed polyQ. In a second step, a new, amyloid-like aggregate is formed with a core composed of both htt(NT) 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.