Department of Structural Biology, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania 15260, USA.
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 exon 1 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.