Lysine residues in the N‐terminal huntingtin amphipathic α‐helix play a key role in peptide aggregation

Huntington's disease is a genetic neurodegenerative disorder caused by an expansion in a polyglutamine domain near the N-terminus of the huntingtin (htt) protein that results in the formation of protein aggregates. Here, htt aggregate structure has been examined using hydrogen–deuterium exchange techniques coupled with tandem mass spectrometry. The focus of the study is on the 17-residue N-terminal flanking region of the peptide that has been shown to alter htt aggregation kinetics and morphology. A top-down sequencing strategy employing electron transfer dissociation is utilized to determine the location of accessible and protected hydrogens. In these experiments, peptides aggregate in a deuterium-rich solvent at neutral pH and are subsequently subjected to deuterium–hydrogen back-exchange followed by rapid quenching, disaggregation, and tandem mass spectrometry analysis. Electrospray ionization of the peptide solution produces the [M + 5H]5+ to [M + 10H]10+ charge states and reveals the presence of multiple peptide sequences differing by single glutamine residues. The [M + 7H]7+ to [M + 9]9+ charge states corresponding to the full peptide are used in the electron transfer dissociation analyses. Evidence for protected residues is observed in the 17-residue N-terminal tract and specifically points to lysine residues as potentially playing a significant role in htt aggregation. Copyright © 2015 John Wiley & Sons, Ltd.