The Role of Protein Context in Disease-Related Huntingtin Protein/Lipid Interface Interactions

Abstract

Huntington's disease (HD) is a neurodegenerative disorder caused by an expansion of a poly-glutamine (poly-Q) region near the N-terminus of the huntingtin (htt) protein. Expansion of the poly-Q region above 35-40 repeats results in the disease that is characterized by inclusion body aggregates of mutated protein. The poly-Q region of htt is flanked by a 17 amino acid N-terminal sequence (N17) and a region of proline repeats (P11). As the cell membrane has been proposed to play a role in mediating htt aggregation, Langmuir trough techniques were used to investigate the effects of flanking regions on the surface activity and insertion of htt peptide constructs into a lipid monolayer. Four peptide constructs were tested: N17-Q35-KK, N17-Q35-P11-KK, KK-Q35-KK and KK-Q35-P11-KK, where the additional lysine residues were attached to improve solubility. Surface activity was measured to determine the affinity of each peptide for the air-water interface. The constructs containing the N-terminal sequence had higher surface activities compared to those without. Inclusion of the polyproline region with the N-terminal sequence contributed to the highest surface activity. To determine peptide association with a cell membrane, insertion of each peptide into a monolayer of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) held at a constant surface pressure was also investigated. Peptides with the flanking N-terminus showed appreciable monolayer insertion. The polyproline region only mediated and increased the degree of insertion when the N-terminus was also present. Rates of insertion of the constructs containing the N-terminus were comparable, while those without this region were markedly slower. Experiments were repeated using monolayers of total brain extract to better model a physiological environment with similar results. The amino acid environment surrounding the poly-Q region dramatically affects peptide association with the cell membrane and this may potentially mediate the aggregation process.