Acetylation Regulates the Interaction of Huntingtin with Lipid Membranes: Implications for Huntington Disease

Abstract

Huntington disease (HD) is a genetic neurodegenerative disorder caused by an expanded polyglutamine (polyQ) domain in the N-terminus of the huntingtin (htt) protein which facilitates its aggregation. The first 17 amino acids (Nt17) in htt is an amphiphathic α-helix lipid-binding domain that promotes the formation of a diverse population of nanoscale aggregates. This domain undergoes numerous posttranslational modifications that modulate htt's toxicity, subcellular localization, and trafficking of vesicles. More specifically, N-terminal acetylation of htt has been implicated in the etiology of HD. Given the importance of acetylation in HD, we employed mass spectrometry (MS), both in situ and ex situ atomic force microscopy (AFM), and spectroscopic techniques to evaluate the impact of lysine acetylation on htt's aggregation kinetics in solution and on model lipid bilayers. Acetylation of htt exon 1 (51Q), and synthetic truncated htt exon 1 peptide (Nt17Q35P10KK) was achieved using a selective covalent label sulfo-N-hydroxysuccinimide (NHSA) in molar ratios of 1x, 2x, and 3x NHSA per peptide. With these molar ratios, all three lysine residues (K6, K9, and K15) in Nt17 were significantly labeled, as verified by MS. N-terminal htt acetylation retarded fibril formation in solution; however, the resulting fibril morphology was unaltered. Htt acetylation strongly impacted the protein's ability to bind lipid membranes, as demonstrated by a combination of lipid binding assays and AFM. Acetylated htt was found to bind to lipid vesicles, and disrupt lipid bilayer morphology less aggressively compared to the unlabeled htt. Our results highlight that N-terminal acetylation influences the aggregation of htt and its interaction with lipid bilayers.