Huntingtin structure is orchestrated by HAP40 and shows a polyglutamine expansion-specific interaction with exon 1

Abstract

Huntington’s disease results from expansion of a glutamine-coding CAG tract in the huntingtin (HTT) gene, producing an aberrantly functioning form of HTT. Both wildtype and disease-state HTT form a hetero-dimer with HAP40 of unknown functional relevance. We demonstrate in vivo and in cell models that HTT and HAP40 cellular abundance are coupled. Integrating data from a 2.6 Å cryo-electron microscopy structure, cross-linking mass spectrometry, small-angle X-ray scattering, and modeling, we provide a near-atomic-level view of HTT, its molecular interaction surfaces and compacted domain architecture, orchestrated by HAP40. Native mass spectrometry reveals a remarkably stable hetero-dimer, potentially explaining the cellular inter-dependence of HTT and HAP40. The exon 1 region of HTT is dynamic but shows greater conformational variety in the polyglutamine expanded mutant than wildtype exon 1. Our data provide a foundation for future functional and drug discovery studies targeting Huntington’s disease and illuminate the structural consequences of HTT polyglutamine expansion.

HTT-OMNI: A Web-based Platform for Huntingtin Interaction Exploration and Multi-omics Data Integration

“…This subset of HIPs formed a highly interconnected network of 128 nodes (STRINGdb interaction scores ≥ 0.4), with only two HIPs (BCAN and MAP3K12) being unconnected. As expected given its large number of known interactors, HTT exhibited many connections within this network, including a connection to the well-known HIP F8A1 (33,34) (also known as HAP40). Within this filtered network, a small subset of these HIPs display high connectivity with other HIPs, suggesting that these targets could serve as hubs of J o u r n a l P r e -p r o o f HTT interactions (Fig.…”

Recent Microscopy Advances and the Applications to Huntington’s Disease Research

“…In the Zeitlin lab, they were able to tag the amino terminus of huntingtin in a mouse model without any negative effects by using small epitope tags as opposed to a 34 kDa fluorescent protein [17]. This suggests that a large fusion of a fluorescent protein at either termini of huntingtin can affect huntingtin function, which is consistent with the known ability of the huntingtin amino terminus to fold back to the more distal regions of the protein [16,18] and suggests that huntingtin function is likely allosterically regulated by either termini.…”

Molecular Pathophysiological Mechanisms in Huntington’s Disease

“…Htt has an α-helical conformation with the HEAT domains connected by smaller bridge domains containing various tandem repeats. It is organized in a complex with Htt-associated protein 40 (HAP40) [ 51 ], which has also an α-helical conformation and interacts with Htt through electrostatic forces and hydrophobic bonds, thereby stabilizing the conformation of Htt [ 52 ].…”