PROBING THE DISTINCT CONFORMATIONAL DYNAMICS OF THE FLANKING POLYQ REGIONS IN HUNTINGTIN EXON 1 AT THE MEMBRANE SURFACE: INSIGHTS INTO HUNTINGTON’S DISEASE

Huntington’s disease (HD) is a severe neurodegenerative disorder caused by an abnormal expansion of the polyglutamine (polyQ) repeat within the first exon of huntingtin protein (httex1). Biological membranes play a critical role in httex1 aggregation in HD. Recent evidence supports that both flanking polyQ regions [the N17 segment and the proline­rich domain (PRD)] strongly modulate the httex1-lipid interaction. However, its structural description is inherently challenging due to the disordered features of httex1 and its high aggregation propensity. Here, we employed acrylodan and Atto-A88 fluorescence to report on membrane binding and conformational dynamics, respectively. We investigated distinct lipid compositions, varying from pure fluid to liquid ordered-containing membranes, and also anionic lipid vesicles. Globally, our data revealed a favored partitioning of httex1 towards negatively-charged vesicles. Still, httex1-lipid interaction has an important hydrophobic component and does not show preference for ordered phases. In addition, both steady­state and time­resolved fluorescence measurements of httex1 specifically labeled at each domain adjacent to the polyQ tract, indicate distinct local polarities and conformational signatures. The N-terminal N17 segment adopts a less flexible state for all lipid compositions, along with a deeper membrane insertion into anionic lipid vesicles. Notably, the C-terminal PRD segment preserves its conformational dynamics and remains highly solvent exposed in the membrane-bound state, independently of the lipid composition. Our work provides unique insights into the distinctive roles of each adjacent polyQ region in regulating httex1­lipid interaction.

This work was supported by FCT­Portugal (2022.01454.PTDC grant and CEECIND/00884/2017 contract to AMM; UIDB/04565/2020 and LA/P/0140/2020 funding to iBB and i4HB) and Bial Foundation/Portuguese Medical Association (grant 45/2022 to AMM).