Lipid Composition Controls the Huntingtin Exon1 Membrane-Association and Differentially Modulates its Flanking Regions Dynamics

The pathological expansion of the polyglutamine (polyQ) repeat within the first exon of huntingtin (Httex1) protein is a defining hallmark of Huntingtons disease (HD). Multiple evidence supports that the membrane recruitment of Httex1 is critical for its self-assembly and related toxicity in HD. In this work, we quantitatively examined the early steps of monomeric Httex1(23Q) association with lipid membranes and its impact on the conformational dynamics of the adjacent polyQ regions - the N-terminal N17 segment and C-terminal proline-rich region (PRR). A broad range of membrane physical properties was explored, including zwitterionic and anionic lipids, and also co-existing liquid-ordered and liquid-disordered phases. Two single cysteine mutants were engineered at the N- and C-termini of Httex1(23Q) and fluorescently-labeled with acrylodan or Atto 488 to probe their local polarity and flexibility, respectively. Our results indicate that Httex1- 23Q preferentially binds to negatively-charged lipid vesicles, and to a lower extent to liquid ordered/disordered phases. The N-terminal N17 segment inserts deeply into anionic membranes, adopting a less flexible state than in aqueous solution. At variance, the C-terminal PRR remains highly dynamic and solvent exposed in the Httex1-23Q membrane-bound state, preserving its intrinsic disordered features across all lipid compositions used. Altogether, our work provides unique insight into the distinct roles of each flanking polyQ region in mediating httex1-lipid binding, and how the lipid composition further modulates these early interaction steps.