Probing initial transient oligomerization events facilitating Huntingtin fibril nucleation at atomic resolution by relaxation-based NMR.

Abstract

The N-terminal region of the huntingtin protein, encoded by exon-1, comprises an amphiphilic domain (htt^NT), a polyglutamine (Q _<i>n</i> ) tract, and a proline-rich sequence. Polyglutamine expansion results in an aggregation-prone protein responsible for Huntington's disease. Here, we study the earliest events involved in oligomerization of a minimalistic construct, htt^NTQ₇, which remains largely monomeric over a sufficiently long period of time to permit detailed quantitative NMR analysis of the kinetics and structure of sparsely populated [Formula: see text] oligomeric states, yet still eventually forms fibrils. Global fitting of concentration-dependent relaxation dispersion, transverse relaxation in the rotating frame, and exchange-induced chemical shift data reveals a bifurcated assembly mechanism in which the NMR observable monomeric species either self-associates to form a productive dimer (τ_ex ∼ 30 μs, <i>K</i>_diss ∼ 0.1 M) that goes on to form a tetramer ([Formula: see text] μs; <i>K</i>_diss ∼ 22 μM), or exchanges with a "nonproductive" dimer that does not oligomerize further (τ_ex ∼ 400 μs; <i>K</i>_diss ∼ 0.3 M). The excited state backbone chemical shifts are indicative of a contiguous helix (residues 3-17) in the productive dimer/tetramer, with only partial helical character in the nonproductive dimer. A structural model of the productive dimer/tetramer was obtained by simulated annealing driven by intermolecular paramagnetic relaxation enhancement data. The tetramer comprises a <i>D</i>₂ symmetric dimer of dimers with largely hydrophobic packing between the helical subunits. The structural model, validated by EPR distance measurements, illuminates the role of the htt^NT domain in the earliest stages of prenucleation and oligomerization, before fibril formation.