Dynamics of the Proline-Rich C-Terminus of Huntingtin Exon-1 Fibrils.

Intrinsically disordered protein domains not only are found in soluble proteins but also can be part of large protein complexes or protein aggregates. For example, several amyloid fibrils have intrinsically disordered domains framing a rigid β-sheet-rich core. These disordered domains can often be observed using solution NMR methods in combination with modest magic angle spinning and without perdeuteration. But how can these regions be detected using solution NMR methods when they are part of a fibril that is not tumbling isotropically in solution? Here we addressed this question by investigating the dynamic C-terminus of huntingtin exon-1 (HTTex1) fibrils that are important in Huntington's disease. We assigned the most dynamic regions of the C-terminus of three HTTex1 variants. On the basis of this assignment, we measured site-specific secondary chemical shifts, peak intensities, and R1, R'2, and R1ρ 15N relaxation rates. In addition, we determined the residual 1H-15N dipolar couplings of this region. Our results show that the dipolar couplings are averaged to a very high degree, resulting in an order parameter that is essentially zero. Together, our data show that the C-terminus of HTTex1 is intrinsically disordered and undergoes motions in the high picosecond to low nanosecond range.