Conformational Shifts in Huntingtin Exon 1 Monomer are Dependent on Temperature and Polyglutamine Length

Expansion of the polyglutamine (polyQ) region in the first exon of huntingtin (htt) is the root of Huntington's disease (HD). Misfolding and aggregation have long been observed to a greater extent in polyQ-expanded htt, but studies have also uncovered many implications of polyQ expansion in alterations to the dynamics and intermolecular interactions of monomeric htt. Here we find that htt exon 1 (HDx1), which is composed of the polyQ tract and flanking regions and in expanded form is sufficient to cause HD-like symptoms in transgenic mice, has a greater proclivity for adopting α-helical structure in the presence of a polyQ expansion. Our circular dichroism (CD) studies show that cooler temperature causes a gain in α-helical structure in HDx1 of a variety of polyQ lengths but a greater gain at disease-level polyQ length. We performed continuous wave electron paramagnetic resonance (EPR) with site-directed spin labeling (SDSL) and found the gain in structure to occur in the HDx1 N-terminus and the N-terminal region of the polyQ tract. Performing pulsed EPR on double spin-labeled mutants of HDx1 confirmed the α-helical structure adopted by the residues in those regions. Our findings fall in step with the previously proposed ‘rusty hinge’ hypothesis, which suggests that the normal, but not expanded, polyQ region serves as a flexible hinge that enables intra- and intermolecular interactions of htt. Stiffening of the hinge by polyQ expansion may lead to gains and losses of physiological functions for the protein. Our next step is to investigate the effect of phosphorylation to the N-terminus. To this end, we have already generated a number of phosphorylation-mimicking HDx1 mutants.Support: HDF, CHDI