Domain Organization of Mutant Huntingtin Fibrils

Huntington’s disease is a progressive, fatal neurodegenerative disorder caused by a polyglutamine (polyQ) expansion in exon 1 of the huntingtin gene (HDx1). A hallmark of the disease is the formation of fibrillar aggregates within cells. In vitro, HDx1 with a polyQ expansion forms fibrils that have a cross beta structure common to amyloid fibrils, but little else is definitively known about HDx1 fibril structure. We used electron paramagnetic resonance spectroscopy to study the organization of the major domains (N-terminus, polyQ, C-terminus) of HDx1 with 46Q within the fibril. Our data show that HDx1 fibrils do not have a parallel, in-register structure like most other disease-associated amyloid fibrils. The C-terminus is highly dynamic and is attached like a tail to the polyQ domain, which is mostly immobilized and forms the core of the fibril. However, the C-terminal portion of the polyQ lies outside the core and has a mobility similar to the C-terminus. The N-terminus produced heterogeneous spectra, indicating that it is able to sample multiple conformations. In sum, our study excluded the parallel, in-register arrangement of beta strands within HDx1 fibrils and represents a first step toward a high-resolution structure of HDx1 fibrils