Huntington’s disease (HD) is caused by expansion of a polyglutamine (polyQ) repeat in the huntingtin protein. A structural basis for the apparent transition between normal and disease-causing expanded polyQ repeats of huntingtin is unknown. The ‘linear lattice’ model proposed random-coil structures for both normal and expanded polyQ in the preaggregation state. Consistent with this model, the affinity and stoichiometry of the anti-polyQ antibody MW1 increased with the number of glutamines. An opposing ‘structural toxic threshold’ model proposed a conformational change above the pathogenic polyQ threshold resulting in a specific toxic conformation for expanded polyQ. Support for this model was provided by the anti-polyQ antibody 3B5H10, which was reported to specifically recognize a distinct pathologic conformation of soluble expanded polyQ. To distinguish between these models, we directly compared binding of MW1 and 3B5H10 to normal and expanded polyQ repeats within huntingtin exon 1 fusion proteins. We found similar binding characteristics for both antibodies. First, both antibodies bound to normal, as well as expanded, polyQ in huntingtin exon 1 fusion proteins. Second, an expanded polyQ tract contained multiple epitopes for antigen-binding fragments (Fabs) of both antibodies, demonstrating that 3B5H10 does not recognize a single epitope specific to expanded polyQ. Finally, small angle X-ray scattering and dynamic light scattering revealed similar binding modes for MW1 and 3B5H10 Fab-huntingtin exon 1 complexes. Together, these results support the linear lattice model for polyQ binding proteins, suggesting that the hypothesized pathologic conformation of soluble expanded polyQ is not a valid target for drug design.

Microgravity was used in an attempt to define the crystal structure of the polyQ repeat of huntingtin alone or bound to MW1, an anti-polyQ Fab. While huntingtin was not crystallized in the experiments, analysis of microgravity-grown and Earth-grown MW1 Fab crystals showed that, on average, microgravity-grown crystals of MW1 Fab showed an increase in size and an improvement in resolution and mosaicity when compared with Earth-grown crystals in one space group, in agreement with data published for other proteins, although the highest overall resolution X-ray data in our experiments were obtained from a crystal grown on Earth.