The structural impact of a polyglutamine tract is location dependent

Polyglutamine (polyQ) expansion leads to protein aggregation and neurodegeneration in Huntington's disease and eight other inherited neurological conditions. Expansion of the polyQ tract beyond a threshold of 37 glutamines leads to the formation of toxic nuclear aggregates, therefore suggesting that polyQ expansion causes a conformational change within the protein the nature of which is unclear. There is a trend in the disease proteins that the polyQ tract is located external to, but not within a structured domain. We have created a model polyQ protein, in which the repeat location mimics the flexible environment of the polyQ tract in the disease proteins. Our model protein recapitulates the aggregation features observed with the clinical proteins, and allows structural characterization. Using NMR spectroscopy and a range of biophysical techniques we demonstrate that polyQ expansion into the pathological range has no effect on the structure, dynamics and stability of a domain adjacent to the polyQ tract. To explore the clinical significance of repeat location we engineered a variant of the model protein with a polyQ tract within the domain, a location that does not mimic physiological context, demonstrating significant destabilization and structural perturbation. These different effects highlight the importance of repeat location. We conclude that protein misfolding within the polyQ tract itself is the driving force behind the key characteristics of polyQ disease and that structural perturbation of flanking domains is not required.