Extended polyglutamine tracts cause aggregation and structural perturbation of an adjacent beta barrel protein

Formation of fibrillar intranuclear inclusions and related neuropathologies of the CAG-repeat disorders are linked to the expansion of a polyglutamine tract. Despite considerable effort, the etiology of these devastating diseases remains unclear. Although polypeptides with glutamine tracts recapitulate many of the observed characteristics of the gene products with CAG repeats, such as in vitro and in vivo aggregation and toxicity in model organisms, extended polyglutamine segments have also been reported to structurally perturb proteins into which they are inserted. Additionally, the sequence context of a polyglutamine tract has recently been shown to modulate its propensity to aggregate. These findings raise the possibility that indirect influences of the repeat tract on adjacent protein domains are contributory to pathologies. Destabilization of an adjacent domain may lead to loss of function, as well as favoring non-native structures in the neighboring domain causing them to be prone to intermolecular association and consequent aggregation. To explore these phenomena, we have used chimeras of a well studied globular protein and exon 1 of huntingtin. We find that expansion of the polyglutamine segment beyond the pathological threshold (>35 glutamines) results in structural perturbation of the neighboring protein whether the huntingtin exon is N- or C-terminal. Elongation of the polyglutamine region also substantially increases the propensity of the chimera to aggregate, both in vitro and in vivo, and in vitro aggregation kinetics of a chimera with a 53-glutamine repeat follow a nucleation polymerization mechanism with a monomeric nucleus.