Side chain to main chain hydrogen bonds stabilize a polyglutamine helix in the activation domain of a transcription factor

Abstract

Polyglutamine (polyQ) tracts are regions of low sequence complexity of variable lengthfound in more than one hundred human proteins. These tracts are frequent in activation domains of transcription factors and their length often correlates with transcriptional activity. In addition, in nine proteins, tract elongation beyond specific thresholds causes polyQ disorders. To study the structural basis of the association between tract length, transcriptional activity and disease, here we addressed how the conformation of the polyQ tract of the androgen receptor (AR), a transcription factor associated with the polyQ disease spinobulbar muscular atrophy (SBMA), depends on its length. We found that the tract folds into a helical structure stabilized by unconventional hydrogen bonds between glutamine side chains and main chain carbonyl groups. These bonds are bifurcate with the conventional main chain to main chain hydrogen bonds stabilizing αhelices. In addition, since tract elongation provides additional interactions, the helicity of the polyQ tract directly correlates with its length. These findings suggest a plausible rationale for the association between polyQ tract length and AR transcriptional activity and have implications for establishing the mechanistic basis of SBMA.Polyglutamine (polyQ) tracts are lowcomplexity regions that are composed almost exclusively of Gln residues. They are frequent in the human proteome, particularly in the intrinsically disordered domains of proteins involved in the regulation of transcription, such as the activation domains of transcription factors 1 . The biological function of polyQ tracts is not wellunderstood, but it has been suggested that they regulate the activity of the proteins that harbor them by modulating the stability of the complexes that they form 2 . The lengths of polyQ tracts are variable because their coding DNA sequences tend to adopt secondary structures that hamper replication and repair 3 . Contractions and expansions in polyQ tracts can have functional consequences, and the lengths of the tracts may have been subject to natural selection 4 . As an example, it has been proposed that the length of the polyQ tract present in the protein huntingtin correlates with the intellectual coefficient 5 , presumably because it plays important although still not welldefined roles in neural plasticity 6 .For nine specific proteins, the variability in the lengths of polyQ tracts has pathogenic implications. Expansions beyond given thresholds are associated with nine rare hereditary neurodegenerative diseases known as polyQ diseases 7 . The mechanistic basis of this phenomenon is a matter of debate. Some authors have proposed that the expanded transcripts themselves are the neurotoxic species 8 due to their propensity to phase separate 9 , while others have suggested that expanded polyQ proteins are inherently neurotoxic 10 . However, it is widely believed that polyQ expansions cause neurotoxicity because they decrease protein solubility, which in turn leads to the formation of...