Consequences of poly-glutamine repeat length for the conformation and folding of the androgen receptor amino-terminal domain

Poly-amino acid repeats, especially long stretches of glutamine (Q), are common features of transcription factors and cell-signalling proteins and are prone to expansion, resulting in neurodegenerative diseases. The amino-terminal domain of the androgen receptor (AR-NTD) has a poly-Q repeat between 9 and 36 residues, which when it expands above 40 residues results in spinal bulbar muscular atrophy. We have used spectroscopy and biochemical analysis to investigate the structural consequences of an expanded repeat (Q45) or removal of the repeat (Delta Q) on the folding of the AR-NTD. Circular dichroism spectroscopy revealed that in aqueous solution, the AR-NTD has a relatively limited amount of stable secondary structure. Expansion of the poly-Q repeat resulted in a modest increase in a-helix structure, while deletion of the repeat resulted in a small loss of a-helix structure. These effects were more pronounced in the presence of the structure-promoting solvent trifluoroethanol or the natural osmolyte trimethylamine N-oxide. Fluorescence spectroscopy showed that the microenvironments of four tryptophan residues were also altered after the deletion of the Q stretch. Other structural changes were observed for the AR-NTDQ45 polypeptide after limited proteolysis; in addition, this polypeptide not only showed enhanced binding of the hydrophobic probe 8-anilinonaphthalene-1-sulphonic acid but was more sensitive to urea-induced unfolding. Taken together, these findings support the view that the presence and length of the poly-Q repeat modulate the folding and structure of the AR-NTD.