Analysis of the conformation of the androgen receptor in spinal bulbar muscular atrophy by atomic force microscopy.

Spinal bulbar muscular atrophy (SBMA) (also known as Kennedy's disease) is a motor degenerative disease caused by an amplification of the polyglutamine stretch at the N-terminus of the human androgen receptor (AR). Amplifications larger than 40 glutamine residues are thought to lead to the disease. A characteristic feature of this disease is a ligand-dependent misfolding and aggregation of the mutant receptor that lead to the death of motor neurons. Initially, large cytoplasmic and nuclear aggregates reaching sizes of 6 μm were thought to be the pathogenic agents. Later studies have suggested that oligomeric species with sizes of less than 1 μm that occur prior to the formation of the larger aggregates are the toxic agents. However, there have been disagreements regarding the shape of these oligomers, as most studies have been carried out with peptide fragments of the androgen receptor containing different lengths of polyglutamine stretch. We have isolated the wild-type AR with a polyglutamine stretch of 22 (ARQ22) and a mutant receptor with a stretch of 65 (ARQ65) using a baculovirus system and have analyzed the oligomeric structures formed by these receptors with atomic force microscopy. This method has allowed us to determine the conformations of the full-length wild-type and mutant AR and revealed the conformation of the mutant AR that causes SBMA.