Polyglutamine diseases are a devastating class of inherited, neurodegenerative disorders, with protein misfolding as a hallmark. Spinobulbar muscular atrophy (SBMA) and Huntington's disease (HD) are two distinct polyglutamine diseases caused by an expanded polyglutamine tract in the androgen receptor (AR) or huntingtin (Htt) protein, respectively. An expanded polyglutamine tract is thought to induce protein misfolding, aggregation, and toxicity. N-terminal fragments of AR or Htt, which contain the polyglutamine stretch, are believed to be the toxic species in these diseases. It has previously been reported that actin-regulatory pathways can modulate intracellular aggregation of N-terminal fragments of AR and Htt. Sequences that flank the polyglutamine tract of AR and Htt might influence protein aggregation and toxicity through direct protein-protein interactions, but this has not been studied in detail.

In this study, we have evaluated an N-terminal 127 amino acid fragment of AR (ARN127) or Htt exon 1. We find that the first 50 amino acids of ARN127 (ARN50) and the first 14 amino acids of Htt exon 1 (HttN14) directly mediate binding to filamentous actin (F-actin) in vitro. ARN127 and Htt exon 1 bind to F-actin in a saturable manner, with affinities that range between 1-2µM. ARN50 and HttN14 are both enriched in Triton-insoluble fractions of cell lysates, indicating tight interactions with membranes and/or large cytoskeletal proteins. ARN50 and HttN14 promote the formation of polyglutamine inclusions and deletion of these actin-binding regions renders the polyglutamine-expanded forms of ARN127 and Htt exon 1 to become more soluble. ARN50 is required for sensitivity to actin-regulatory compounds, regulates the type, number, and distribution of polyglutamine inclusions, and promotes SDS-insolubility of aggregates. Though HttN14 does not appear to drastically regulate the types of inclusions formed, it too promotes a biochemically distinct SDS-insoluble form of aggregate. The F-actin binding regions on ARN127 and Htt exon 1 thus appear to alter the aggregation frequency and type of polyglutamine-induced aggregation. These findings highlight the importance of protein interactions via flanking sequences in determining the propensity of unstable proteins to misfold.