Musashi-1: An Example of How Polyalanine Tracts Contribute to Self-Association in the Intrinsically Disordered Regions of RNA-Binding Proteins

RNA-binding proteins (RBPs) have intrinsically disordered regions (IDRs) whose biophysical properties have yet to be explored to the same extent as those of the folded RNA interacting domains. These IDRs are essential to the formation of biomolecular condensates, such as stress and RNA granules, but dysregulated assembly can be pathological. Because of their structural heterogeneity, IDRs are best studied by NMR spectroscopy. In this study, we used NMR spectroscopy to investigate the structural propensity and self-association of the IDR of the RBP Musashi-1. We identified two transient α-helical regions (residues ~208-218 and ~270-284 in the IDR, the latter with a polyalanine tract). Strong NMR line broadening in these regions and circular dichroism and micrography data suggest that the two α-helical elements and the hydrophobic residues in between may contribute to the formation of oligomers found in stress granules and implicated in Alzheimer's disease. Bioinformatics analysis suggests that polyalanine stretches in the IDRs of RBPs may have evolved to promote RBP assembly.