Characterization of the Roles of Intrinsically Disordered Regions from RNA-Binding Proteins in Phase Separation

Eukaryotic cells organize complex biochemical reactions through compartmentalization. While many intracellular compartments are enclosed by membranes, others are not. Messenger ribonucleoprotein (mRNP) granules are membrane-less organelles that enrich RNA and RNA-binding proteins containing intrinsically disordered regions (IDRs). I demonstrate that IDRs, coupled with RNA binding domain and RNA, can phase separate in vitro, producing dynamic liquid droplets. Over time, these droplets mature into more stable states, as assessed by slowed fluorescence recovery after photobleaching and resistance to salt. Maturation often coincides with the formation of fibrous structures. Pathological mutation within IDRs leads to the acceleration of maturation. Different disordered domains can co-assemble into phase-separated droplets. In the case of the IDR from FUS (fused in sarcoma), I show that tyrosine residues are important in promoting phase separation. Either mutation of these aromatic residues or phosphorylation of the IDR disassembles liquid droplets. I further discover that the disassembly is due to the disruption of aromatic interactions mediated by critical tyrosine residues and therefore an increase in the overall solubility of proteins. Taken together, these studies demonstrate a plausible mechanism by which interactions between IDRs, coupled with RNA binding, could contribute to mRNP granule assembly in vivo by promoting phase separation. Progression from dynamic liquids to stable fibers may be regulated to produce cellular structures with diverse physiochemical properties and functions. Misregulation of maturation could contribute to diseases that are associated with aberrant mRNP granules. Posttranslational modifications of IDRs could modulate the assembly and disassembly of mRNP granules by altering the solubility of IDRs.