Phase Separation of Heterogeneous Nuclear Ribonucleoprotein A1 upon Specific RNA‐Binding Observed by Magnetic Resonance

Interaction of heterogeneous nuclear ribonucleoprotein A1 (hnRNP A1) with specific single‐stranded RNA and its relation to liquid‐liquid phase separation (LLPS) were studied in vitro by magnetic resonance based on site‐directed spin labelling. An ensemble model of dispersed hnRNP A1 in the absence of RNA was derived from distance distributions between spin labelled sites and small angle X‐ray scattering. This model revealed a compact state of the low‐complexity domain and its interaction with the RNA recognition motifs. Paramagnetic relaxation enhancement NMR spectroscopy confirmed this interaction. Addition of RNA to dispersed hnRNP A1 induced liquid‐droplet formation. Such LLPS depended on RNA concentration and sequence, with continuous wave EPR spectroscopy showing an influence of RNA point mutations on local protein dynamics. We propose that an interplay of sequence‐specific RNA binding and LLPS contributes to regulation of specific RNA segregation during stress response.