NMR Experiments for Studies of Dilute and Condensed Protein Phases: Application to the Phase-Separating Protein CAPRIN1.

Intrinsically disordered proteins (IDPs) or regions of intrinsic disorder in otherwise folded proteins (IDRs) play important roles in many different biological processes, including formation of biological condensates via liquid-liquid phase separation. NMR spectroscopy is a powerful tool for obtaining site-specific structural and dynamical information on IDPs/IDRs, and recent efforts have focused on the development of experiments for atomic-resolution studies of these molecules. These include triple-resonance experiments that are based on ¹³ CO-direct detection of magnetization, exploiting increased sensitivity of cryogenically cooled probes. In order to evaluate the different classes of experiment for studies of IDRs or IDPs in both dilute and phase-separated environments, in particular at neutral and higher pHs where many of these proteins phase separate, we compared ¹³ CO-detect versus ¹ H ^α -detect experiments, showing that significant sensitivity gains are achieved via proton detection under the conditions of our experiments. A suite of ¹ H ^α -detect experiments was subsequently developed for studies of IDPs/IDRs and applied to the dilute phase of a 103-residue disordered region of CAPRIN1 that phase separates at neutral pH. Residue-specific chemical shifts derived from our study enable the accurate prediction of the importance of the N-terminal Arg-containing region of this construct for promoting phase separation relative to other Arg-rich stretches of sequence, subsequently confirmed by mutagenesis. Our study emphasizes that the sequence positions of key residues can be a critical factor in controlling phase separation and highlights the unique role of NMR in establishing the relations between amino acid sequence and phase-separation propensity.