Multivalent Electrostatic Interactions and Autoinhibition Underlie Phase Separation of Npm1 with R-rich Nucleolar Protein Surf6

The nucleolus, the largest membrane-less organelle, is critical to cellular functioning as the site of ribosome biogenesis and as a fundamental cell stress sensor. It has previously been shown that the nucleolus assembles via liquid-liquid phase separation of its components from the surrounding nucleoplasm. Here, we show that two abundant nucleolar proteins often upregulated in certain cancers, Nucleophosmin (NPM1) and Surfeit locus protein 6 (Surf6), interact via multivalent electrostatic interactions and form phase-separated droplets. Utilizing microscopic techniques, we propose a novel interaction of the C-terminal nucleic acid binding domain of NPM1 to modulate self valency through autoinhibition of its acidic residues.