The stability of NPM1 oligomers regulated by acidic disordered regions controls the quality of liquid droplets

A nucleolus is a typical membrane-less nuclear body that is formed by liquid-liquid phase separation (LLPS) of its components. A major component that drives LLPS in the nucleolus is nucleophosmin (NPM1). The oligomer formation and inter-oligomer interactions of NPM1 are suggested to cooperatively contribute to the induction of LLPS. However, the molecular mechanism of how the quality of the liquid droplets formed by NPM1 is regulated is currently not well understood. In this manuscript, we revealed the regulatory mechanism of NPM1 oligomer formation and its relationship with the ability to form liquid droplets. Molecular dynamics simulations and mutant protein analyses suggest that the acidic amino acids in the N-terminal and central disordered regions of NPM1 disturb the key interactions between monomers. We also demonstrate that mutants with attenuated oligomer stability form liquid droplets as do the wild-type; the fluidity of the formed liquid droplets was greater than that of the wild-type. These results suggest that the stability of NPM1 oligomers is a critical determinant of liquid droplet quality. Furthermore, we observed that when the net negative charges in the acidic disordered regions were increased by phosphomimetic mutations at Ser125, the NPM1 oligomer stability decreased, which increased the fluidity of the liquid droplets. Our results provide a novel mechanistic insight into how nucleolar dynamics are regulated during the cell cycle.