Phase Transition of a Disordered Nuage Protein Generates Environmentally Responsive Membraneless Organelles

Cells chemically isolate molecules in compartments to both facilitate and regulate their interactions. In addition to membrane-encapsulated compartments, cells can form proteinaceous and membraneless organelles, including nucleoli, Cajal and PML bodies, and stress granules. The principles that determine when and why these structures form have remained elusive. Here, we demonstrate that the disordered tails of Ddx4, a primary constituent of nuage or germ granules, form phase-separated organelles both in live cells and in vitro. These bodies are stabilized by patterned electrostatic interactions that are highly sensitive to temperature, ionic strength, arginine methylation, and splicing. Sequence determinants are used to identify proteins found in both membraneless organelles and cell adhesion. Moreover, the bodies provide an alternative solvent environment that can concentrate single-stranded DNA but largely exclude double-stranded DNA. We propose that phase separation of disordered proteins containing weakly interacting blocks is a general mechanism for forming regulated, membraneless organelles. [Display omitted] •Intrinsically disordered N terminus of Ddx4 forms organelles in cells and in vitro•Phase transition to form organelles is driven by electrostatic interactions•Methylation, ionic strength, and temperature changes can dissolve the organelles•Sequence determinants of formation are common in membraneless organelle proteins Nott et al. demonstrate that a single protein constituent can reversibly form membraneless organelles both in vitro and in cells. The bodies provide an alternative solvent environment that concentrates single-stranded, but exclude double-stranded DNA. We propose that phase separation of disordered proteins is a general mechanism for forming regulated, membraneless organelles.