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

TLDR

Cells use both membrane-bound and proteinaceous membraneless organelles to compartmentalize molecules, but the principles governing their formation are not well understood. The study proposes that phase separation of disordered proteins with weakly interacting blocks underlies regulated membraneless organelle formation. The authors demonstrate that electrostatic interactions among disordered protein tails, modulated by temperature, ionic strength, arginine methylation, and splicing, stabilize phase‑separated organelles, and sequence motifs identify proteins common to membraneless organelles and cell adhesion. They show that Ddx4 disordered tails form phase‑separated organelles in cells and in vitro, and these organelles selectively concentrate single‑stranded DNA while excluding double‑stranded DNA.

Abstract

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.