Phase transitions in the assembly of multivalent signalling proteins

The mechanisms by which the ångström-scale molecular properties of cells are translated to micrometre-scale macroscopic properties have not been well understood, but this study shows that when multivalent proteins interact with each other, they undergo a switch-like phase separation, which is concomitant with a transition from small complexes to huge polymeric assemblies, as the concentration increases. The translation of molecular-scale structures into the macroscopic world of organelles and tissues is a little-understood aspect of cellular organization. Here, Michael Rosen and colleagues show that when multivalent proteins interact with each other, they undergo a switch-like phase transition from small complexes to huge polymeric assemblies as concentration increases. At the same time, they undergo a macroscopic liquid–liquid phase separation. This produces micrometre-sized suspended liquid droplets that resemble cellular structures such as P bodies, P granules and Cajal bodies. Such switch-like phase separations and transitions from small complexes to large assemblies may be a general feature of interactions between multivalent molecules. Cells are organized on length scales ranging from ångström to micrometres. However, the mechanisms by which ångström-scale molecular properties are translated to micrometre-scale macroscopic properties are not well understood. Here we show that interactions between diverse synthetic, multivalent macromolecules (including multi-domain proteins and RNA) produce sharp liquid–liquid-demixing phase separations, generating micrometre-sized liquid droplets in aqueous solution. This macroscopic transition corresponds to a molecular transition between small complexes and large, dynamic supramolecular polymers. The concentrations needed for phase transition are directly related to the valency of the interacting species. In the case of the actin-regulatory protein called neural Wiskott–Aldrich syndrome protein (N-WASP) interacting with its established biological partners NCK and phosphorylated nephrin1, the phase transition corresponds to a sharp increase in activity towards an actin nucleation factor, the Arp2/3 complex. The transition is governed by the degree of phosphorylation of nephrin, explaining how this property of the system can be controlled to regulatory effect by kinases. The widespread occurrence of multivalent systems suggests that phase transitions may be used to spatially organize and biochemically regulate information throughout biology.