Compositional Control of Phase-Separated Cellular Bodies

Cellular bodies such as P bodies and PML nuclear bodies (PML NBs) appear to be phase separated liquids organized by multivalent interactions among proteins and RNA molecules. Although many components of various cellular bodies are known, general principles that define body composition are lacking. We have proposed a model for the formation of cellular bodies that is based on the polymerization-driven phase separation of key scaffold components of cellular bodies. We modeled cellular bodies using several engineered multivalent proteins and RNA. \textit{In vitro} and in cells these scaffold molecules form phase separated liquid droplets that are strongly enriched with the scaffold molecules. Analytical theories of polymerization suggest the resulting second phase contains large polymers of the multivalent scaffolds. Low valency client molecules partition differently into these structures depending on the stoichoimetric ratio of the scaffolds, with a sharp switch in recruitment across the phase diagram diagonal. Composition can switch rapidly through changes in scaffold concentration or valency. Natural PML NBs and P bodies show analogous partitioning behavior, suggesting how their compositions could be controlled by levels of PML SUMOylation or cellular mRNA concentration, respectively. Indeed, the engineered polySUMO/polySIM engineered scaffolds recruit many of the natural PML NB clients in a manner that depends on the SUMO:SIM stoichiometric ratio. Together, these data suggest a conceptual framework for considering the composition and control thereof of cellular bodies assembled through heterotypic multivalent interactions.