Multiphase Complex Coacervate Droplets

Liquid–liquid
phase separation plays an important role in
cellular organization. Many subcellular condensed bodies are hierarchically
organized into multiple coexisting domains or layers. However, our
molecular understanding of the assembly and internal organization
of these multicomponent droplets is still incomplete, and rules for
the coexistence of condensed phases are lacking. Here, we show that
the formation of hierarchically organized multiphase droplets with
up to three coexisting layers is a generic phenomenon in mixtures
of complex coacervates, which serve as models of charge-driven liquid–liquid
phase separated systems. We present simple theoretical guidelines
to explain both the hierarchical arrangement and the demixing transition in multiphase droplets using
the interfacial tensions and critical salt concentration as inputs.
Multiple coacervates can coexist if they differ sufficiently in macromolecular
density, and we show that the associated differences in critical salt
concentration can be used to predict multiphase droplet formation.
We also show that the coexisting coacervates present distinct chemical
environments that can concentrate guest molecules to different extents.
Our findings suggest that condensate immiscibility may be a very general
feature in biological systems, which could be exploited to design
self-organized synthetic compartments to control biomolecular processes.