Liquid-Liquid Phase Separation Enhances the Self-Assembly of Viral Capsids*

In many virus families, assembly of the capsid and/or genome packaging occurs within liquid-liquid phase-separated (LLPS) ‘membrane-less compartments’ known as replication sites, Negri bodies or viro-plasms. Capsid proteins and other assembly components preferentially partition into these compartments, leading to high local concentrations and thus potentially facilitating faster and more efficient assembly. However, while LLPS has been the subject of intense research, its effects on self-assembly behaviors have yet to be understood in detail. Here, we use rate equations and a coarse-grained computational model to study the effect of attractive compartments formed by LLPS on capsid assembly thermodynamics and kinetics. We demonstrate that preferential partitioning of assembly components into phase- separated compartments leads to a larger achievable yield, robust assembly over a wider range of binding affinities and concentrations, and assembly rates that are increased by orders of magnitude in comparison to assembly in bulk solution.  However, we also identify regimes where assembly is misdirected to malformations or rapid nucleation leads to monomer starvation kinetic traps. The results identify advantages and limitations for natural viruses or synthetic assembly systems to exploit LLPS to control assembly behaviors.