Interfacial Assembly of Bacterial Microcompartment Shell Proteins in Aqueous Multiphase Systems

Abstract Compartments are a fundamental feature of life, based variously on lipid membranes, protein shells, or biopolymer phase separation. Here, this combines self‐assembling bacterial microcompartment (BMC) shell proteins and liquid‐liquid phase separation (LLPS) to develop new forms of compartmentalization. It is found that BMC shell proteins assemble at the liquid‐liquid interfaces between either 1) the dextran‐rich droplets and PEG‐rich continuous phase of a poly(ethyleneglycol)(PEG)/dextran aqueous two‐phase system, or 2) the polypeptide‐rich coacervate droplets and continuous dilute phase of a polylysine/polyaspartate complex coacervate system. Interfacial protein assemblies in the coacervate system are sensitive to the ratio of cationic to anionic polypeptides, consistent with electrostatically‐driven assembly. In both systems, interfacial protein assembly competes with aggregation, with protein concentration and polycation availability impacting coating. These two LLPS systems are then combined to form a three‐phase system wherein coacervate droplets are contained within dextran‐rich phase droplets. Interfacial localization of BMC hexameric shell proteins is tunable in a three‐phase system by changing the polyelectrolyte charge ratio. The tens‐of‐micron scale BMC shell protein‐coated droplets introduced here can accommodate bioactive cargo such as enzymes or RNA and represent a new synthetic cell strategy for organizing biomimetic functionality.