Probing the surface charge of condensates using microelectrophoresis

Biomolecular condensates play an important role in cellular organization. Coacervates are commonly used as model systems that mimic the physiochemical properties of biomolecular condensates, and are simultaneously studied as protocell models. The surface of condensates plays a key role in governing the exchange of molecules between condensates and their surroundings, the accumulation of species at the interface, and the stability of condensates against coalescence. However, most important surface properties, including the surface charge, remain poorly characterized and understood. The surface potential of coacervates is often measured using laser doppler electrophoresis, which assumes a size-independent electrophoretic mobility. Here, we show that this assumption is incorrect for condensates and present an alternative method to study the electrophoretic mobility of coacervates by microelectrophoresis and single-particle tracking, using electric fields of 1-15 V/cm. Coacervates have a size-dependent electrophoretic mobility, originating from their fluid nature, from which a well-defined zeta potential is calculated. We demonstrate the performance of our methodology on several complex coacervates and condensate models, which all exhibit size-dependent mobility. Interestingly, microelectrophoresis measurements revealed that polylysine chains are enriched at the surface of polylysine/polyaspartic acid complex coacervates, which causes the negatively charged protein ɑ-synuclein to adsorb and accumulate at the interface. Addition of ATP was able to invert the surface charge and displace the adsorbed ɑ-synuclein from the surface, which could suppress its interface-catalyzed aggregation. Together, these findings show how condensate surface charge can be measured and affected, making this microelectrophoresis platform combined with automated single-particle tracking a promising characterization technique for both biomolecular condensates and coacervate protocells.