Revealing nanoscale structure and interfaces of protein and polymer condensates via cryo-electron microscopy

Abstract

Liquid-liquid phase separation (LLPS) is a ubiquitous demixing phenomenon observed in various molecular solutions, including in polymer and protein solutions. Demixing of solutions results in condensed, phase separated droplets which exhibit a range of liquid-like properties driven by transient intermolecular interactions. Understanding the organization within these condensates is crucial for deciphering their material properties and functions. This study introduces a streamlined methodology for imaging three different classes of condensates, proteins, complex coacervates, and non-ionic block copolymer coacervates, using cryo-electron microscopy (cryo-EM). The method involves initiating condensate formation on electron microscopy grids to control droplet size and stage in the phase separation process. Results from cryo-EM reveal distinct nanoscale networks and interfaces in the condensate samples. We further investigate these structures using cryo-electron tomography which provides 3D reconstructions, uncovering porous internal structures, unique core-shell morphologies, and inhomogeneities within the nanoscale organization of protein condensates. Comparison with dry-state transmission electron microscopy emphasizes the importance of preserving the hydrated structure for accurate structural analysis. We correlate the internal structure of condensates with their material properties by performing viscosity measurements that support that more viscous condensates exhibit denser internal assemblies. Our findings contribute to a comprehensive understanding of condensate structure and this streamlined methodology provides a versatile tool for exploring various phase-separated systems and their nanoscale structures.