(716i) Partitioning and Enhanced Self-Assembly of Actin in Polypeptide Coacervates

Biomolecules exist and function in cellular microenvironments that control their spatial organization, local concentration, and biochemical reactivity. Due to the versatility and efficiency of native cytoplasm, the development of artificial bioreactors and cellular mimics to compartmentalize, concentrate, and control the local physico-chemical properties is of great interest. Building on recent successes in partitioning proteins in the crowded interior of liquid polyelectrolyte-complex coacervates, we employed polypeptide coacervates as a platform to examine the electrostatically-driven self-assembly of the ubiquitous cytoskeletal protein actin into linear filaments. We find that actin spontaneously partitions into coacervate droplets and is enriched by up to 30-fold. Actin polymerizes into micrometer-long filaments and, in contrast to the globular protein BSA, these filaments localize predominantly to the droplet periphery. We observe up to a 50-fold enhancement in the actin filament assembly rate inside coacervate droplets, consistent with the enrichment of actin within the coacervate phase. While the polycationic component of these coacervates, poly-L-lysine, is known to nucleate actin filaments in solution, we did not observe the hallmarks of lysine-mediated nucleation in coacervates, suggesting alternate mechanisms for coacervate-facilitated actin assembly. Together these results suggest that coacervates can serve as a versatile platform to localize and enrich biomolecules to study their reactivity in physiological environments.