Driving biomolecular droplets through polymer meshes

Biomolecular condensates are commonly thought as being formed through phase separation from a dilute bulk phase. However, inside cells, biomolecular condensates reside in crowded environments or dense polymer meshes (e.g., cytoskeleton or nucleoskeleton plus chromatin). To characterize the effects of such dense polymer meshes, here we used optical tweezers to trap and drive biomolecular droplets through the meshes formed by a synthetic polymer, Ficoll70. The movement of solid polystyrene beads requires the displacement of Ficoll70 chains and thus encounters the full viscous resistance. By contrast, due to their liquid nature, biomolecular droplets can transiently deform when passing through polymer meshes, and thus experiences reduced viscous resistance. When the polymer meshes are dense (as found at 200 g/L Ficoll), polystyrene beads, but not biomolecular droplets, are elastically coupled to the meshes. The liquid nature biomolecular droplets thus enable them to easily migrate through dense polymer meshes, which in turn may facilitate their cellular functions.