Relationship Between Filament-Polymerizing Muscle Myosin and Droplets Generated by Liquid-Liquid Phase Separation

From individual living cells to the tissues and organs of multicellular organisms, the internal fluids contain numerous macromolecules such as nucleic acids and proteins. Liquid-liquid phase separation (LLPS) is proving to be useful in understanding macromolecular-mediated phenomena that occur within cells, which have been hard to explain through conventional interactions between biological factors. This LLPS is applicable not only to phenomena at scales equivalent to cells but also to those at much larger scales. We studied the association between the behaviors of myosin, a representative molecular motor isolated from muscle, and LLPS in a binary polymer solution using polyethylene glycol and dextran. Myosin localized in the droplets of the dextran-rich phase and polymerized to filaments and formed larger assemblies regardless of whether the salt strength did or did not allow polymerization. Those assemblies resembled a coarse mesh of tangled myosin filaments. Conversely, when myosin was inside, the droplets deformed into a non-spherical morphology. Notably, at a salt strength where myosin normally polymerizes to filaments, some of the deformed droplets even produced sharp protrusions. These findings suggest that not only LLPS modifies the behavior of myosin but also, conversely, myosin affects the nature of the droplets formed by LLPS.