Regulating Shape of Biopolymer Liquid Crystal Droplets

Cells have densely packed biopolymers that self-organize through mechanisms including liquid-liquid phase separation. We investigate phase separation in mixtures of biopolymers with different rigidities. When polymers are much shorter than their persistence length, they behave like stiff filaments, while much longer than their persistence length are semiflexible or flexible filaments. Actin is a biopolymer filament whose length can be controlled by capping protein, resulting in short filaments that are known to order into a nematic liquid crystal when crowded together. Using fluorescence microscopy, we observe separation in a dense suspension of short actin filaments and flexible biopolymers. We find that actin filaments phase separate into liquid crystal droplets, with tactoid shape, against a relatively isotropic background of flexible biopolymers. We vary actin filament length and find that the tactoid aspect ratio increases with increasing actin filament length. This research could expand our understanding of the role of polymer length and rigidity in regulation of subcellular assembly shape and pattern formation at the nanoscale.