Abstract
Protein engineering enables the creation of materials with designer functionality and tailored responsiveness. Here, we design a protein with two control motifs for its phase separation into micron sized liquid droplets - one driven by a hydrophobic domain and the other by oxidation of a disulfide bond. Our work is based on the plant surfactant protein, oleosin, which has a hydrophobic domain but no cysteines. Oleosin phase separates to form liquid droplets below a critical temperature akin to many naturally occurring membrane-less organelles. Sequence mutations are made to introduce a cysteine residue into oleosin. The addition of a cysteine causes phase separation at a lower concentration and increases the phase transition temperature. Adding a reducing agent to phase-separated, cysteine-containing oleosin rapidly dissolves the droplets. The transition temperature is tuned by varying the location of the cysteine or by blending the parent cysteine-less molecule with the cysteine-containing mutant. This provides a novel way to control protein droplet formation and dissolution. We envision this work having applications as a system for the release of a protein or drug with engineered sensitivity to reducing conditions and as a mimic of membrane-less organelles in synthetic protocells.