Simultaneous induction of distinct protein phase separation events in multiple subcellular compartments of a single cell.

Intracellular protein crystallization occurs under various physiological and pathological settings, yet the underlying cellular processes remain enigmatic. After validating individual crystallization events using cellular proteins that readily crystallize in the ER (NEU1), cytosol (crystallin-γD mutant) or nucleus (CLC protein), I demonstrate three independent crystallization events can take place concurrently in different subcellular compartments of a single cell without compromising cell viability. By co-expressing NEU1 and previously reported two human monoclonal antibodies that undergo crystallization and liquid-liquid phase separation in the ER, I additionally demonstrate two independent phase separation events can be simultaneously induced in the ER lumen of a single cell without mixing or interfering each other's phase separation behaviors. Intracellular protein crystallization thus takes place in a crowded physiological cellular environment and does not require high protein purity. Furthermore, I report a simple method to increase the yield of intracellular protein crystals by treating the cells with a topoisomerase II inhibitor that blocks cell division without preventing cell size growth. This study not only presents accessible model tools for studying cryptic in vivo protein crystallization events, but also paves a way toward establishing the intracellular protein crystallization as a novel platform for recombinant protein expression and purification.