Biomolecular Condensation and L-Cysteine Signaling Activates Dormant Protease Activity of Papain Droplets

Proteases are an important class of proteolytic enzymes having great importance in both basic science and industrial applications. While cells tightly regulate the spatio-temporal activity of different proteases for cellular homeostasis, mis-regulation often leads to adverse effects. In this context, the protease activity of papain and its activation by L-cysteine is poorly understood in the literature. Herein, we discover that the protease activity of papain can be effectively regulated via a spontaneous liquid-liquid phase separation (LLPS) pathway. We show that papain undergoes biomolecular condensation via spontaneous LLPS under macromolecular crowding through the involvement of intermolecular hydrophobic interactions. Secondary structure analyses revealed a compact conformation of phase-separated papain with increased α -helix content. Although native free papain is found to be active towards synthetic and protein substrates, the proteolytic digestion produces heterogeneous peptide aggregates. In contrast, we found that papain droplets remain dormant toward protein digestion due to the disulfide linkage of the active cysteine residue (Cys-25) in its compact conformational state. More importantly, we show that the protease activity of phase-separated papain can be reactivated in the presence of L-cysteine to produce uniform soluble peptide fragments. Our findings indicate that although disulfide linkages are not necessary for the phase separation of papain, upon phase separation, intermolecular interactions between phase-separated papain result in the formation of disulfide linkages involving active Cys-25 residues. The present discovery has tremendous technological importance to boost the efficacy of meat tenderization in the food industry.