Phase separation and ageing of glycine-rich protein from tick adhesive.

Hard ticks feed on their host for multiple days. To ensure firm attachment, they secrete a protein-rich saliva that eventually forms a solid cement cone. The underlying mechanism of this liquid-to-solid transition is currently not understood. This study focuses on the phase transitions of a disordered glycine-rich protein (GRP) found in tick saliva. We show that GRP undergoes liquid-liquid phase separation via simple coacervation to form biomolecular condensates in salty environments. Cation-π and π-π interactions mediated by periodically placed arginine and aromatic amino-acid residues are the primary driving forces that promote phase separation. Interestingly, GRP condensates exhibit ageing by undergoing liquid-to-gel transition over time and exhibit adhesive properties, similar to the naturally occurring cement cone. Finally, we provide evidence for protein-rich condensates in natural tick saliva. Our findings provide a starting point to gain further insights into the bioadhesion of ticks, to develop novel tick control strategies, and towards achieving biomedical applications such as tissue sealants.