Coacervation of mussel adhesive proteins (MAPs) has been suggested as a formulation that mussels use during secretion due to its high density, no dispersion into seawater, and low interfacial tension. However, absence of anionic partner among MAPs made it difficult to explain how mussels formulate coacervates. Thus, simple coacervation, the liquid-liquid phase separation with only one component, was considered instead of complex coacervation and recent report of simple coacervate formation using fp-3S (slow variant), unique among MAPs with low charge density, supported the possibility of MAP simple coacervation. This intrigues simple coacervation of other MAPs that have high positive charge density. Limited availability of MAPs from extremely difficult extraction has restricted investigation on not only how biophysical properties of MAP simple coacervate can contribute to mussel adhesion but also how MAP simple coacervate can be formed. In this work, we used recombinant MAPs for characterization of MAP simple coacervation with great assistance from mass production. Simple coacervates of interfacial MAPs, fp-3F (fast variant) and fp-5, were successfully demonstrated, which is the first report of MAP simple coacervation other than fp-3S, to the best our knowledge. Using recombinant fp-3F, we additionally showed salt-induced simple coacervation at low pH corresponding to acidified environment of distal depression during mussel secretion. Rheological analysis confirmed viscous liquid properties and shear-thinning properties. Surface forces apparatus (SFA) analysis confirmed stronger adhesion force compared to surface-deposited protein film. In addition, possible driving forces were suggested based on the investigation of effects from Dopa, His<sub>6</sub> tag, negatively charged amino acids, temperature, and salt. Collectively, we believe that this study has broadened the scope of understanding for simple coacervation of MAPs.

Biophysical properties of salt-induced simple coacervation of interfacial mussel adhesive proteins

253rd National Meeting (2017)

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