Junctional Role of Anionic Domain of Mussel Foot Protein Type 4 in Underwater Mussel Adhesion

Mussel byssi form a robust underwater adhesive system, anchoring to various surfaces in harsh marine environments. Central to byssus is foot protein type 4 (fp-4), a junction protein connecting collagenous threads to proteinaceous plaque. This study investigated an anionic plaque-binding domain of fp-4 (fp-4a) and its interactions with cationic foot proteins (fp-1, fp-5, and fp-151 as model substitutes for fp-2) and metal ions (Ca²⁺, Fe³⁺, and V³⁺). Aggregation, a liquid-solid phase transition, was confirmed for recombinant fp-4a (rfp-4a) with rfp-5, rfp-151, and metal ions using turbidity measurements and microscopy. Molecular cohesion forces were measured by the surface forces apparatus, while dynamic light scattering, circular dichroism spectroscopy, and chaotropic agent assay clarified the aggregation mechanisms. Collectively, we discovered that rfp-4a formed aggregates with cationic rfps through electrostatic interactions and hydrogen bonding, further stabilized by metal ion incorporation, emphasizing its critical role in mussel adhesion systems and its potential for bioadhesive applications.