Investigating the complexity of tyrosinase(s) in the adhesive system of blue mussels

The blue mussel, Mytilus edulis, uses its foot to secrete numerous protein threads collectively known as byssus, each terminating in a plaque that allows adhesion to the substrate. This byssus enables the animal to resist the hydrodynamic forces of waves in the intertidal zone and is considered one of the most powerful adhesive secretions among marine invertebrates, which explains why mussel adhesion has been studied for several decades. It is well established that the blue mussel has a catechol-based adhesive system. Catechol groups are produced by the enzymatic post-translational hydroxylation of tyrosine residues into DOPA (3,4-dihydroxy-L-phenylalanine) within the byssal adhesive proteins. DOPA has crucial roles in both interfacial adhesive and bulk cohesive interactions within their adhesive secretions. However, the enzymes responsible for its biosynthesis, the tyrosinases, have been poorly studied. Tyrosinases are oxygen-transferring, copper metalloproteins that catalyse the o-hydroxylation of monophenols (e.g., tyrosine) into o-diphenols (e.g., DOPA) and further oxidise o-diphenols into o-quinones. Our research integrates foot transcriptomics with mass spectrometry analyses of byssal threads to identify the tyrosinase enzyme(s) potentially involved in DOPA synthesis in the adhesive proteins of the blue mussel. Using this approach, several tyrosinases have been identified. Their role in byssus formation was subsequently confirmed through their localisation in the foot glands via in situ hybridization. In parallel, proteomics confirmed the presence of one of the tyrosinase candidates in the vesicles of the plaque gland. Therefore, to gain deeper insights into this candidate, this enzyme was recombinantly produced. Its enzymatic activity would be evaluated by measuring DOPA production in comparison with the well-established mushroom tyrosinase. Our investigations contribute to our understanding of the molecular basis of adhesion in marine organisms, particularly helping to advance innovative DOPA-based adhesive materials.