The impacts of flagellin on larval metamorphosis of mussel Mytilus coruscus varied with protein structure

Flagellin produced by Pseudoalteromonas marina has been shown to induce larval metamorphosis in mussels. The flagellin protein FliC was composed of four subunits encoded by the genes fliC-02330, fliC-02345, fliC-02346, and fliC-02347. In this study, we investigated the effects of these four FliC proteins on biofilm formation and larval metamorphosis. The inducing activity of the recombinant proteins FliC-02345, FliC-02346, and FliC-02347 was similar and significantly higher (p < 0.05) than that of FliC-02330. When compared to the wild-type strain, biofilm formation was significantly enhanced in the four fliC mutants, whereas the inducing activity of four ΔfliC biofilms decreased. Among these mutants, ΔfliC-02330 exhibited the highest thickness, density, and c-di-GMP levels among its respective biofilms. The inducing activity of the ΔfliC-02330 biofilm increased by 30–40% compared to the inducing activities of other mutant strains. Furthermore, structural analysis revealed differences in protein structure and phosphorylation sites between FliC-02330 and its counterparts (FliC-02345, FliC-02346, and FliC-02347), suggesting that variations in protein structure contribute to differing impacts on both biofilm formation and larval metamorphosis. These findings provide new insights into the interaction between bacterial flagellin protein and the larval metamorphosis of marine invertebrates.