Molecular characterization and immunological functions of the antimicrobial peptide theromacin from razor clam Sinonovacula constricta.

Antimicrobial peptides are natural immune effector molecules that protect the host from pathogenic microorganisms and play crucial roles in the innate immune system. Herein, the functions of theromacin from the razor clam Sinonovacula constricta (ScTM) were identified. Multiple sequence alignment revealed that the structure of ScTM was similar to that of a theromacin homolog with a signal peptide and conserved Macin domain. Tissue distribution analysis indicated that ScTM mRNA was expressed in all tested tissues. Moreover, ScTM transcripts were upregulated in hemocytes and hepatopancreas after challenge with Vibrio parahaemolyticus. Importantly, interfering with ScTM expression through siRNA transfection reduced hemocyte clearance ability and razor clam survival rates. Further functional analysis indicated that recombinant ScTM protein possessed considerable binding abilities to pathogens and PAMPs, showing the strongest binding ability to LPS. Minimum inhibitory concentration assays revealed that rScTm exerted stronger antimicrobial activities against Gram-negative bacteria than against Gram-positive bacteria. Moreover, rScTM exhibited antimicrobial activity against V. parahaemolyticus by increasing extracellular membrane permeability, impairing inner membrane integrity, and binding to genomic DNA, leading to cytoplasmic efflux and bacterial death. All of the results of this work provide a new perspective for the development of an effective strategy to control razor clam bacterial infection.