Big defensins and mytimacins, new AMP families of the Mediterranean mussel Mytilus galloprovincialis

► Identification of macin and big defensin in Mytilus galloprovincialis. ► They exhibit a signal peptide, a positive net charge and a high content in cysteines. ► Macins are widespread in Metazoans. ► Big defensins are limited to Bivalves, Merostomata and amphioxus. Antimicrobial peptides (AMPs) play a fundamental role in the innate immunity of invertebrates, preventing the invasion of potential pathogens. Mussels can express a surprising abundance of cysteine-rich AMPs pertaining to the defensin, myticin, mytilin and mytimycin families, particularly in the circulating hemocytes. Based on deep RNA sequencing of Mytilus galloprovincialis, we describe the identification, molecular diversity and constitutive expression in different tissues of five novel transcripts pertaining to the macin family (named mytimacins) and eight novel transcripts pertaining to the big defensins family (named MgBDs). The predicted antimicrobial peptides exhibit a N-terminal signal peptide, a positive net charge and a high content in cysteines, allegedly organized in intra-molecular disulfide bridges. Mytimacins and big defensins therefore represent two novel AMP families of M. galloprovincialis which extend the repertoire of cysteine-rich AMPs in this bivalve mollusk.