Evolution and molecular diversity of mytilin-like defense peptides in marine mussels

The cysteine –stabilized alpha/beta motif (CS-αβ) is a widespread structural scaffold found in several low-molecular weight cationic peptides with defense function. While many CS-αβ antimicrobial peptides have been described as major players in bivalve immune response, several aspects concerning their evolution are still unclear and a clear-cut definition of the relationship among defensins, mytilins, myticins and other structurally similar peptides is still lacking. Here, we redefine the distribution of mytilin-like CS-αβ peptides thanks to a bioinformatic screening of the sequence resources available for Mytilida. We highlight that, in spite of limited primary sequence similarity, these AMPs retain a nearly identical three-dimensional folding, stabilized by eight highly conserved cysteine residues. We discuss that the variable position of the C1-C5 disulfide bond has a significant effect on the structural flexibility of the mytilins identified in Perna spp., as well as in a few novel Mytilus spp. mytilins. We further describe the organization of the Mytilus galloprovincialis mytilin gene cluster, reporting the presence of two pseudogenes in addition to the four canonical genes previously identified, and discuss the presence of additional dispensable mytilin genes subject to presence/absence variation.