Expansion and neofunctionalization of actinoporin-like genes in Mediterranean mussel (Mytilus galloprovincialis)

Littoporins: Novel actinoporin-like proteins in caenogastropod genus Littorina

In the course of searching for genes controlling the immune system in caenogastropod mollusks, we characterized and phylogenetically placed five new actinoporin-like cytolysins expressed in periwinkles of the genus Littorina. These newly discovered proteins, named littoporins (LitP), contain a central cytolysin/lectin domain and exhibit a predicted protein fold that is almost identical to the three-dimensional structures of actinoporins. Two of these proteins, LitP-1 and LitP-2, were found to be upregulated in L. littorea kidney tissues and immune cells in response to natural and experimental infection with the trematode Himasthla elongata, suggesting their potential role as perforins in the systemic anti-trematode immune response. The primary sequence divergence of littoporins is hypothesized to be attributed to the taxonomic range of cell membranes they can recognize and permeabilize.

The evolutionary diversification and antimicrobial potential of MPEG1 in Metazoa

Macrophage-expressed gene 1 (MPEG1) is an ancient immune effector known to exist in Cnidaria, Mollusca, Actinopterygii, and Mammalia. In this study, we examined the evolution and antibacterial potential of MPEG1 across Metazoa. By unbiased data-mining, MPEG1 orthologs were found in 11 of 34 screened phyla. In invertebrates, MPEG1 is present in the major phyla and exhibits intensive duplication. In vertebrates, class-based clades were formed by the major, generic MPEG1 (gMPEG1) in each class. However, there is a minority of unique MPEG1 (uMPEG1) from 71 species of 4 classes that clustered into a separate clade detached from all major class-based clades. gMPEG1 and uMPEG1 exhibit strong genomic collinearity and are surrounded by high-density transposons. gMPEG1 and uMPEG1 transcript expressions were most abundant in immune organs, but differed markedly in tissue specificity. Systematic analysis identified an antimicrobial peptide (AMP)-like segment in the C-terminal (CT) tail of MPEG1. Peptides based on the AMP-like regions of 35 representative MPEG1 were synthesized. Bactericidal activities were displayed by all peptides. Together these results suggest transposon-propelled evolutionary diversification of MPEG1 in Metazoa that has likely led to functional specialisation. This study also reveals a possible antimicrobial mechanism mediated directly and solely by the CT tail of MPEG1.