Expansion and Neofunctionalization of Actinoporin-like Genes in Mediterranean Mussel (Mytilus galloprovincialis)

Abstract

Pore-forming toxins are an important component of the venom of many animals. Actinoporins are potent cytolysins that were first detected in the venom of sea anemones; however, they are occasionally found in animals other than cnidarians and are expanded in a few predatory gastropods. Here, we report the presence of 27 unique actinoporin-like genes with monophyletic origin in Mytilus galloprovincialis, which we have termed mytiporins. These mytiporins exhibited a remarkable level of molecular diversity and gene presence-absence variation, which warranted further studies aimed at elucidating their functional role. We structurally and functionally characterized mytiporin-1 and found significant differences from the archetypal actinoporin fragaceatoxin C. Mytiporin-1 showed weaker permeabilization activity, no specificity towards sphingomyelin, and weak activity in model lipid systems with negatively charged lipids. In contrast to fragaceatoxin C, which forms octameric pores, functional mytiporin-1 pores on negatively charged lipid membranes were hexameric. Similar hexameric pores were observed for coluporin-26 from Cumia reticulata and a conoporin from Conus andremenezi. This indicates that also other molluscan actinoporin-like proteins differ from fragaceatoxin C. Although the functional role of mytiporins in the context of molluscan physiology remains to be elucidated, the lineage-specific gene family expansion event that characterizes mytiporins indicates that strong selective forces acted on their molecular diversification. Given the tissue distribution of mytiporins, this process may have broadened the taxonomic breadth of their biological targets, which would have important implications for digestive processes or mucosal immunity.

The evolutionary diversification and antimicrobial potential of MPEG1 in Metazoa

“…Phylogenetically, the MPEG1 in invertebrate are distant from that in vertebrates, implying diversified evolution of MPEG1 in invertebrate and vertebrate. Previous works have shown that the innate immune genes, such as TLR, NLR, and the pore-forming toxin actinoporin, are prone to expansion and duplication in invertebrate, as a compensation for the lack of adaptive immunity [34] , [35] , [36] . For MPEG1, its duplication in Euteleostomi led to the emergence of perforin-1 [37] .…”