The multigene families of actinoporins (part I): Isoforms and genetic structure

Actinoporins are basic pore-forming proteins produced by sea anemones, with molecular weight around 20 kDa showing high affinity for sphingomyelin-containing membranes. Most sea anemones produce more than one actinoporin isoform differing in isoelectric point, molecular weigth and cytolytic activity. Examples of sea anemones with actinoporin isoforms are: Actinia equina with at least five isoform genes; Actinia tenebrosa, three isoforms; Actinia fragacea, five isoforms; Actineria villosa, Phyllodiscus semoni, Stichodactyla helianthus and Oulactis orientalis, with two isoforms each one, and Heteractis crispa with twenty-four isoforms. Additionally, thirty-four different amino acid sequences were deduced from fifty-two nucleotide sequences of Heteractis magnifica toxins suggesting the presence of a large number of isoforms or allelic variants. Many amino acidic changes in the isoforms are located in important regions for pore formation. The genetic structure of actinoporins comprises a pre-propeptide and a mature toxin region; therefore, actinoporins could be synthetized in the Golgi apparatus as precursor forms. The subsequent maturation of the toxins involves a proteolytic processing during secretion. Here we hypothesize that sea anemones could have suffered duplication, conversion and mutation of genes that produced multigene families as an efficient response to evolutionary pressure, leading to successful strategies of predatory and defensive function.