Primary structure of echotoxin 2, an actinoporin-like hemolytic toxin from the salivary gland of the marine gastropod Monoplex echo.

Echotoxins are 25 kDa proteins with both hemolytic and lethal activities, previously purified from the salivary gland of the marine gastropod Monoplex echo. In this study, a cDNA encoding echotoxin 2 was cloned by RT-PCR, 3'-RACE and 5'-RACE, based on its partial amino acid sequence. The full-length echotoxin 2 cDNA (1000 bp) obtained contains an open reading frame (825 bp) coding for a precursor protein of 274 amino acid residues. Mature echotoxin 2 composed of 226 amino acid residues is assumed to be produced by post-translational removal of N-terminal 23 residues (predicted as a signal peptide) and C-terminal 25 residues from the precursor protein. Very interestingly, a homology search revealed that echotoxin 2 is analogous to actinoporins, 20 kDa pore-forming hemolysins reported from various sea anemones. In addition to the similarities in biological activity, molecular size and basicity between echotoxin 2 and actinoporins, two prominent structural features, an N-terminal amphiphilic alpha-helix and an aromatic patch comprising Trp and Tyr residues, both of which are important for the pore-forming activity of actinoporins, are also recognized in echotoxin 2. However, echotoxin 2 is distinguishable from actinoporins in having Cys residues and lacking an RGD motif.