Primary structures and conformations of stonefish toxin-like toxins from three species of rabbitfish, Siganus puellus, Siganus unimaculatus, and Siganus virgatus

This study was undertaken on three species of rabbitfish, Siganus puellus, Siganus unimaculatus, and Siganus virgatus, to clarify whether stonefish toxin-like toxins are distributed in rabbitfish. The primary structures of toxins from these three species of rabbitfish were successfully elucidated by complementary DNA cloning and found to comprise two different subunits with almost the same molecular mass, i.e., 70–80 kDa. Based on the heterodimeric structure, molecular size, and amino acid sequence features, the toxins from the three species of rabbitfish were judged to be analogues of the stonefish toxins. The amino acid sequences of the toxins from S. puellus and S. unimaculatus were very similar, and distinguishable from those of S. virgatus and S. fuscescens. Homology modeling indicated that the predicted three-dimensional structures of the rabbitfish toxins are comparable to that of stonustoxin (a toxin of the stonefish Synanceia horrida), as they are composed of four domains. It is thus assumed that the rabbitfish toxins form pores in cell membranes by essentially the same mechanism as stonustoxin, although the rabbitfish toxins have alterations of some amino acid residues that are suggested to be critical for the interaction between the two subunits of stonustoxin.