The venom gland transcriptome of the Desert Massasauga Rattlesnake (Sistrurus catenatus edwardsii): towards an understanding of venom composition among advanced snakes (Superfamily Colubroidea)

Abstract

BackgroundSnake venoms are complex mixtures of pharmacologically active proteins and peptides which belong to a small number of superfamilies. Global cataloguing of the venom transcriptome facilitates the identification of new families of toxins as well as helps in understanding the evolution of venom proteomes.ResultsWe have constructed a cDNA library of the venom gland of a threatened rattlesnake (a pitviper), Sistrurus catenatus edwardsii (Desert Massasauga), and sequenced 576 ESTs. Our results demonstrate a high abundance of serine proteinase and metalloproteinase transcripts, indicating that the disruption of hemostasis is a principle mechanism of action of the venom. In addition to the transcripts encoding common venom proteins, we detected two varieties of low abundance unique transcripts in the library; these encode for three-finger toxins and a novel toxin possibly generated from the fusion of two genes. We also observed polyadenylated ribosomal RNAs in the venom gland library, an interesting preliminary obsevation of this unusual phenomenon in a reptilian system.ConclusionThe three-finger toxins are characteristic of most elapid venoms but are rare in viperid venoms. We detected several ESTs encoding this group of toxins in this study. We also observed the presence of a transcript encoding a fused protein of two well-characterized toxins (Kunitz/BPTI and Waprins), and this is the first report of this kind of fusion in a snake toxin transcriptome. We propose that these new venom proteins may have ancillary functions for envenomation. The presence of a fused toxin indicates that in addition to gene duplication and accelerated evolution, exon shuffling or transcriptional splicing may also contribute to generating the diversity of toxins and toxin isoforms observed among snake venoms. The detection of low abundance toxins, as observed in this and other studies, indicates a greater compositional similarity of venoms (though potency will differ) among advanced snakes than has been previously recognized.

Common evolution of waprin and kunitz-like toxin families in Australian venomous snakes

“…Indeed, the three dimensional conformation and the presence of a number of N-terminal positively charged amino acid residues have previously been shown to be important for the antimicrobial activities of Omwaprin [17]. A recent report of a fused toxin protein cloned from the venom gland of the viper Sistrurus catenatus edwardsii (Desert Massasauga rattlesnake, DQ464286) demonstrates some sequence homology to the Australian elapid waprins [33]. This clone contains a leader sequence which shares 83 % identity to the propeptide sequence reported here, followed by a fusion of two sequences that have homology to a kunitz-type inhibitor and a waprin, respectively.…”

Venom evolution through gene duplications

“…Gene fusion events may be mediated by trans-splicing (splicing events between genes) or unequal crossing-over and can result in the creation of novel genes with unknown function. For instance, a novel EST containing two discrete and well-characterized toxin gene family domains, Kunitz and waprin, has been identified in the venom gland transcriptome of the desert rattlesnake (Pahari et al, 2007). The protein sequence of this transcript has not been isolated but both Kunitz and waprin domains are capable of inhibiting proteases.…”

Structural and Functional Characterization of a Novel Homodimeric Three-finger Neurotoxin from the Venom of Ophiophagus hannah (King Cobra)

“…They are most commonly found in the venoms of elapid and hydrophiid snakes. Recently, our laboratory has also demonstrated the presence of 3FTxs from colubrid venoms (12,13), and 3FTx transcripts have been found in the venom gland transcriptome of viperid snakes (14,15). The proteins in this family of toxins share a common structural scaffold of three ␤-sheeted loops emerging from a central core (11,16).…”