Accelerated exchange of exon segments in Viperid three-finger toxin genes (Sistrurus catenatus edwardsii; Desert Massasauga)

Abstract

BackgroundSnake venoms consist primarily of proteins and peptides showing a myriad of potent biological activities which have been shaped by both adaptive and neutral selective forces. Venom proteins are encoded by multigene families that have evolved through a process of gene duplication followed by accelerated evolution in the protein coding region.ResultsHere we report five gene structures of three-finger toxins from a viperid snake, Sistrurus catenatus edwardsii. These toxin genes are structured similarly to elapid and hydrophiid three-finger toxin genes, with two introns and three exons. Both introns and exons show distinct patterns of segmentation, and the insertion/deletion of segments may define their evolutionary history. The segments in introns, when present, are highly similar to their corresponding segments in other members of the gene family. In contrast, some segments in the exons show high similarity, while others are often distinctly different among corresponding regions of the isoforms.ConclusionOrdered, conserved exon structure strongly suggests that segments in corresponding regions in exons have been exchanged with distinctly different ones during the evolution of these genes. Such a "switching" of segments in exons may result in drastically altering the molecular surface topology and charge, and hence the molecular targets of these three-finger toxins. Thus the phenomenon of accelerated segment switch in exons to alter targeting (ASSET) may play an important role in the evolution of three-finger toxins, resulting in a family of toxins with a highly conserved structural fold but widely varying biological activities.

Evolution of Conus Peptide Genes: Duplication and Positive Selection in the A-Superfamily

“…A second hypothesis would involve recombination events between different genes, resulting in a sequence with a α 4/4 pattern for the mature sequence but a signal and propeptide sequences similar to the α 4/7 ones (as for the Bu1.3 toxin). Such events have already been reported in literature for multi-gene families including toxin genes (e.g., Doley 2008). Finally, convergent evolution, although not common between closely related genes within a single species, may also explain the results obtained.…”

Role of accelerated segment switch in exons to alter targeting (ASSET) in the molecular evolution of snake venom proteins

“…These transcripts showed very low sequence similarity with elapid 3FTxs except for the conserved signal peptide and the number and position of cysteines. A systematic comparison of their sequences revealed that some of the segments in the mature proteins were 80–100% identical, whereas other segments were only 12.5–50% similar [ 46 ]. Some segments in the protein coding region seem to be exchanged with distinctly different segments, keeping the structural fold intact during their evolution.…”

Venom gland transcriptomics for identifying, cataloging, and characterizing venom proteins in snakes

“…Using the cDNA of venom gland tissue from S. c. edwardsii (desert massasauga), gene structures of viperid 3FTxs were reported for the first time (Doley et al, 2008a). Based on gene sequences, viperid 3FTxs appear to have evolved through a phenomenon called accelerated segment switch in exons to alter targeting (ASSET).…”