title: Accelerated exchange of exon segments in Viperid three-finger toxin genes (Sistrurus catenatus edwardsii; Desert Massasauga).
abstract: BACKGROUND
  Snake 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.
  
  
  RESULTS
  Here 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.
  
  
  CONCLUSIONS
  Ordered, 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.
authors.affiliation: Protein Science Laboratory, Department of Biological Sciences, National University of Singapore, 117543, Singapore. dbsrd@nus.edu.sg
authors.forename: Robin
authors.initials: R
authors.lastname: Doley
authors.forename: Susanta
authors.initials: S
authors.lastname: Pahari
authors.forename: Stephen P
authors.initials: SP
authors.lastname: Mackessy
authors.forename: R Manjunatha
authors.initials: RM
authors.lastname: Kini
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D019143:Evolution, Molecular; D005091:Exons / Q000235:genetics*; D023061:Gene Order; D007438:Introns / Q000235:genetics; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010802:Phylogeny; D017434:Protein Structure, Tertiary; D016415:Sequence Alignment; D014757:Viper Venoms / Q000737:chemistry / Q000235:genetics*; D017819:Viperidae / Q000235:genetics*
chemical_list: D014757:Viper Venoms