Combined snake venomics and venom gland transcriptomic analysis of the ocellated carpet viper, Echis ocellatus

Abstract

Snakebite in Africa causes thousands of deaths annually and considerable permanent physical disability. The saw-scaled viper, Echis ocellatus, represents the single most medically important snake species in West Africa. To provide a detailed compositional analysis of the venom of E. ocellatus for designing novel toxin-specific immunotherapy and to delineate sequence structure-function relationships of individual toxins, we characterised the venom proteome and the venom gland transcriptome. Whole E. ocellatus venom was fractionated by reverse-phase HPLC, followed by analysis of each chromatographic fraction using a combination of SDS-PAGE, N-terminal sequencing, MALDI-TOF mass fingerprinting, and CID-MS/MS of tryptic peptides. This analysis identified around 35 distinct proteins of molecular masses in the range of 5.5-110 kDa belonging to 8 different toxin families (disintegrin, DC-fragment, phospholipase A(2), cysteine-rich secretory protein, serine proteinase, C-type lectin, l-amino acid oxidase, and Zn(2+)-dependent metalloprotease). Comparison of the toxin composition of E. ocellatus venom determined using a proteomic approach, with the predicted proteome derived from assembly of 1000 EST sequences from a E. ocellatus venom gland cDNA library, shows some differences. Most notably, peptides derived from 26% of the venom proteins could not be ascribed an exact match in the transcriptome. Similarly, 64 (67%) out of the 95 putative toxin clusters reported in the transcriptome did not match to peptides detected in the venom proteome. These data suggest that the final composition of venom is influenced by transcriptional and post-translational mechanisms that may be more complex than previously appreciated. This, in turn, emphasises the value of combining proteomic and transcriptomic approaches to acquire a more complete understanding of the precise composition of snake venom, than would be gleaned from using one analysis alone. From a clinical perspective, the large amount of SVMPs (66.5% of the total venom proteins) is consistent with the haemorrhagic pathology associated with E. ocellatus envenoming. More significantly, whilst the proteomic analysis confirms the majority of these metalloproteinases (58%) belong to the SVMP PIII class, MS/MS derived peptide sequencing also demonstrates a major constituent (32%) of E. ocellatus venom is a PIV-SVMP with a quaternary structure comprising a 48 kDa (Q2UXQ4 or Q2UXQ5) PIII-SVMP subunit, and two 14-16 kDa C-type lectin-like domains [EOC_00087 and EOC_00124] which display similarity to echicetin alpha [P81017] and beta [P81996] subunits.