Adaptive evolution in the toxicity of a spider's venom enzymes.

Aurélio Pedroso,Mario Tyago Murakami,Denise V Tambourgi,Giselle Pidde-Queiroz,Sergio Russo Matioli

doi:10.1186/s12862-015-0561-4

Abstract

BackgroundSphingomyelinase D is the main toxin present in the venom of Loxosceles spiders. Several isoforms present in these venoms can be structurally classified in two groups. Class I Sphingomyelinase D contains a single disulphide bridge and variable loop. Class II Sphingomyelinase D presents an additional intrachain disulphide bridge that links a flexible loop with a catalytic loop. These classes exhibit differences in their toxic potential. In this paper we address the distribution of the structural classes of SMase D within and among species of spiders and also their evolutionary origin by means of phylogenetic analyses. We also conducted tests to assess the action of natural selection in their evolution combined to structural modelling of the affected sites.ResultsThe majority of the Class I enzymes belong to the same clade, which indicates a recent evolution from a single common ancestor. Positively selected sites are located on the catalytic interface, which contributes to a distinct surface charge distribution between the classes. Sites that may prevent the formation of an additional bridge were found in Class I enzymes.ConclusionsThe evolution of Sphingomyelinase D has been driven by natural selection toward an increase in noxiousness, and this might help explain the toxic variation between classes.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-015-0561-4) contains supplementary material, which is available to authorized users.

Highlights

Sphingomyelinase D is the main toxin present in the venom of Loxosceles spiders
SMase D toxins were detected in the genera Ixodes and Rhipicephalus, in the family Ixodidae [11], and as exotoxins produced by certain pathogenic bacteria
We found three sequences from I. scapularis annotated as Sphingomyelinase D and Phospholipase D and one from R. pulchellus annotated as Sphingomyelinase D

Summary

Introduction

Sphingomyelinase D is the main toxin present in the venom of Loxosceles spiders. Several isoforms present in these venoms can be structurally classified in two groups. Class II Sphingomyelinase D presents an additional intrachain disulphide bridge that links a flexible loop with a catalytic loop. SMase D is a phospholipase that catalyses the hydrolysis of sphingomyelin (SM), the major constituent in the outer surface of the lipid bilayer of most eukaryotic plasma membranes. The effects of this enzyme result in the formation of ceramide 1-phosphate (N-acylsphingosine 1-phosphate) and choline, whereas mammalian sphingomyelinases convert SM in phosphocholine and ceramide [1,2,3,4,5]. SMase D toxins were detected in the genera Ixodes and Rhipicephalus, in the family Ixodidae [11], and as exotoxins produced by certain pathogenic bacteria. The bacterial and spider venom SMases D possess similar molecular masses (31–35 kDa)

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