title: Adaptive evolution in the toxicity of a spider's venom enzymes.
abstract: BACKGROUND
  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 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.
  
  
  RESULTS
  The 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.
  
  
  CONCLUSIONS
  The 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.
authors.affiliation: Laboratório de Imunoquímica, Instituto Butantan, São Paulo, S.P., Brazil. aurelio.junior@butantan.gov.br.
authors.forename: Aurélio
authors.initials: A
authors.lastname: Pedroso
authors.affiliation: Departamento de Genética e Biologia Evolutiva, Instituto de Biociências, Universidade de São Paulo, São Paulo, S.P., Brazil. srmatiol@ib.usp.br.
authors.forename: Sergio Russo
authors.initials: SR
authors.lastname: Matioli
authors.affiliation: Laboratório Nacional de Biociências, Centro Nacional de Pesquisa em Energia e Materiais, Campinas, S.P., Brazil. mario.murakami@lnbio.cnpem.br.
authors.forename: Mario Tyago
authors.initials: MT
authors.lastname: Murakami
authors.affiliation: Laboratório de Imunoquímica, Instituto Butantan, São Paulo, S.P., Brazil. giselle.queiroz@butantan.gov.br.
authors.forename: Giselle
authors.initials: G
authors.lastname: Pidde-Queiroz
authors.affiliation: Laboratório de Imunoquímica, Instituto Butantan, São Paulo, S.P., Brazil. denise.tambourgi@butantan.gov.br.
authors.forename: Denise V
authors.initials: DV
authors.lastname: Tambourgi
mesh_terms: D000818:Animals; D019143:Evolution, Molecular; D008958:Models, Molecular; D010727:Phosphoric Diester Hydrolases / Q000737:chemistry / Q000235:genetics*; D010802:Phylogeny; D012641:Selection, Genetic; D013111:Spider Venoms / Q000201:enzymology* / Q000235:genetics; D013112:Spiders / Q000145:classification* / Q000201:enzymology / Q000235:genetics*
chemical_list: D013111:Spider Venoms; D010727:Phosphoric Diester Hydrolases; C055160:sphingomyelin phosphodiesterase D