Crystallization and preliminary X-ray analysis of the human-specific toxin intermedilysin

Intermedilysin is a human-specific toxin from Streptococcus intermedius, which is part of normal human oral flora. The bacterium is an opportunistic pathogen with a tendency for deep-seated infection in the brain and liver. Intermedilysin belongs to the cholesterol-dependent cytolysin (CDCs) family of toxins, which have been identified in several different bacteria including the serious human pathogens S. pneumoniae and Clostridium perfringens. Intermedilysin, however, is the only member that shows exclusive specificity for human cells. The toxin has a couple of non-conservative amino-acid substitutions in a tryptophan-rich region of the molecule (Cys to Ala and Trp to Pro), the most conserved region amongst the CDCs. Mutations in this region are known to render other CDCs inactive. In order to investigate the structure-function relationships of the unusual features of intermedilysin, which will help us to understand the molecular mechanism of the toxin family in general, recombinant intermedilysin has been crystallized. The crystals belong to an orthorhombic space group and contain two molecules per asymmetric unit. Diffraction data were collected to 2.3 A using synchrotron radiation.