Detecting the nature and solving the crystal structure of a contaminant protein from an opportunistic pathogen

The unintentional crystallization of contaminant proteins in the place of target recombinant proteins is sporadically reported, despite the availability of stringent expression/purification protocols and of software for the detection of contaminants. Typically, the contaminant protein originates from the expression organism (for example Escherichia coli), but in rare circumstances contaminants from different sources have been reported. Here, a case of contamination from a Serratia bacterial strain that occurred while attempting to crystallize an unrelated protein from Burkholderia pseudomallei (overexpressed in E. coli) is presented. The contamination led to the unintended crystallization and structure analysis of a cyanase hydratase from a bacterial strain of the Serratia genus, an opportunistic enterobacterium that grows under conditions similar to those of E. coli and that is found in a variety of habitats, including the laboratory environment. In this context, the procedures that were adopted to identify the contaminant based on crystallographic data only are presented and the crystal structure of Serrata spp. cyanase hydratase is briefly discussed.