Institute for Molecular Bioscience, University of Queensland^* School for Integrative Biology, University of Queensland, Australia^**
○Mareike Kurz^* Begona Heras^* Inaki Iturbe-Ormaetxe^** Scott O'Neill^** Jenny Martin^*

The correct folding of proteins into their secondary, tertiary and quaternary structure is essential in order to attain a native functional conformation. Many secreted proteins are structurally stabilized by intramolecular disulfide bridges, which, linked in the correct way, are essential for activity and stability. For prokaryotes, the mechanism of disulfide bond formation is well characterised only for the Gram-negative γ -proteobacteria Escherichia coli. Its folding system is composed of two pathways, the oxidizing and the isomerizing pathway. DsbA and DsbB introduce disulfide bonds into proteins (oxidative pathway), while DsbC and DsbD catalyse the rearrangement of mismatched cysteines (isomerizing pathway).

Can findings from studies on E. coli be translated to other bacteria? To address this question a comprehensive analysis of DsbA-like sequences over the entire bacterial kingdom was performed. Based on the results from this study we infer that the classical E. coli-like DsbA is present only in the γ -proteobacteria subclass and that at least two other DsbA classes exist. We have provisionally named theses classes 2 and 3.

Two DsbA proteins from the α -proteobacteria Wolbachia pipientis, DsbA3 and DsbA4, were chosen for further study. DsbA3 is a class 3 DsbA, characterized by the presence of a second conserved pair of cysteines in the sequence. DsbA4 is a hybrid DsbA with an E. coli-like active site (Cys-Pro-His-Cys) and sequence motifs characteristic of Dsb isomerases. Structural and biochemical characterization of these proteins will be presented.