Characterization of new DsbB-like thioloxidoreductases of Campylobacter jejuni and Helicobacter pylori and classification of the DsbB family based on phylogenomic, structural and functional criteria

In Gram-negative bacterial cells, disulfide bond formation occurs in the oxidative environment of the periplasm and is catalysed by Dsb (disulfide bond) proteins found in the periplasm and in the inner membrane. In this report the identification of a new subfamily of disulfide oxidoreductases encoded by a gene denoted dsbl, and functional characterization of Dsbl proteins from Campylobacterjejuni and Helicobacter pylori, as well as DsbB from C. jejuni, are described. The N-terminal domain of Dsbl is related to DsbB proteins and comprises five predicted transmembrane segments, while the C-terminal domain is predicted to locate to the periplasm and to fold into a beta-propeller structure. The dsbl gene is co-transcribed with a small ORF designated dba (dsbl-accessory). Based on a series of deletion and complementation experiments it is proposed that DsbB can complement the lack of Dsbl but not the converse. In the presence of DsbB, the activity of Dsbl was undetectable, hence it probably acts only on a subset of possible substrates of DsbB. To reconstruct the principal events in the evolution of DsbB and Dsbl proteins, sequences of all their homologues identifiable in databases were analysed. In the course of this study, previously undetected variations on the common thiol-oxidoreductase theme were identified, such as development of an additional transmembrane helix and loss or migration of the second pair of Cys residues between two distinct periplasmic loops. In conjunction with the experimental characterization of two members of the Dsbl lineage, this analysis has resulted in the first comprehensive classification of the DsbB/Dsbl family based on structural, functional and evolutionary criteria.