Bacterial pathogens utilise surface bound and secreted proteins in order to promote infections and disease, with the largest family of such proteins being the autotransporters. Autotransporters are used by a wide range of important bacterial pathogens to facilitate colonisation, biofilm formation, spread and tissue destruction. However, the vast majority of autotransporters remain uncharacterised.
Aims: We sought to reveal the molecular mechanisms of 2 new toxins from completely different autotransporter groups in order to define their roles in bacterial infections, and provide information that can be applied towards new medical outcomes.
Methods: We employed a comprehensive array of methods to explore autotransporters including X-ray crystallography, enzyme assays, mutational analysis, cell culture, invasion assays, infection models along with bioinformatics.
Results/Discussion: Driven by our own accumulation of autotransporter structural and mechanistic data we decided to perform an extensive phylogenetic study of functionally described autotransporters. This updated analysis allowed us to define new and under-studied autotransporter groups [1].
The subtilase autotransporters were identified as a large structurally unknown autotransporter group. Hence we determined the first crystal structure of a subtilase autotransporter Ssp from the pathogen Serratia marcescens [2]. We found that Ssp was a potent toxin, capable of entering human epithelium to cause cytotoxic effects along with being highly lethal in a Galleria mellonella infection model. Importantly, the Ssp structure revealed a completely new structural layout, amongst the backdrop of the many well known bacterial toxins. Moreover, many of the unique structural attributes of Ssp were required for its novel mode of cellular entry and toxicity.
By comparison, we also determined the first crystal structure and mechanism of action of the main toxin EspC from enteropathogenic E. coli (unpublished). The structure showed another type of toxin framework that alongside our specialised cellular toxicity studies, uncovered yet another mode of cellular entry/toxicity. These stories are further additions to our growing list of new mechanisms of action that we have uncovered for this large family of bacterial virulence factors [3-5]. Of significance is that we are now using this information to develop new therapies such as biofilm inhibitors [6] and new cell penetrating drug carriers.