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 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 better define the relationships between autotransporters, in order to find new mechanisms of action that can be applied towards new medical outcomes.
Methods: We employed a comprehensive array of methods to explore autotransporter function including X-ray crystallography, enzyme assays, mutational analysis, cell culture, invasion assays, infection models along with bioinformatics.
Results/Discussion: Partly 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.
This Ssp story is another addition to our growing list of new molecular mechanisms that we have uncovered for this large family of bacterial virulence factors [3-5], and show that the autotransporter family still contains many more unique structures and mechanisms that await to be revealed. Of significance is that we are now using these molecular mechanisms to develop new therapies such as biofilm inhibitors [6] and new cell penetrating drug carriers.