The Scatterlings of Virulence: towards a complete type-III secretion effector repertoire in Escherichia coli

Some strains of the bacterium Escherichia coli (or E. coli) can inject proteins into human cells, using a specialized molecular syringe known as a 'type-III secretion system'. Once inside human cells, the bacterial proteins then produce effects that help the bacterium colonize the gut and/or cause disease. Until recently it was thought that only a handful of these so-called 'effector proteins' passed through the molecular syringe. However, we have uncovered evidence from analysis of genome sequences to suggest that as many as a hundred different proteins are injected through the type-III secretion system of one particularly virulent strain, E. coli O157. In other words, instead of a using a sextet of proteins, E. coli is employed a symphony orchestra of proteins to seduce human cells into doing what it wants. These new effector proteins are encoded by genes scattered throughout the E. coli chromosome, hence our evocative epithet, 'the scatterlings of virulence'. In this proposal, we will draw on the combined expertise of a bacteriologist and a cell biologist to confirm that the new candidate effector proteins are indeed targeted into human cells, to determine where they go within these cells and what effects they produce once inside human cells. In so doing we will shed light on how bacteria cause disease and on the normal functioning of the systems they disturb.

Selected strains of the model organism and formidable pathogen, Escherichia coli, are able to translocate so-called 'effector proteins' into human cells through a specialized molecular syringe, 'the type-III secretion system'. Translocated effectors are thought to influence the biology of the target cell to the bacterium's advantage. Targets of E. coli effectors include the cytoskeleton (microtubules and actin), inflammatory responses, tight junction barrier function, the cell cycle, mitochondrial function and apoptosis. Type-III secretion thus represents a case study in integrative biology, linking bacteriology with eukaryotic cell biology. Until recently, it was thought that only a handful of effectors were translocated through type-III secretion in E. coli. However, as an exercise in predictive biology, through genomic data-mining, we have identified nearly a hundred novel effector candidates, scattered around the E. coli O157 genome. In pilot studies, six out of seven of the new candidate effectors that we tested were indeed translocated. Drawing on the expertise of a bacteriologist with an interest in pathogenesis, genomics and bioinformatics and a cell biologist, who is an expert on the cytoskeleton, we now wish to determine (1) how many of our candidate effectors are translocated into human cells, (2) where they localize, (3) what effects they have and (4) what eukaryotic proteins they bind to. In addition, we aim to shed light on the evolution of the effector repertoire, on the nature of the secretion and translocation signals, and on the normal functioning of the pathways targeted by effectors.