Investigation of the function and secretion of CexE in enterotoxigenic Escherichia coli

Enteroaggregative Escherichia coli (EAEC) utilises an atypical type I secretion system, AatPABCD, for the secretion of dispersin (Aap). AggR is known to regulate the transcription of the aat system and aap in EAEC 042. Using RNA-Seq, the transcriptomes of EAEC 042 and EAEC 042ΔaggR were compared for differential gene expression. As well as new targets for AggR activation, aap was found to have the greatest difference in transcription. Whilst the function of Aap is understood, the role of the homolog, CexE, in enterotoxigenic E. coli H10407 is not. It was found that CexE could not complement the aap mutant phenotype instead having the opposite effect, resulting in an increase in aggregation. Aggregation can indicate a role in biofilm. It was found that a cexE mutant could not form a biofilm. The investigation of CexE structure revealed that CexE could be secreted by the Aat system. Therefore, Aat-mediated secretion of CexE was investigated. A modification by AatD of Aap/CexE proteins was identified and a new model for Aat secretion is proposed. In summary, the function of Aap and CexE are different but they are both secreted by the Aat system.