Cells must secrete proteins to alter their environment. Bacteria use a diverse set of protein machinery to secrete a myriad of protein substrates. Many of these proteins are involved in the causation of disease. One such protein is CexE, which is encoded by Enterotoxigenic Escherichia coli (ETEC); one of the most common causes of bacterial mediated diarrhoeal disease. CexE is a surface expose lipoprotein required for colonisation and increased disease severity. CexE is secreted and displayed on the cell surface by a novel composite secretion system, the Aat system. Composed of five proteins, AatPABCD, the Aat system incorporates elements of a Type I secretion system and the Lol lipoprotein trafficking pathway. CexE is post-translationally modified by AatD, which adds a single acyl chain to the N-terminal glycine. CexE is extracted from the inner membrane then secreted via AatA, as outer membrane protein. AatA is homologous to TolC, a β-barrel pore with an α-helical tunnel domain that extends into the periplasm. AatA is unique amongst TolC family proteins in that it secretes a lipoprotein substrate. How a lipoprotein is secreted and inserted into the outer membrane is unknown. To understand this, we will use a mixture of biochemical, molecular, and structural techniques to determine the mechanism of AatA function. Initial predicted structures suggest modifications on the conserved TolC-type fold. Through this work, we aim to determine the structure of AatA by cryogenic electron microscopy and investigate the molecular mechanisms of this protein by comparison with TolC. Understanding the mechanism of CexE secretion could result in new treatment options for diarrhoeal pathogens that are increasingly resistance to antimicrobial therapy.