| Legionella pneumophila (Lp) is the pathogen of Legionnaires' disease. Lp infects human alveolar macrophages by secreting a group of effector proteins, which can alter the host endocytic pathway. A Type IVb secretion system (T4bSS) is required for effector protein translocation. The T4bSS is encoded by ∼27 dot ( defective organelle trafficking) and icm (intracellular multiplication) genes. The goals of this thesis were to evaluate the role of IcmS and IcmW, two co-receptor/chaperone proteins from the T4bSS, and investigate their interactions with WipA, an effector protein. Biochemical and biophysical approaches were used to provide insights into the role of IcmS-IcmW in the T4bSS.;This work showed that IcmS forms a dimer. Studies of wild type IcmS and mutants revealed possible locations of four cysteine residues. Cys8 and Cys50 are probably located on the surface of the dimer, causing endogenous cross-linking between dimers. Cys53 and Cys88 may be located inside the dimer and serine mutations of these residues are destabilizing. An organic osmolyte, trimethylamine N-oxide (TMAO), functioned as a stabilizer for IcmS mutants and wild type proteins and increased their melting temperatures. The IcmS dimer tends to form a dimer of dimers and oligomers when concentrated and may have hydrophobic regions on its surface.;Refolded IcmW apparently forms a dimer by endogenous cross-linking, and requires IcmS to prevent its aggregation. The IcmS-IcmW complex may be a hetero-tetramer, composed of two IcmS subunits and two IcmW subunits. The IcmS-IcmW complex also tends to aggregate when concentrated because of disulfide cross-links between different complexes.;Binding studies suggest that WipA has weak and reversible interactions with the IcmS dimer and the IcmS-IcmW complex. IcmS dimer and IcmS-IcmW complexes also show a ubiquitous binding to denatured proteins, possibly with hydrophobic regions on the surfaces of these complexes. We suggest that binding sites for IcmS-IcmW on WipA are located in flexible loops. As a coreceptor/chaperone, the IcmS-IcmW complex may bind with effector proteins like WipA and then present them to the transmembrane complex of the T4bSS. After bound effector proteins have been released and transported out of Lp bacteria, IcmS-IcmW complexes become available to recruit other effector substrates. |
