Biochemical and molecular characterisation of FliI and FliH from Helicobacter pylori : a thesis presented in partial fulfilment of Doctor of Philosophy in Microbiology at the Institute of Molecular BioSciences, Massey University, Palmerston North, New Zealand
The bacterium Helicobacter pylori is a human pathogen that infects a large proportion of the world's population and is associated with serious diseases such as gastric ulcers and adenocarcinoma. The motility of this organism, by virtue of sheathed polar flagella is essential to colonisation and persistence in the human host. The sequencing of the H. pylori genome in 1996 identified homologues of the majority of the flagellar genes found in S. enterica serovai typhimurium. These included genes encoding the flagellum ATPase, FliI and FliH a presumptive inhibitor, the primary focus of this study. Sequencing did not originally identify an H. pylori homologue of the flagellar chaperone FliJ, and this is also considered in this study. Bioinformatic analysis and modeling suggests a structural and functional relationship between FliI and homologues such as F1-ATPase α- and β-subunit. In particular, residues 2-91 of FliI resemble the N-terminal domain of the F1-ATPase α- and β-subunits. Biochemical analyses reported in this thesis showed that a truncated FliI-(2- 91) protein was folded, although the N-terminal 18 residues were likely unstructured. Furthermore, deletion mutagenesis showed that this disordered segment of the protein mediates interaction with FliH and very likely forms an amphipathic α-helix upon forming of the FliI-FliH complex. The scanning mutagenesis of this interaction segment of FliI identified a cluster of conserved hydrophobic residues that was critical for the interaction with FliH. Thus, the interaction between FliI and FliH has similarities to the interaction between the N-terminal α-helix of the α-subunit and the globular domain of the δ-subunit of the F1-ATPase. This similarity suggests that FliH, by analogy with the δ-subunit of the F1-ATPase, may function as a molecular stator of the flagellum. The findings presented above have been published (96). The function of a putative H. pylori FliJ homologue, HP0256, was also investigated by knock-out mutagenesis. Disruption of this gene does not abolish flagellar assembly, however further research continued beyond this thesis showed that the knock-out mutant results in impaired motility.