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Journal Article

Structural and functional studies on the N-terminal domain of the Shigella type III secretion protein MxiG

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McDowell, M. A., Johnson, S., Deane, J. E., Cheung, M., Roehrich, A. D., Blocker, A. J., et al. (2011). Structural and functional studies on the N-terminal domain of the Shigella type III secretion protein MxiG. The Journal of Biological Chemistry, 286(35), 30606-30614. doi:10.1074/jbc.M111.243865.

Cite as: https://hdl.handle.net/21.11116/0000-0009-696F-6
MxiG is a single-pass membrane protein that oligomerizes within the inner membrane ring of the Shigella flexneri type III secretion system (T3SS). The MxiG N-terminal domain (MxiG-N) is the predominant cytoplasmic structure; however, its role in T3SS assembly and secretion is largely uncharacterized. We have determined the solution structure of MxiG-N residues 6-112 (MxiG-N(6-112)), representing the first published structure of this T3SS domain. The structure shows strong structural homology to forkhead-associated (FHA) domains. Canonically, these cell-signaling modules bind phosphothreonine (Thr(P)) via highly conserved residues. However, the putative phosphate-binding pocket of MxiG-N(6-112) does not align with other FHA domain structures or interact with Thr(P). Furthermore, mutagenesis of potential phosphate-binding residues has no effect on S. flexneri T3SS assembly and function. Therefore, MxiG-N has a novel function for an FHA domain. Positioning of MxiG-N(6-112) within the EM density of the S. flexneri needle complex gives insight into the ambiguous stoichiometry of the T3SS, supporting models with 24 MxiG subunits in the inner membrane ring.