Interaction of MxiG with the cytosolic complex of the type III secretion system controls Shigella virulence

Department of Cellular Microbiology, Max-Planck-Institute for Infection Biology, Charitéplatz 1, 10117, Berlin, Germany.
The FASEB Journal (Impact Factor: 5.48). 01/2012; 26(4):1717-26. DOI: 10.1096/fj.11-197160
Source: PubMed

ABSTRACT Gram-negative bacteria use the type 3 secretion system (T3SS) to colonize host cells. T3SSs are ring-shaped macromolecular complexes specific for the transport of effector molecules into host cells. It was recently suggested that a cytosolic ring-shaped protein complex delivers effector molecules to the T3SS. However, how transport of effector proteins is regulated is not known. Here, we report the high-resolution X-ray crystal structure of the whole cytosolic domain of MxiG (MxiG(1-126)), a major component of the inner T3SS rings in Shigella flexneri. MxiG(1-126) folds as an FHA domain, which specifically binds phosphorylated threonines. Indeed, MxiG(1-126) binds to Spa33, a cytoplasmic-ring component of Shigella, as revealed in pulldown studies. Surface plasmon resonance analysis showed specific interaction of MxiG with a Spa33 peptide only if phosphorylated. In total, 24 copies of the MxiG(1-126) crystal structure were fitted into the cryo-EM map of the Shigella T3SS. The phosphoprotein binding site of each MxiG molecule faces the channel of the T3SS, allowing interaction with cytosolic binding partners. Secretion assays and host cell invasion studies of complemented Shigella knockout cells indicated that the phosphoprotein binding of MxiG is essential for bacterial virulence. Our findings suggest that MxiG is involved in T3SS regulation.

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    • "Its cytoplasmic domain includes a forkhead-associated (FHA) phosphothreoninebinding domain, which binds phosphorylated peptides from Spa33 bearing a FHA recognition motif (Barison et al., 2012), but not phosphothreonine amino acid (McDowell et al., 2011). Indeed, it was recently demonstrated that mutations in MxiG that affect phosphoprotein binding reduce secretion of IpaB and impair invasion of HeLa cells (Barison et al., 2012). These findings suggest that the interaction of phosphorylated proteins, possibly SP components like Spa33, with the T3SS proximal ring is required for translocator secretion (Fig. 2E). "
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