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J Mol Biol. 2014 Aug 12;426(16):2958-69. doi: 10.1016/j.jmb.2014.06.009. Epub 2014 Jun 18.

NMR model of PrgI-SipD interaction and its implications in the needle-tip assembly of the Salmonella type III secretion system.

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Department of Molecular Biosciences, University of Kansas, Lawrence, KS 66045, USA.
Department of Microbial Pathogenesis, Yale University School of Medicine, 295 Congress Avenue, New Haven, CT 06536, USA.
Key Laboratory of Magnetic Resonance in Biological Systems, State Key Laboratory of Magnetic Resonance and Atomic Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan, Hubei Province 430071, China.
Department of Molecular Biosciences, University of Kansas, Lawrence, KS 66045, USA. Electronic address:


Salmonella and other pathogenic bacteria use the type III secretion system (T3SS) to inject virulence proteins into human cells to initiate infections. The structural component of the T3SS contains a needle and a needle tip. The needle is assembled from PrgI needle protomers and the needle tip is capped with several copies of the SipD tip protein. How a tip protein docks on the needle is unclear. A crystal structure of a PrgI-SipD fusion protein docked on the PrgI needle results in steric clash of SipD at the needle tip when modeled on the recent atomic structure of the needle. Thus, there is currently no good model of how SipD is docked on the PrgI needle tip. Previously, we showed by NMR paramagnetic relaxation enhancement (PRE) methods that a specific region in the SipD coiled coil is the binding site for PrgI. Others have hypothesized that a domain of the tip protein-the N-terminal α-helical hairpin-has to swing away during the assembly of the needle apparatus. Here, we show by PRE methods that a truncated form of SipD lacking the α-helical hairpin domain binds more tightly to PrgI. Further, PRE-based structure calculations revealed multiple PrgI binding sites on the SipD coiled coil. Our PRE results together with the recent NMR-derived atomic structure of the Salmonella needle suggest a possible model of how SipD might dock at the PrgI needle tip. SipD and PrgI are conserved in other bacterial T3SSs; thus, our results have wider implication in understanding other needle-tip complexes.


NMR; PrgI; Salmonella; SipD; type III secretion

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