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

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 Prgl 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 Prgl SipD fusion protein docked on the Prgl 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 Prgl 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 Prgl. Others have hypothesized that a domain of the tip protein-the N-terminal alpha-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 alpha-helical hairpin domain binds more tightly to Prgl. Further, PRE-based structure calculations revealed multiple Prgl 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 Prgl needle tip. SipD and Prgl are conserved in other bacterial T3SSs; thus, our results have wider implication in understanding other needle-tip complexes. (C) 2014 Elsevier Ltd. All rights reserved.