Lunelli M, Hurwitz R, Lambers J, Kolbe M. Crystal structure of PrgI-SipD: insight into a secretion competent state of the type three secretion system needle tip and its interaction with host ligands.
PLoS Pathog 2011;
7:e1002163. [PMID:
21829362 PMCID:
PMC3150277 DOI:
10.1371/journal.ppat.1002163]
[Citation(s) in RCA: 48] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/13/2010] [Accepted: 05/28/2011] [Indexed: 11/18/2022] Open
Abstract
Many infectious Gram-negative bacteria, including Salmonella typhimurium, require a Type Three Secretion System (T3SS) to translocate virulence factors into host cells. The T3SS consists of a membrane protein complex and an extracellular needle together that form a continuous channel. Regulated secretion of virulence factors requires the presence of SipD at the T3SS needle tip in S. typhimurium. Here we report three-dimensional structures of individual SipD, SipD in fusion with the needle subunit PrgI, and of SipD:PrgI in complex with the bile salt, deoxycholate. Assembly of the complex involves major conformational changes in both SipD and PrgI. This rearrangement is mediated via a π bulge in the central SipD helix and is stabilized by conserved amino acids that may allow for specificity in the assembly and composition of the tip proteins. Five copies each of the needle subunit PrgI and SipD form the T3SS needle tip complex. Using surface plasmon resonance spectroscopy and crystal structure analysis we found that the T3SS needle tip complex binds deoxycholate with micromolar affinity via a cleft formed at the SipD:PrgI interface. In the structure-based three-dimensional model of the T3SS needle tip, the bound deoxycholate faces the host membrane. Recently, binding of SipD with bile salts present in the gut was shown to impede bacterial infection. Binding of bile salts to the SipD:PrgI interface in this particular arrangement may thus inhibit the T3SS function. The structures presented in this study provide insight into the open state of the T3SS needle tip. Our findings present the atomic details of the T3SS arrangement occurring at the pathogen-host interface.
Since the rise of pathogenic bacterial strains resistant to antibiotics, the need to develop potent anti-infective drugs is continually increasing. This necessitates a detailed knowledge of the bacterial host invasion process. Gram-negative bacteria have evolved a protein transport system through which they deliver virulence factors into host cells. These virulence factors influence the signal transduction cascade and metabolism inside host cells in a way that is beneficial for the invading bacteria. The proteins at the transport system needle tip mediate contact with host cells and spatiotemporal coordinated release of virulence factors. In this study, we used biophysical and biochemical methods to understand the structure and function of proteins present at the needle tip of such a virulence factor transport system in Salmonella species. We could show that two different proteins, structurally conserved in many pathogenic bacteria, bind each other to constitute the needle tip of the transport system. Multiple copies of both proteins constitute the tip of the transport system in what may represent the open state of the needle. Our study will serve to provide new insights into the virulence factor transport system essential for many different pathogenic bacteria, and may thus offer novel targets to fight infection.
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