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.
[Show abstract][Hide abstract] ABSTRACT: The Type III Secretion System (T3SS) is a multi-mega Dalton apparatus assembled from more than twenty components and is found in many species of animal and plant bacterial pathogens. The T3SS creates a contiguous channel through the bacterial and host membranes, allowing injection of specialized bacterial effector proteins directly to the host cell. In this review, we discuss our current understanding T3SS assembly and structure, as well as highlight structurally characterized Salmonella effectors. This article is part of a Special Issue entitled: Protein Trafficking & Secretion.
Biochimica et Biophysica Acta (BBA) - Molecular Cell Research 08/2014; 1843(8). DOI:10.1016/j.bbamcr.2014.01.035 · 5.02 Impact Factor
"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). "
[Show abstract][Hide abstract] ABSTRACT: Many Gram-negative pathogens utilize Type 3 Secretion Systems (T3SSs) for a successful infection. The T3SS is a large macromolecular complex which spans both bacterial membranes and delivers effector proteins into the host cell. The infection requires spatio-temporal control of diverse sets of secreted effectors and various mechanisms have evolved to regulate T3SS in response to external stimuli. This review will describe mechanisms that may control T3 secretion, revealing a multi-step regulatory strategy. We then propose an updated model of T3SS that illustrates different stages of secretion and integrates the most recent structural and functional data.
[Show abstract][Hide abstract] ABSTRACT: Flagellar and translocation-associated type III secretion (T3S) systems are present in most gram-negative plant- and animal-pathogenic bacteria and are often essential for bacterial motility or pathogenicity. The architectures of the complex membrane-spanning secretion apparatuses of both systems are similar, but they are associated with different extracellular appendages, including the flagellar hook and filament or the needle/pilus structures of translocation-associated T3S systems. The needle/pilus is connected to a bacterial translocon that is inserted into the host plasma membrane and mediates the transkingdom transport of bacterial effector proteins into eukaryotic cells. During the last 3 to 5 years, significant progress has been made in the characterization of membrane-associated core components and extracellular structures of T3S systems. Furthermore, transcriptional and posttranscriptional regulators that control T3S gene expression and substrate specificity have been described. Given the architecture of the T3S system, it is assumed that extracellular components of the secretion apparatus are secreted prior to effector proteins, suggesting that there is a hierarchy in T3S. The aim of this review is to summarize our current knowledge of T3S system components and associated control proteins from both plant- and animal-pathogenic bacteria.
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