A refined model of the prototypical Salmonella SPI-1 T3SS basal body reveals the molecular basis for its assembly.

The T3SS injectisome is a syringe-shaped macromolecular assembly found in pathogenic Gram-negative bacteria that allows for the direct delivery of virulence effectors into host cells. It is composed of a "basal body", a lock-nut structure spanning both bacterial membranes, and a "needle" that protrudes away from the bacterial surface. A hollow channel spans throughout the apparatus, permitting the translocation of effector proteins from the bacterial cytosol to the host plasma membrane. The basal body is composed largely of three membrane-embedded proteins that form oligomerized concentric rings. Here, we report the crystal structures of three domains of the prototypical Salmonella SPI-1 basal body, and use a new approach incorporating symmetric flexible backbone docking and EM data to produce a model for their oligomeric assembly. The obtained models, validated by biochemical and in vivo assays, reveal the molecular details of the interactions driving basal body assembly, and notably demonstrate a conserved oligomerization mechanism.

Structural analysis of the essential self-cleaving type III secretion proteins EscU and SpaS.

During infection by Gram-negative pathogenic bacteria, the type III secretion system (T3SS) is assembled to allow for the direct transmission of bacterial virulence effectors into the host cell. The T3SS system is characterized by a series of prominent multi-component rings in the inner and outer bacterial membranes, as well as a translocation pore in the host cell membrane. These are all connected by a series of polymerized tubes that act as the direct conduit for the T3SS proteins to pass through to the host cell. During assembly of the T3SS, as well as the evolutionarily related flagellar apparatus, a post-translational cleavage event within the inner membrane proteins EscU/FlhB is required to promote a secretion-competent state. These proteins have long been proposed to act as a part of a molecular switch, which would regulate the appropriate chronological secretion of the various T3SS apparatus components during assembly and subsequently the transported virulence effectors. Here we show that a surface type II beta-turn in the Escherichia coli protein EscU undergoes auto-cleavage by a mechanism involving cyclization of a strictly conserved asparagine residue. Structural and in vivo analysis of point and deletion mutations illustrates the subtle conformational effects of auto-cleavage in modulating the molecular features of a highly conserved surface region of EscU, a potential point of interaction with other T3SS components at the inner membrane. In addition, this work provides new structural insight into the distinct conformational requirements for a large class of self-cleaving reactions involving asparagine cyclization.

Prospects for the development and use of gram-negative bacterial virulence protein secretion inhibitors for prophylaxis and treatment of infectious diseases.

As the breadth and number of bacterial pathogens exhibiting antibiotic resistance is rapidly increasing, our ability to treat new and re-emerging infectious diseases is being threatened. Therefore, the development of new therapeutic strategies, including drugs acting on new targets, is required. In this review, the feasibility of gram-negative bacterial secretion systems for the development of anti-virulence agents and possible arenas for their utility in therapeutic intervention of gram-negative bacterial infections such as gastroenteritis, nosocomial infections, and venereal diseases are discussed.

Structural characterization of the molecular platform for type III secretion system assembly.

Type III secretion systems (TTSSs) are multi-protein macromolecular 'machines' that have a central function in the virulence of many Gram-negative pathogens by directly mediating the secretion and translocation of bacterial proteins (termed effectors) into the cytoplasm of eukaryotic cells. Most of the 20 unique structural components constituting this secretion apparatus are highly conserved among animal and plant pathogens and are also evolutionarily related to proteins in the flagellar-specific export system. Recent electron microscopy experiments have revealed the gross 'needle-shaped' morphology of the TTSS, yet a detailed understanding of the structural characteristics and organization of these protein components within the bacterial membranes is lacking. Here we report the 1.8-A crystal structure of EscJ from enteropathogenic Escherichia coli (EPEC), a member of the YscJ/PrgK family whose oligomerization represents one of the earliest events in TTSS assembly. Crystal packing analysis and molecular modelling indicate that EscJ could form a large 24-subunit 'ring' superstructure with extensive grooves, ridges and electrostatic features. Electron microscopy, labelling and mass spectrometry studies on the orthologous Salmonella typhimurium PrgK within the context of the assembled TTSS support the stoichiometry, membrane association and surface accessibility of the modelled ring. We propose that the YscJ/PrgK protein family functions as an essential molecular platform for TTSS assembly.

Tethered thiazolidinone dimers as inhibitors of the bacterial type III secretion system.

Disruption of protein-protein interactions by small molecules is achievable but presents significant hurdles for effective compound design. In earlier work we identified a series of thiazolidinone inhibitors of the bacterial type III secretion system (T3SS) and demonstrated that this scaffold had the potential to be expanded into molecules with broad-spectrum anti-Gram negative activity. We now report on one series of thiazolidinone analogs in which the heterocycle is presented as a dimer at the termini of a series of linkers. Many of these dimers inhibited the T3SS-dependent secretion of a virulence protein at concentrations lower than that of the original monomeric compound identified in our screen.

The type III secretion system as a source of novel antibacterial drug targets.

Type III Secretion Systems (T3SSs) are highly organized multi-protein nanomachines which translocate effector proteins from the bacterial cytosol directly into host cells. These systems are required for the pathogenesis of a wide array of Gram-negative bacterial pathogens, and thus have attracted attention as potential antibacterial drug targets. A decade of research has enabled the identification of natural products, conventional small molecule drug-like structures, and proteins that inhibit T3SSs. The mechanism(s) of action and molecular target(s) of the majority of these inhibitors remain to be determined. At the same time, structural biology methods are providing an increasingly detailed picture of the functional arrangement of the T3SS component proteins. The confluence of these two research areas may ultimately identify non-classical drug targets and facilitate the development of novel therapeutics.

Substituted 2-imino-5-arylidenethiazolidin-4-one inhibitors of bacterial type III secretion.

Diverse species of pathogenic Gram-negative bacteria use secretion systems to export a variety of protein toxins and virulence factors that help establish and maintain infection. Disruption of such secretion systems is a potentially effective therapeutic strategy. We developed a high-throughput screen and identified a tris-aryl substituted 2-imino-5-arylidenethiazolidin-4-one, compound 1, as an inhibitor of the type III secretion system. Expansion of this chemotype enabled us to define the essential pharmacophore for type III secretion inhibition by this structural class. A synthetic diversity set helped us identify N-3 as the most permissive locus and led to the design of a panel of novel N-3-dipeptide-modified congeners with improved activity and physiochemical properties. We now report on the synthesis of these compounds, including a novel solid phase approach to the rapid generation of the dipeptide-thiazolidinone hybrids, and their in vitro characterization as inhibitors of type III secretion in Salmonella enterica serovar Typhimurium.

An inhibitor of gram-negative bacterial virulence protein secretion.

Bacterial virulence mechanisms are attractive targets for antibiotic development because they are required for the pathogenesis of numerous global infectious disease agents. The bacterial secretion systems used to assemble the surface structures that promote adherence and deliver protein virulence effectors to host cells could comprise one such therapeutic target. In this study, we developed and performed a high-throughput screen of small molecule libraries and identified one compound, a 2-imino-5-arylidene thiazolidinone that blocked secretion and virulence functions of a wide array of animal and plant Gram-negative bacterial pathogens. This compound inhibited type III secretion-dependent functions, with the exception of flagellar motility, and type II secretion-dependent functions, suggesting that its target could be an outer membrane component conserved between these two secretion systems. This work provides a proof of concept that compounds with a broad spectrum of activity against Gram-negative bacterial secretion systems could be developed to prevent and treat bacterial diseases.