Bacterial type IV secretion systems in human disease

Type IV secretion (T4S) systems are versatile machines involved in many processes relevant to bacterial virulence, such as horizontal DNA transfer and effector translocation into human cells. A recent workshop organized by the International University of Andalousia in Baeza, Spain, covered most aspects of bacterial T4S relevant to human disease, ranging from the structural and mechanistic analysis of the T4S systems to the physiological roles of the translocated effector proteins in subverting cellular functions in infected humans. This review reports the highlights from this workshop, which include the first visualization of a T4S system core complex spanning both membranes of Gram-negative bacteria, the identification of the first host receptors for T4S systems, the identification and characterization of novel T4S effector proteins, the analysis of the molecular function of effector proteins in subverting human cellular functions and an analysis of the role of T4S systems in the evolution of pathogenic bacteria. Our increasing knowledge of the biology of T4S systems improves our ability to exploit them as biotechnological tools or to use them as novel targets for a new generation of antimicrobials.     

Bacterial type IV secretion: conjugation systems adapted to deliver effector molecules to host cells

Several bacterial pathogens utilize conjugation machines to export effector molecules during infection. Such systems are members of the type IV or 'adapted conjugation' secretion family. The prototypical type IV system is the Agrobacterium tumefaciens T-DNA transfer machine, which delivers oncogenic nucleoprotein particles to plant cells. Other pathogens, including Bordetella pertussis, Legionella pneumophila, Brucellaspp. and Helicobacter pylori, use type IV machines to export effector proteins to the extracellular milieu or the mammalian cell cytosol.     

Bacterial Type IV secretion systems: versatile virulence machines

Many bacterial pathogens employ multicomponent protein complexes to deliver macromolecules directly into their eukaryotic host cell to promote infection. Some Gram-negative pathogens use a versatile Type IV secretion system (T4SS) that can translocate DNA or proteins into host cells. T4SSs represent major bacterial virulence determinants and have recently been the focus of intense research efforts designed to better understand and combat infectious diseases. Interestingly, although the two major classes of T4SSs function in a similar manner to secrete proteins, the translocated 'effectors' vary substantially from one organism to another. In fact, differing effector repertoires likely contribute to organism-specific host cell interactions and disease outcomes. In this review, we discuss the current state of T4SS research, with an emphasis on intracellular bacterial pathogens of humans and the diverse array of translocated effectors used to manipulate host cells.     

Towards a systems biology approach to study type II/IV secretion systems

Many gram-negative bacteria produce thin protein filaments, named pili, which extend beyond the confines of the outer membrane. The importance of these pili is illustrated by the fact that highly complex, multi-protein pilus-assembly machines have evolved, not once, but several times. Their many functions include motility, adhesion, secretion, and DNA transfer, all of which can contribute to the virulence of bacterial pathogens or to the spread of virulence factors by horizontal gene transfer. The medical importance has stimulated extensive biochemical and genetic studies but the assembly and function of pili remains an enigma. It is clear that progress in this field requires a more holistic approach where the entire molecular apparatus that forms the pilus is studied as a system. In recent years systems biology approaches have started to complement classical studies of pili and their assembly. Moreover, continued progress in structural biology is building a picture of the components that make up the assembly machine. However, the complexity and multiple-membrane spanning nature of these secretion systems pose formidable technical challenges, and it will require a concerted effort before we can create comprehensive and predictive models of these remarkable molecular machines.     

Towards a systems biology approach to study type II/IV secretion systems

Many gram-negative bacteria produce thin protein filaments, named pili, which extend beyond the confines of the outer membrane. The importance of these pili is illustrated by the fact that highly complex, multi-protein pilus-assembly machines have evolved, not once, but several times. Their many functions include motility, adhesion, secretion, and DNA transfer, all of which can contribute to the virulence of bacterial pathogens or to the spread of virulence factors by horizontal gene transfer. The medical importance has stimulated extensive biochemical and genetic studies but the assembly and function of pili remains an enigma. It is clear that progress in this field requires a more holistic approach where the entire molecular apparatus that forms the pilus is studied as a system. In recent years systems biology approaches have started to complement classical studies of pili and their assembly. Moreover, continued progress in structural biology is building a picture of the components that make up the assembly machine. However, the complexity and multiple-membrane spanning nature of these secretion systems pose formidable technical challenges, and it will require a concerted effort before we can create comprehensive and predictive models of these remarkable molecular machines.     

Biochemistry of type IV secretion

In the past year, our knowledge of type IV transporters of Gram-negative bacteria has further expanded. Advances include the discovery of additional members of this family of proteins, increased knowledge of the morphologies of type IV transporters, and a better understanding of the mechanisms by which macromolecules are exported by these systems.     

Structural biology of type VI secretion systems

Type VI secretion systems (T6SSs) are transenvelope complexes specialized in the transport of proteins or domains directly into target cells. These systems are versatile as they can target either eukaryotic host cells and therefore modulate the bacteria-host interaction and pathogenesis or bacterial cells and therefore facilitate access to a specific niche. These molecular machines comprise at least 13 proteins. Although recent years have witnessed advances in the role and function of these secretion systems, little is known about how these complexes assemble in the cell envelope. Interestingly, the current information converges to the idea that T6SSs are composed of two subassemblies, one resembling the contractile bacteriophage tail, whereas the other subunits are embedded in the inner and outer membranes and anchor the bacteriophage-like structure to the cell envelope. In this review, we summarize recent structural information on individual T6SS components emphasizing the fact that T6SSs are composite systems, adapting subunits from various origins.     

Monoderm bacteria: the new frontier for type IV pilus biology

In the diverse world of bacterial pili, type IV pili (Tfp) are unique for two reasons: their multifunctionality and ubiquity. This latter feature offers an extraordinary possibility, that is, to perform comparative studies in evolutionarily distant species in order to improve our fragmentary understanding of Tfp biology. Regrettably, such potential has remained largely untapped, because, for 20 years, Tfp have only been characterised in diderm bacteria. However, recent studies of Tfp in monoderms have started closing the gap, revealing many interesting commonalities and a few significant differences, extending the frontiers of knowledge of Tfp biology. Here, I review the current state of the art of the Tfp field in monoderm bacteria and discuss resulting implications for our general understanding of the assembly and function of these widespread filamentous nanomachines.     

Virulence-associated type IV secretion systems of Bartonella

Type IV secretion systems (T4SSs) are transport machineries of Gram-negative bacteria that mediate interbacterial DNA-transfer, and secretion of virulence factors into eukaryotic target cells. A growing number of human pathogenic bacteria use T4SSs for intercellular delivery of effector molecules that modify host cellular functions in favour of the pathogen. Recent advances in studying the molecular mechanisms of Bartonella pathogenesis have provided evidence for the central roles of two distinct T4SSs, VirB/VirD4 and Trw, in the ability of the bacteria to colonize, invade and persist within either vascular endothelial cells or erythrocytes, respectively. The identification of VirB/VirD4-transported substrates and the delineation of their secretion signal have paved the way towards understanding the molecular mechanisms underlying Bartonella-host cell interaction and modulation, as well as the exploitation of this system for engineered substrate delivery into mammalian target cells.     

Bacterial flagella and type III secretion systems

Certain classes of pathogenic bacteria secrete virulence proteins in a Sec-independent manner, by a mechanism known as type III secretion. The main body of the export apparatus specific for virulence proteins is identified as a needle complex, which has a similar structural organization to flagella. The two structures share several proteins with highly homologous amino acid sequences. Even where the sequence identity is low among flagellar proteins from various species, the physico-chemical properties of each protein remain homologous. Therefore, by comparing the physico-chemical properties of unidentified proteins, it is possible to find homologs among type III secretion systems.     

The versatile bacterial type IV secretion systems

Bacteria use type IV secretion systems for two fundamental objectives related to pathogenesis--genetic exchange and the delivery of effector molecules to eukaryotic target cells. Whereas gene acquisition is an important adaptive mechanism that enables pathogens to cope with a changing environment during invasion of the host, interactions between effector and host molecules can suppress defence mechanisms, facilitate intracellular growth and even induce the synthesis of nutrients that are beneficial to bacterial colonization. Rapid progress has been made towards defining the structures and functions of type IV secretion machines, identifying the effector molecules, and elucidating the mechanisms by which the translocated effectors subvert eukaryotic cellular processes during infection.     

细菌的IV型分泌系统

细菌的分泌系统与细菌的生存及致病性密切相关。细菌的分泌系统包括I-VI型,其中,IV型分泌系统是与细菌接合机制有关的一类分泌系统。IV型分泌系统不但可以转运DNA,还可以转运蛋白质及核糖核蛋白复合物等大分子物质,这点区别于其他几种分泌系统。IV型分泌系统介导基因水平转移,通过细菌间接合作用,传递抗性基因和毒力基因,有利于细菌进化;另一方面,IV型分泌系统转运效应蛋白质分子到宿主细胞,参与细菌致病。本文着重从IV型分泌系统几种主要类型的分泌机制等方面对IV型分泌系统进行概述。     

Structural definition on the surface of Helicobacter pylori type IV secretion apparatus

Genetic and functional studies have indicated that the type IV secretion system (TFSS) of Helicobacter pylori forms a secretion complex in the cell envelope that protrudes towards the outside in order to inject CagA protein into gastric epithelial cells. However, the proposed structural model is based on partial amino acid homology with the components of the Agrobacterium tumefaciens TFSS. Therefore, we undertook the identification of the structural features of the TFSS exposed on the surface of H. pylori and found that filamentous structures present on the bacterial surface are related to the secretion apparatus. Using immunofluorescence microscopy with antibodies directed to tyrosine-phosphorylated CagA (pY-CagA) and Hp0532 (VirB7) in the infection assay, pY-CagA signals were detected just below the host cell-attached bacteria, where Hp0532 (VirB7) signals were detected as co-localized, suggesting that the CagA injected into the host cell through the TFSS apparatus is still mostly confined to the areas just below the attached bacteria after being phosphorylated. Furthermore, the filamentous structures on bacterium were found to be associated with Hp0532 (VirB7) or Hp0528 (VirB9), the major components of TFSS, by immunogold electron microscopy. These results strongly suggest that the H. pylori TFSS apparatus is a filamentous macromolecular structure protruding from the bacterial envelope.