A large domain swap in the VirB11 ATPase of Brucella suis leaves the hexameric assembly intact

Biological diversity of prokaryotic type IV secretion systems

Type IV secretion systems (T4SS) translocate DNA and protein substrates across prokaryotic cell envelopes generally by a mechanism requiring direct contact with a target cell. Three types of T4SS have been described: (i) conjugation systems, operationally defined as machines that translocate DNA substrates intercellularly by a contact-dependent process; (ii) effector translocator systems, functioning to deliver proteins or other macromolecules to eukaryotic target cells; and (iii) DNA release/uptake systems, which translocate DNA to or from the extracellular milieu. Studies of a few paradigmatic systems, notably the conjugation systems of plasmids F, R388, RP4, and pKM101 and the Agrobacterium tumefaciens VirB/VirD4 system, have supplied important insights into the structure, function, and mechanism of action of type IV secretion machines. Information on these systems is updated, with emphasis on recent exciting structural advances. An underappreciated feature of T4SS, most notably of the conjugation subfamily, is that they are widely distributed among many species of gram-negative and -positive bacteria, wall-less bacteria, and the Archaea. Conjugation-mediated lateral gene transfer has shaped the genomes of most if not all prokaryotes over evolutionary time and also contributed in the short term to the dissemination of antibiotic resistance and other virulence traits among medically important pathogens. How have these machines adapted to function across envelopes of distantly related microorganisms? A survey of T4SS functioning in phylogenetically diverse species highlights the biological complexity of these translocation systems and identifies common mechanistic themes as well as novel adaptations for specialized purposes relating to the modulation of the donor-target cell interaction.

The structural biology of type IV secretion systems

Type IV secretion systems (T4SSs) are versatile secretion systems that are found in both Gram-negative and Gram-positive bacteria and secrete a wide range of substrates, from single proteins to protein-protein and protein-DNA complexes. They usually consist of 12 components that are organized into ATP-powered, double-membrane-spanning complexes. The structures of single soluble components or domains have been solved, but an understanding of how these structures come together has only recently begun to emerge. This Review focuses on the structural advances that have been made over the past 10 years and how the corresponding structural insights have helped to elucidate many of the details of the mechanism of type IV secretion.

An anomalous type IV secretion system in Rickettsia is evolutionarily conserved

BACKGROUND

Bacterial type IV secretion systems (T4SSs) comprise a diverse transporter family functioning in conjugation, competence, and effector molecule (DNA and/or protein) translocation. Thirteen genome sequences from Rickettsia, obligate intracellular symbionts/pathogens of a wide range of eukaryotes, have revealed a reduced T4SS relative to the Agrobacterium tumefaciens archetype (vir). However, the Rickettsia T4SS has not been functionally characterized for its role in symbiosis/virulence, and none of its substrates are known.

RESULTS

Superimposition of T4SS structural/functional information over previously identified Rickettsia components implicate a functional Rickettsia T4SS. virB4, virB8 and virB9 are duplicated, yet only one copy of each has the conserved features of similar genes in other T4SSs. An extraordinarily duplicated VirB6 gene encodes five hydrophobic proteins conserved only in a short region known to be involved in DNA transfer in A. tumefaciens. virB1, virB2 and virB7 are newly identified, revealing a Rickettsia T4SS lacking only virB5 relative to the vir archetype. Phylogeny estimation suggests vertical inheritance of all components, despite gene rearrangements into an archipelago of five islets. Similarities of Rickettsia VirB7/VirB9 to ComB7/ComB9 proteins of epsilon-proteobacteria, as well as phylogenetic affinities to the Legionella lvh T4SS, imply the Rickettsiales ancestor acquired a vir-like locus from distantly related bacteria, perhaps while residing in a protozoan host. Modern modifications of these systems likely reflect diversification with various eukaryotic host cells.

CONCLUSION

We present the rvh (Rickettsiales vir homolog) T4SS, an evolutionary conserved transporter with an unknown role in rickettsial biology. This work lays the foundation for future laboratory characterization of this system, and also identifies the Legionella lvh T4SS as a suitable genetic model.

PilB and PilT are ATPases acting antagonistically in type IV pilus function in Myxococcus xanthus

Type IV pili (T4P) are dynamic surface structures that undergo cycles of extension and retraction. T4P dynamics center on the PilB and PilT proteins, which are members of the secretion ATPase superfamily of proteins. Here, we show that PilB and PilT of the T4P system in Myxococcus xanthus have ATPase activity in vitro. Using a structure-guided approach, we systematically mutagenized PilB and PilT to resolve whether both ATP binding and hydrolysis are important for PilB and PilT function in vivo. PilB as well as PilT ATPase activity was abolished in vitro by replacement of conserved residues in the Walker A and Walker B boxes that are involved in ATP binding and hydrolysis, respectively. PilB proteins containing mutant Walker A or Walker B boxes were nonfunctional in vivo and unable to support T4P extension. PilT proteins containing mutant Walker A or Walker B boxes were also nonfunctional in vivo and unable to support T4P retraction. These data provide genetic evidence that both ATP binding and hydrolysis by PilB are essential for T4P extension and that both ATP binding and hydrolysis by PilT are essential for T4P retraction. Thus, PilB and PilT are ATPases that act at distinct steps in the T4P extension/retraction cycle in vivo.

Identification, structure and mode of action of a new regulator of the Helicobacter pylori HP0525 ATPase

Helicobacter pylori is one of the world's most successful human pathogens causing gastric ulcers and cancers. A key virulence factor of H. pylori is the Cag pathogenicity island, which encodes a type IV secretion system. HP0525 is an essential component of the Cag system and acts as an inner membrane associated ATPase. HP0525 forms double hexameric ring structures, with the C-terminal domains (CTDs) forming a closed ring and the N-terminal domains (NTDs) forming a dynamic, open ring. Here, the crystal structure of HP0525 in complex with a fragment of HP1451, a protein of previously unknown function, is reported. The HP1451 construct consists of two domains similar to nucleic acid-binding domains. Two HP1451 molecules bind to the HP0525 NTDs on opposite sides of the hexamer, locking it in the closed form and forming a partial lid over the HP0525 chamber. From the structure, it is suggested that HP1451 acts as an inhibitory factor of HP0525 to regulate Cag-mediated secretion, a suggestion confirmed by results of in vitro ATPase assay and in vivo pull-down experiments.

VirB8: a conserved type IV secretion system assembly factor and drug target

Type IV secretion systems are used by many gram-negative bacteria for the translocation of macromolecules (proteins, DNA, or DNA-protein complexes) across the cell envelope. Among them are many pathogens for which type IV secretion systems are essential virulence factors. Type IV secretion systems comprise 8-12 conserved proteins, which assemble into a complex spanning the inner and the outer membrane, and many assemble extracellular appendages, such as pili, which initiate contact with host and recipient cells followed by substrate translocation. VirB8 is an essential assembly factor for all type IV secretion systems. Biochemical, cell biological, genetic, and yeast two-hybrid analyses showed that VirB8 undergoes multiple interactions with other type IV secretion system components and that it directs polar assembly of the membrane-spanning complex in the model organism Agrobacterium tumefaciens. The availability of the VirB8 X-ray structure has enabled a detailed structure-function analysis, which identified sites for the binding of VirB4 and VirB10 and for self-interaction. Due to its multiple interactions, VirB8 is an excellent model for the analysis of assembly factors of multiprotein complexes. In addition, VirB8 is a possible target for drugs that target its protein-protein interactions, which would disarm bacteria by depriving them of their essential virulence functions.

Crystal structures of the pilus retraction motor PilT suggest large domain movements and subunit cooperation drive motility

PilT is a hexameric ATPase required for bacterial type IV pilus retraction and surface motility. Crystal structures of ADP- and ATP-bound Aquifex aeolicus PilT at 2.8 and 3.2 A resolution show N-terminal PAS-like and C-terminal RecA-like ATPase domains followed by a set of short C-terminal helices. The hexamer is formed by extensive polar subunit interactions between the ATPase core of one monomer and the N-terminal domain of the next. An additional structure captures a nonsymmetric PilT hexamer in which approach of invariant arginines from two subunits to the bound nucleotide forms an enzymatically competent active site. A panel of pilT mutations highlights the importance of the arginines, the PAS-like domain, the polar subunit interface, and the C-terminal helices for retraction. We present a model for ATP binding leading to dramatic PilT domain motions, engagement of the arginine wire, and subunit communication in this hexameric motor. Our conclusions apply to the entire type II/IV secretion ATPase family.