The type II secretion arrowhead: the structure of GspI-GspJ-GspK

Architecture, Function, and Substrates of the Type II Secretion System

The type II secretion system (T2SS) delivers toxins and a range of hydrolytic enzymes, including proteases, lipases, and carbohydrate-active enzymes, to the cell surface or extracellular space of Gram-negative bacteria. Its contribution to survival of both extracellular and intracellular pathogens as well as environmental species of proteobacteria is evident. This dynamic, multicomponent machinery spans the entire cell envelope and consists of a cytoplasmic ATPase, several inner membrane proteins, a periplasmic pseudopilus, and a secretin pore embedded in the outer membrane. Despite the trans-envelope configuration of the T2S nanomachine, proteins to be secreted engage with the system first once they enter the periplasmic compartment via the Sec or TAT export system. Thus, the T2SS is specifically dedicated to their outer membrane translocation. The many sequence and structural similarities between the T2SS and type IV pili suggest a common origin and argue for a pilus-mediated mechanism of secretion. This minireview describes the structures, functions, and interactions of the individual T2SS components and the general architecture of the assembled T2SS machinery and briefly summarizes the transport and function of a growing list of T2SS exoproteins. Recent advances in cryo-electron microscopy, which have led to an increased understanding of the structure-function relationship of the secretin channel and the pseudopilus, are emphasized.

Pseudomonas aeruginosa minor pilins prime type IVa pilus assembly and promote surface display of the PilY1 adhesin

Type IV pili (T4P) contain hundreds of major subunits, but minor subunits are also required for assembly and function. Here we show that Pseudomonas aeruginosa minor pilins prime pilus assembly and traffic the pilus-associated adhesin and anti-retraction protein, PilY1, to the cell surface. PilV, PilW, and PilX require PilY1 for inclusion in surface pili and vice versa, suggestive of complex formation. PilE requires PilVWXY1 for inclusion, suggesting that it binds a novel interface created by two or more components. FimU is incorporated independently of the others and is proposed to couple the putative minor pilin-PilY1 complex to the major subunit. The production of small amounts of T4P by a mutant lacking the minor pilin operon was traced to expression of minor pseudopilins from the P. aeruginosa type II secretion (T2S) system, showing that under retraction-deficient conditions, T2S minor subunits can prime T4P assembly. Deletion of all minor subunits abrogated pilus assembly. In a strain lacking the minor pseudopilins, PilVWXY1 and either FimU or PilE comprised the minimal set of components required for pilus assembly. Supporting functional conservation of T2S and T4P minor components, our 1.4 Å crystal structure of FimU revealed striking architectural similarity to its T2S ortholog GspH, despite minimal sequence identity. We propose that PilVWXY1 form a priming complex for assembly and that PilE and FimU together stably couple the complex to the major subunit. Trafficking of the anti-retraction factor PilY1 to the cell surface allows for production of pili of sufficient length to support adherence and motility.

Distinct docking and stabilization steps of the Pseudopilus conformational transition path suggest rotational assembly of type IV pilus-like fibers

The closely related bacterial type II secretion (T2S) and type IV pilus (T4P) systems are sophisticated machines that assemble dynamic fibers promoting protein transport, motility, or adhesion. Despite their essential role in virulence, the molecular mechanisms underlying helical fiber assembly remain unknown. Here, we use electron microscopy and flexible modeling to study conformational changes of PulG pili assembled by the Klebsiella oxytoca T2SS. Neural network analysis of 3,900 pilus models suggested a transition path toward low-energy conformations driven by progressive increase in fiber helical twist. Detailed predictions of interprotomer contacts along this path were tested by site-directed mutagenesis, pilus assembly, and protein secretion analyses. We demonstrate that electrostatic interactions between adjacent protomers (P-P+1) in the membrane drive pseudopilin docking, while P-P+3 and P-P+4 contacts determine downstream fiber stabilization steps. These results support a model of a spool-like assembly mechanism for fibers of the T2SS-T4P superfamily.

Complex formation and processing of the minor transformation pilins of Bacillus subtilis

Transformation in most bacteria is dependent on orthologues of Type 2 secretion and Type 4 pilus system proteins. In each system, pilin proteins (major and minor) are required to make the pilus structure and are essential to the process, although the precise roles of the minor pilins remain unclear. We have explored protein-protein interactions among the competence minor pilins of Bacillus subtilis through in vitro binding studies, immunopurification and mass spectrometry. We demonstrate that the minor pilins directly interact, and the minor pilin ComGG interacts with most of the known proteins required for transformation. We find that ComGG requires other ComG proteins for its stabilization and for processing by the pre-pilin peptidase. These observations, C-terminal mutations in ComGG that prevent processing and the inaccessibility of pre-ComGG to externally added protease suggest a model in which pre-ComGG must be associated with other minor pilins for processing to take place. We propose that ComGG does not become a transmembrane protein until after processing. These behaviours contrast with that of pre-ComGC, the major pilin, which is accessible to externally added protease and requires only the peptidase to be processed. The roles of the pilins and of the pilus in transformation are discussed.

Type IV pilin proteins: versatile molecular modules

Type IV pili (T4P) are multifunctional protein fibers produced on the surfaces of a wide variety of bacteria and archaea. The major subunit of T4P is the type IV pilin, and structurally related proteins are found as components of the type II secretion (T2S) system, where they are called pseudopilins; of DNA uptake/competence systems in both Gram-negative and Gram-positive species; and of flagella, pili, and sugar-binding systems in the archaea. This broad distribution of a single protein family implies both a common evolutionary origin and a highly adaptable functional plan. The type IV pilin is a remarkably versatile architectural module that has been adopted widely for a variety of functions, including motility, attachment to chemically diverse surfaces, electrical conductance, acquisition of DNA, and secretion of a broad range of structurally distinct protein substrates. In this review, we consider recent advances in this research area, from structural revelations to insights into diversity, posttranslational modifications, regulation, and function.

Secretion of TcpF by the Vibrio cholerae toxin-coregulated pilus biogenesis apparatus requires an N-terminal determinant

Type IV pili are important for microcolony formation, biofilm formation, twitching motility, and attachment. We and others have shown that type IV pili are important for protein secretion across the outer membrane, similar to type II secretion systems. This study explored the relationship between protein secretion and pilus formation in Vibrio cholerae. The toxin-coregulated pilus (TCP), a type IV pilus required for V. cholerae pathogenesis, is necessary for the secretion of the colonization factor TcpF (T. J. Kirn, N. Bose, and R. K. Taylor, Mol. Microbiol. 49:81-92, 2003). This phenomenon is not unique to V. cholerae; secreted virulence factors that are dependent on the presence of components of the type IV pilus biogenesis apparatus for secretion have been reported with Dichelobacter nodosus (R. M. Kennan, O. P. Dhungyel, R. J. Whittington, J. R. Egerton, and J. I. Rood, J. Bacteriol. 183:4451-4458, 2001) and Francisella tularensis (A. J. Hager et al., Mol. Microbiol. 62:227-237, 2006). Using site-directed mutagenesis, we demonstrated that the secretion of TcpF is dependent on the presence of selected amino acid R groups at position five. We were unable to find other secretion determinants, suggesting that Y5 is the major secretion determinant within TcpF. We also report that proteins secreted in a type IV pilus biogenesis apparatus-dependent manner have a YXS motif within the first 15 amino acids following the Sec cleavage site. The YXS motif is not present in proteins secreted by type II secretion systems, indicating that this is unique to type IV pilus-mediated secretion. Moreover, we show that TcpF interacts with the pilin TcpA, suggesting that these proteins are secreted by the type IV pilus biogenesis system. These data provide a starting point for understanding how type IV pili can mediate secretion of virulence factors important for bacterial pathogenesis.

Type II secretion in Yersinia-a secretion system for pathogenicity and environmental fitness

In Yersinia species, type III secretion (T3S) is the most prominent and best studied secretion system and a hallmark for the infection process of pathogenic Yersinia species. Type II secretion (T2S), on the other hand, is less well-characterized, although all Yersinia species, pathogenic as well as non-pathogenic, possess one or even two T2S systems. The only Yersinia strain in which T2S has so far been studied is the human pathogenic strain Y. enterocolitica 1b. Mouse infection experiments showed that at least one of the two T2S systems of Y. enterocolitica 1b, termed Yts1, is involved in dissemination and colonization of deeper tissues like liver and spleen. Interestingly, in vitro studies revealed a complex regulation of the Yts1 system, which is mainly active at low temperatures and high Mg²⁺-levels. Furthermore, the functional characterization of the proteins secreted in vitro indicates a role of the Yts1 machinery in survival of the bacteria in an environmental habitat. In silico analyses identified Yts1 homologous systems in bacteria that are known as plant symbionts or plant pathogens. Thus, the recent studies point to a dual function of the Yts1 T2S systems, playing a role in virulence of humans and animals, as well as in the survival of the bacteria outside of the mammalian host. In contrast, the role of the second T2S system, Yts2, remains ill defined. Whereas the T3S system and its virulence-mediating role has been intensively studied, it might now be time to also focus on the T2S system and its role in the Yersinia lifestyle, especially considering that most of the Yersinia isolates are not found in infected humans but have been gathered from various environmental samples.

Prime time for minor subunits of the type II secretion and type IV pilus systems

The type II secretion system (T2SS) exports folded proteins from the periplasms of Gram-negative bacteria. The type IV pilus system (T4PS) is a multifunctional machine used for adherence, motility and DNA transfer in bacteria and archaea. Partial sequence identity between the two systems suggests that they are related and might function via a similar mechanism, the dynamic assembly and disassembly of pseudopilus (T2SS) or pilus (T4PS) filaments. The major subunit in each system is thought to form the bulk of the (pseudo)pilus, while minor (low-abundance) subunits have proposed roles in assembly initiation, antagonism of disassembly, or modulation of (pseudo)pilus functional properties. In this issue, Cisneros et al. () extend their previous finding that pseudopilus assembly is primed by the minor pseudopilins, showing that the same proteins can initiate assembly of Escherichia coli T4P. Similarly, they show that the E. coli minor pilins prime the polymerization of T2S pseudopili, although unlike genuine pseudopili, the chimeric filaments did not support secretion. This work reinforces the notion of a common assembly mechanism for the T2S and T4P systems.

Minor pseudopilin self-assembly primes type II secretion pseudopilus elongation

In Gram-negative bacteria, type II secretion systems (T2SS) assemble inner membrane proteins of the major pseudopilin PulG (GspG) family into periplasmic filaments, which could drive protein secretion in a piston-like manner. Three minor pseudopilins PulI, PulJ and PulK are essential for protein secretion in the Klebsiella oxytoca T2SS, but their molecular function is unknown. Here, we demonstrate that together these proteins prime pseudopilus assembly, without actively controlling its length or secretin channel opening. Using molecular dynamics, bacterial two-hybrid assays, cysteine crosslinking and functional analysis, we show that PulI and PulJ nucleate filament assembly by forming a staggered complex in the plasma membrane. Binding of PulK to this complex results in its partial extraction from the membrane and in a 1-nm shift between their transmembrane segments, equivalent to the major pseudopilin register in the assembled PulG filament. This promotes fully efficient pseudopilus assembly and protein secretion. Therefore, we propose that PulI, PulJ and PulK self-assembly is thermodynamically coupled to the initiation of pseudopilus assembly, possibly setting the assembly machinery in motion.

Evolutionary and functional diversity of the Pseudomonas type IVa pilin island

In Pseudomonas aeruginosa, most proteins involved in type IVa pilus (T4aP) biogenesis are highly conserved except for the major pilin PilA and the minor pilins involved in pilus assembly. Here we show that each of the five major pilin alleles is associated with a specific set of minor pilins, and unrelated strains with the same major pilin type have identical minor pilin genes. The sequences of the minor pilin genes of strains with group III and V pilins are identical, suggesting that these groups diverged recently through further evolution of the major pilin cluster. Both gene clusters are localized on a single 'pilin island' containing putative tRNA recombinational hotspots, and a similar organization of pilin genes was identified in other Pseudomonas species. To address the biological significance of group-specific differences, cross-complementation studies using group II (PAO1) and group III (PA14) minor pilins were performed. Heterologous minor pilins complemented twitching motility to various extents except in the case of PilX, which was non-functional in non-native backgrounds. A recombinant PA14 strain expressing the PAO1 minor pilins regained motility only upon co-introduction of the PA14 pilX gene. Comparison of PilX and PilQ secretin sequences from group II, III and V genomes revealed discrete regions of sequence that co-varied between groups. Our data suggest that changes in PilX sequence have led to compensatory changes in the PilQ secretin monomer such that heterologous PilX proteins are no longer able to promote opening of the secretin to allow pili to appear on the cell surface.

Detailed structural and assembly model of the type II secretion pilus from sparse data

Many gram-negative bacteria secrete specific proteins via the type II secretion systems (T2SS). These complex machineries share with the related archaeal flagella and type IV pilus (T4P) biogenesis systems the ability to assemble thin, flexible filaments composed of small, initially inner membrane-localized proteins called "pilins." In the T2SS from Klebsiella oxytoca, periplasmic pseudopili that are essential for pullulanase (PulA) secretion extend beyond the bacterial surface and form pili when the major pilin PulG is overproduced. Here, we describe the detailed, experimentally validated structure of the PulG pilus generated from crystallographic and electron microscopy data by a molecular modeling approach. Two intermolecular salt bridges crucial for function were demonstrated using single and complementary charge inversions. Double-cysteine substitutions in the transmembrane segment of PulG led to position-specific cross-linking of protomers in assembled pili. These biochemical data provided information on residue distances in the filament that were used to derive a refined model of the T2SS pilus at pseudoatomic resolution. PulG is organized as a right-handed helix of subunits, consistent with protomer organization in gonococcal T4P. The conserved character of residues involved in key hydrophobic and electrostatic interactions within the major pseudopilin family supports the general relevance of this model for T2SS pseudopilus structure.

Many substrates and functions of type II secretion: lessons learned from Legionella pneumophila

Type II secretion is one of six systems that exist in Gram-negative bacteria for the purpose of secreting proteins into the extracellular milieu and/or into host cells. This article will review the various recent studies of Legionella pneumophila that have increased our appreciation of the numbers, types and novelties of proteins that can be secreted via the type II system, as well as the many ways in which type II secretion can promote bacterial physiology, growth, ecology, intracellular infection and virulence. In this context, type II secretion represents a potentially important target for industrial and biomedical applications.

A type II secreted RNase of Legionella pneumophila facilitates optimal intracellular infection of Hartmannella vermiformis

Type II protein secretion plays a role in a wide variety of functions that are important for the ecology and pathogenesis of Legionella pneumophila. Perhaps most dramatic is the critical role that this secretion pathway has in L. pneumophila intracellular infection of aquatic protozoa. Recently, we showed that virulent L. pneumophila strain 130b secretes RNase activity through its type II secretion system. We now report the cloning and mutational analysis of the gene (srnA) encoding that novel type of secreted activity. The SrnA protein was defined as being a member of the T2 family of secreted RNases. Supernatants from mutants inactivated for srnA completely lacked RNase activity, indicating that SrnA is the major secreted RNase of L. pneumophila. Although srnA mutants grew normally in bacteriological media and human U937 cell macrophages, they were impaired in their ability to grow within Hartmannella vermiformis amoebae. This finding represents the second identification of a L. pneumophila type II effector being necessary for optimal intracellular infection of amoebae, with the first being the ProA zinc metalloprotease. Newly constructed srnA proA double mutants displayed an even larger infection defect that appeared to be the additive result of losing both SrnA and ProA. Overall, these data represent the first demonstration of a secreted RNase promoting an intracellular infection event, and support our long-standing hypothesis that the infection defects of L. pneumophila type II secretion mutants are due to the loss of multiple secreted effectors.