The passenger-associated transport repeat promotes virulence factor secretion efficiency and delineates a distinct autotransporter subtype

Crystal structure of a subtilisin-like autotransporter passenger domain reveals insights into its cytotoxic function

Autotransporters (ATs) are a large family of bacterial secreted and outer membrane proteins that encompass a wide range of enzymatic activities frequently associated with pathogenic phenotypes. We present the structural and functional characterisation of a subtilase autotransporter, Ssp, from the opportunistic pathogen Serratia marcescens. Although the structures of subtilases have been well documented, this subtilisin-like protein is associated with a 248 residue β-helix and itself includes three finger-like protrusions around its active site involved in substrate interactions. We further reveal that the activity of the subtilase AT is required for entry into epithelial cells as well as causing cellular toxicity. The Ssp structure not only provides details about the subtilase ATs, but also reveals a common framework and function to more distantly related ATs. As such these findings also represent a significant step forward toward understanding the molecular mechanisms underlying the functional divergence in the large AT superfamily.

Adhesive Functions or Pseudogenization of Type Va Autotransporters in Brucella Species

Adhesion to host cells is a key step for successful infection of many bacterial pathogens and may define tropism to different host tissues. To do so, bacteria display adhesins on their surfaces. Brucella is an intracellular pathogen capable of proliferating in a wide variety of cell types. It has been described that BmaC, a large protein that belongs to the classical (type Va) autotransporter family, is required for efficient adhesion of Brucella suis strain 1330 to epithelial cells and fibronectin. Here we show that B. suis 1330 harbors two other type Va autotransporters (BmaA and BmaB), which, although much smaller, share significant sequence similarities with BmaC and contain the essential domains to mediate proper protein translocation to the bacterial surface. Gain and loss of function studies indicated that BmaA, BmaB, and BmaC contribute, to a greater or lesser degree, to adhesion of B. suis 1330 to different cells such as synovial fibroblasts, osteoblasts, trophoblasts, and polarized epithelial cells as well as to extracellular matrix components. It was previously shown that BmaC localizes to a single bacterial pole. Interestingly, we observed here that, similar to BmaC, the BmaB adhesin is localized mostly at a single cell pole, reinforcing the hypothesis that Brucella displays an adhesive pole. Although Brucella species have strikingly similar genomes, they clearly differ in their host preferences. Mainly, the differences identified between species appear to be at loci encoding surface proteins. A careful in silico analysis of the putative type Va autotransporter orthologues from several Brucella strains showed that the bmaB locus from Brucella abortus and both, the bmaA and bmaC loci from Brucella melitensis are pseudogenes in all strains analyzed. Results reported here evidence that all three autotransporters play a role in the adhesion properties of B. suis 1330. However, Brucella spp. exhibit extensive variations in the repertoire of functional adhesins of the classical autotransporter family that can be displayed on the bacterial surface, making them an interesting target for future studies on host preference and tropism.

The virulence domain of Shigella IcsA contains a subregion with specific host cell adhesion function

Shigella species cause bacillary dysentery, especially among young individuals. Shigellae target the human colon for invasion; however, the initial adhesion mechanism is poorly understood. The Shigella surface protein IcsA, in addition to its role in actin-based motility, acts as a host cell adhesin through unknown mechanism(s). Here we confirmed the role of IcsA in cell adhesion and defined the region required for IcsA adhesin activity. Purified IcsA passenger domain was able block S. flexneri adherence and was also used as a molecular probe that recognised multiple components from host cells. The region within IcsA's functional passenger domain (aa 138-148) was identified by mutagenesis. Upon the deletion of this region, the purified IcsAΔ138-148 was found to no longer block S. flexneri adherence and had reduced ability to interact with host molecules. Furthermore, S. flexneri expressing IcsAΔ138-148 was found to be significantly defective in both cell adherence and invasion. Taken together, our data identify an adherence region within the IcsA functional domain and provides useful information for designing therapeutics for Shigella infection.

Type V Secretion in Gram-Negative Bacteria

Type V, or "autotransporter," secretion is a term used to refer to several simple protein export pathways that are found in a wide range of Gram-negative bacteria. Autotransporters are generally single polypeptides that consist of an extracellular ("passenger") domain and a β barrel domain that anchors the protein to the outer membrane (OM). Although it was originally proposed that the passenger domain is secreted through a channel formed solely by the covalently linked β barrel domain, experiments performed primarily on the type Va, or "classical," autotransporter pathway have challenged this hypothesis. Several lines of evidence strongly suggest that both the secretion of the passenger domain and the membrane integration of the β barrel domain are catalyzed by the barrel assembly machinery (Bam) complex, a conserved hetero-oligomer that plays an essential role in the assembly of most integral OM proteins. The secretion reaction appears to be driven at least in part by the folding of the passenger domain in the extracellular space. Although many aspects of autotransporter biogenesis remain to be elucidated, it will be especially interesting to determine whether the different classes of proteins that fall under the type V rubric-most of which have not been examined in detail-are assembled by the same basic mechanism as classical autotransporters.

Structural insights into the architecture of the Shigella flexneri virulence factor IcsA/VirG and motifs involved in polar distribution and secretion

IcsA/VirG is a key virulence factor of the human pathogen Shigella flexneri, acting as both an adhesin and actin-polymerizing factor during infection. We identified a soluble expression construct of the IcsA/VirG α-domain using the ESPRIT library screening system and determined its structure to 1.9Å resolution. In addition to the previously characterized autochaperone domain, our structure reveals a new domain, which shares a common fold with the autochaperone domains of various autotransporters. We further provide insight into the previously structurally uncharacterized β-helix domain that harbors the polar targeting motif and passenger-associated transport repeat. This structure is the first of any member of the recently identified passenger-associated transport repeat-containing autotransporters. Thus, it provides new insights into the overall architecture of this class of autotransporters, the function of the identified additional autochaperone domain and the structural properties of motifs involved in polar targeting and secretion of the Shigella flexneri virulence factor IcsA/VirG.

Secretion of the Intimin Passenger Domain Is Driven by Protein Folding

Intimin is an essential adhesin of attaching and effacing organisms such as entropathogenic Escherichia coli It is also the prototype of type Ve secretion or inverse autotransport, where the extracellular C-terminal region or passenger is exported with the help of an N-terminal transmembrane β-barrel domain. We recently reported a stalled secretion intermediate of intimin, where the passenger is located in the periplasm but the β-barrel is already inserted into the membrane. Stalling of this mutant is due to the insertion of an epitope tag at the very N terminus of the passenger. Here, we examined how this insertion disrupts autotransport and found that it causes misfolding of the N-terminal immunoglobulin (Ig)-like domain D00. We could also stall the secretion by making an internal deletion in D00, and introducing the epitope tag into the second Ig-like domain, D0, also resulted in reduced passenger secretion. In contrast to many classical autotransporters, where a proximal folding core in the passenger is required for secretion, the D00 domain is dispensable, as the passenger of an intimin mutant lacking D00 entirely is efficiently exported. Furthermore, the D00 domain is slightly less stable than the D0 and D1 domains, unfolding at ∼200 piconewtons (pN) compared with ∼250 pN for D0 and D1 domains as measured by atomic force microscopy. Our results support a model where the secretion of the passenger is driven by sequential folding of the extracellular Ig-like domains, leading to vectorial transport of the passenger domain across the outer membrane in an N to C direction.

A small conserved motif supports polarity augmentation of Shigella flexneri IcsA

The rod-shaped enteric intracellular pathogen Shigella flexneri and other Shigella species are the causative agents of bacillary dysentery. S. flexneri are able to spread within the epithelial lining of the gut, resulting in lesion formation, cramps and bloody stools. The outer membrane protein IcsA is essential for this spreading process. IcsA is the initiator of an actin-based form of motility whereby it allows the formation of a filamentous actin 'tail' at the bacterial pole. Importantly, IcsA is specifically positioned at the bacterial pole such that this process occurs asymmetrically. The mechanism of IcsA polarity is not completely understood, but it appears to be a multifactorial process involving factors intrinsic to IcsA and other regulating factors. In this study, we further investigated IcsA polarization by its intramolecular N-terminal and central polar-targeting (PT) regions (nPT and cPT regions, respectively). The results obtained support a role in polar localization for the cPT region and contend the role of the nPT region. We identified single IcsA residues that have measurable impacts on IcsA polarity augmentation, resulting in decreased S. flexneri sprading efficiency. Intriguingly, regions and residues involved in PT clustered around a highly conserved motif which may provide a functional scaffold for polarity-augmenting residues. How these results fit with the current model of IcsA polarity determination is discussed.