Kingella kingae RtxA Cytotoxin in the Context of Other RTX Toxins

The Gram-negative bacterium Kingella kingae is part of the commensal oropharyngeal flora of young children. As detection methods have improved, K. kingae has been increasingly recognized as an emerging invasive pathogen that frequently causes skeletal system infections, bacteremia, and severe forms of infective endocarditis. K. kingae secretes an RtxA cytotoxin, which is involved in the development of clinical infection and belongs to an ever-growing family of cytolytic RTX (Repeats in ToXin) toxins secreted by Gram-negative pathogens. All RTX cytolysins share several characteristic structural features: (i) a hydrophobic pore-forming domain in the N-terminal part of the molecule; (ii) an acylated segment where the activation of the inactive protoxin to the toxin occurs by a co-expressed toxin-activating acyltransferase; (iii) a typical calcium-binding RTX domain in the C-terminal portion of the molecule with the characteristic glycine- and aspartate-rich nonapeptide repeats; and (iv) a C-proximal secretion signal recognized by the type I secretion system. RTX toxins, including RtxA from K. kingae, have been shown to act as highly efficient 'contact weapons' that penetrate and permeabilize host cell membranes and thus contribute to the pathogenesis of bacterial infections. RtxA was discovered relatively recently and the knowledge of its biological role remains limited. This review describes the structure and function of RtxA in the context of the most studied RTX toxins, the knowledge of which may contribute to a better understanding of the action of RtxA in the pathogenesis of K. kingae infections.

Identity Determinants of the Translocation Signal for a Type 1 Secretion System

The toxin hemolysin A was first identified in uropathogenic E. coli strains and shown to be secreted in a one-step mechanism by a dedicated secretion machinery. This machinery, which belongs to the Type I secretion system family of the Gram-negative bacteria, is composed of the outer membrane protein TolC, the membrane fusion protein HlyD and the ABC transporter HlyB. The N-terminal domain of HlyA represents the toxin which is followed by a RTX (Repeats in Toxins) domain harboring nonapeptide repeat sequences and the secretion signal at the extreme C-terminus. This secretion signal, which is necessary and sufficient for secretion, does not appear to require a defined sequence, and the nature of the encoded signal remains unknown. Here, we have combined structure prediction based on the AlphaFold algorithm together with functional and in silico data to examine the role of secondary structure in secretion. Based on the presented data, a C-terminal, amphipathic helix is proposed between residues 975 and 987 that plays an essential role in the early steps of the secretion process.

An A/U-Rich Enhancer Region Is Required for High-Level Protein Secretion through the HlyA Type I Secretion System

Efficient protein secretion is often a valuable alternative to classic cellular expression to obtain homogenous protein samples. Early on, bacterial type I secretion systems (T1SS) were employed to allow heterologous secretion of fusion proteins. However, this approach was not fully exploited, as many proteins could not be secreted at all or only at low levels. Here, we present an engineered microbial secretion system which allows the effective production of proteins up to a molecular mass of 88 kDa. This system is based on the hemolysin A (HlyA) T1SS of the Gram-negative bacterium Escherichia coli, which exports polypeptides when fused to a hemolysin secretion signal. We identified an A/U-rich enhancer region upstream of hlyA required for effective expression and secretion of selected heterologous proteins irrespective of their prokaryotic, viral, or eukaryotic origin. We further demonstrate that the ribosomal protein S1 binds to the hlyA A/U-rich enhancer region and that this region is involved in the high yields of secretion of functional proteins, like maltose-binding protein or human interferon alpha-2.IMPORTANCE A 5' untranslated region of the mRNA of substrates of type I secretion systems (T1SS) drastically enhanced the secretion efficiency of the endogenously secreted protein. The identification of ribosomal protein S1 as the interaction partner of this 5' untranslated region provides a rationale for the enhancement. This strategy furthermore can be transferred to fusion proteins allowing a broader, and eventually a more general, application of this system for secreting heterologous fusion proteins.

Type I Protein Secretion-Deceptively Simple yet with a Wide Range of Mechanistic Variability across the Family

A very large type I polypeptide begins to reel out from a ribosome; minutes later, the still unidentifiable polypeptide, largely lacking secondary structure, is now in some cases a thousand or more residues longer. Synthesis of the final hundred C-terminal residues commences. This includes the identity code, the secretion signal within the last 50 amino acids, designed to dock with a waiting ATP binding cassette (ABC) transporter. What happens next is the subject of this review, with the main, but not the only focus on hemolysin HlyA, an RTX protein toxin secreted by the type I system. Transport substrates range from small peptides to giant proteins produced by many pathogens. These molecules, without detectable cellular chaperones, overcome enormous barriers, crossing two membranes before final folding on the cell surface, involving a unique autocatalytic process.Unfolded HlyA is extruded posttranslationally, C-terminal first. The transenvelope "tunnel" is formed by HlyB (ABC transporter), HlyD (membrane fusion protein) straddling the inner membrane and periplasm and TolC (outer membrane). We present a new evaluation of the C-terminal secretion code, and the structure function of HlyD and HlyB at the heart of this nanomachine. Surprisingly, key details of the secretion mechanism are remarkably variable in the many type I secretion system subtypes. These include alternative folding processes, an apparently distinctive secretion code for each type I subfamily, and alternative forms of the ABC transporter; most remarkably, the ABC protein probably transports peptides or polypeptides by quite different mechanisms. Finally, we suggest a putative structure for the Hly-translocon, HlyB, the multijointed HlyD, and the TolC exit.

Interdomain regulation of the ATPase activity of the ABC transporter haemolysin B from Escherichia coli

Type 1 secretion systems (T1SS) transport a wide range of substrates across both membranes of Gram-negative bacteria and are composed of an outer membrane protein, a membrane fusion protein and an ABC (ATP-binding cassette) transporter. The ABC transporter HlyB (haemolysin B) is part of a T1SS catalysing the export of the toxin HlyA in E. coli. HlyB consists of the canonical transmembrane and nucleotide-binding domains. Additionally, HlyB contains an N-terminal CLD (C39-peptidase-like domain) that interacts with the transport substrate, but its functional relevance is still not precisely defined. In the present paper, we describe the purification and biochemical characterization of detergent-solubilized HlyB in the presence of its transport substrate. Our results exhibit a positive co-operativity in ATP hydrolysis. We characterized further the influence of the CLD on kinetic parameters by using an HlyB variant lacking the CLD (HlyB∆CLD). The biochemical parameters of HlyB∆CLD revealed an increased basal maximum velocity but no change in substrate-binding affinity in comparison with full-length HlyB. We also assigned a distinct interaction of the CLD and a transport substrate (HlyA1), leading to an inhibition of HlyB hydrolytic activity at low HlyA1 concentrations. At higher HlyA1 concentrations, we observed a stimulation of the hydrolytic activities of both HlyB and HlyB∆CLD, which was completely independent of the interaction of HlyA1 with the CLD. Notably, all observed effects on ATPase activity, which were also analysed in detail by mass spectrometry, were independent of the HlyA1 secretion signal. These results assign an interdomain regulatory role for the CLD modulating the hydrolytic activity of HlyB.

Equilibrium folding of pro-HlyA from Escherichia coli reveals a stable calcium ion dependent folding intermediate

HlyA from Escherichia coli is a member of the repeats in toxin (RTX) protein family, produced by a wide range of Gram-negative bacteria and secreted by a dedicated Type 1 Secretion System (T1SS). RTX proteins are thought to be secreted in an unfolded conformation and to fold upon secretion by Ca(2+) binding. However, the exact mechanism of secretion, ion binding and folding to the correct native state remains largely unknown. In this study we provide an easy protocol for high-level pro-HlyA purification from E. coli. Equilibrium folding studies, using intrinsic tryptophan fluorescence, revealed the well-known fact that Ca(2+) is essential for stability as well as correct folding of the whole protein. In the absence of Ca(2+), pro-HlyA adopts a non-native conformation. Such molecules could however be rescued by Ca(2+) addition, indicating that these are not dead-end species and that Ca(2+) drives pro-HlyA folding. More importantly, pro-HlyA unfolded via a two-state mechanism, whereas folding was a three-state process. The latter is indicative of the presence of a stable folding intermediate. Analysis of deletion and Trp mutants revealed that the first folding transition, at 6-7M urea, relates to Ca(2+) dependent structural changes at the extreme C-terminus of pro-HlyA, sensed exclusively by Trp914. Since all Trp residues of HlyA are located outside the RTX domain, our results demonstrate that Ca(2+) induced folding is not restricted to the RTX domain. Taken together, Ca(2+) binding to the pro-HlyA RTX domain is required to drive the folding of the entire protein to its native conformation.

The Type 1 secretion pathway - the hemolysin system and beyond

Type 1 secretion systems (T1SS) are wide-spread among Gram-negative bacteria. An important example is the secretion of the hemolytic toxin HlyA from uropathogenic strains. Secretion is achieved in a single step directly from the cytosol to the extracellular space. The translocation machinery is composed of three indispensable membrane proteins, two in the inner membrane, and the third in the outer membrane. The inner membrane proteins belong to the ABC transporter and membrane fusion protein families (MFPs), respectively, while the outer membrane component is a porin-like protein. Assembly of the three proteins is triggered by accumulation of the transport substrate (HlyA) in the cytoplasm, to form a continuous channel from the inner membrane, bridging the periplasm and finally to the exterior. Interestingly, the majority of substrates of T1SS contain all the information necessary for targeting the polypeptide to the translocation channel - a specific sequence at the extreme C-terminus. Here, we summarize our current knowledge of regulation, channel assembly, translocation of substrates, and in the case of the HlyA toxin, its interaction with host membranes. We try to provide a complete picture of structure function of the components of the translocation channel and their interaction with the substrate. Although we will place the emphasis on the paradigm of Type 1 secretion systems, the hemolysin A secretion machinery from E. coli, we also cover as completely as possible current knowledge of other examples of these fascinating translocation systems. This article is part of a Special Issue entitled: Protein trafficking and secretion in bacteria. Guest Editors: Anastassios Economou and Ross Dalbey.

An RTX transporter tethers its unfolded substrate during secretion via a unique N-terminal domain

Type 1 secretion systems (T1SS) catalyze the one step protein transport across the membranes of Gram-negative bacteria and are composed of an outer membrane protein, a membrane fusion protein and an ABC transporter. The ABC transporter consists of the canonical nucleotide binding and transmembrane domains. For the toxin hemolysin A (HlyA), the ABC transporter HlyB carries an additional, N-terminal domain sharing about 40% homology to C39 peptidases, but this "C39-like domain" (CLD) is suggested to feature another, yet unknown function. Our functional and structural analysis demonstrates that the CLD is essential for secretion and that it specifically interacts with the unfolded state of HlyA. We determined the nuclear magnetic resonance structure of the CLD as well as the substrate-binding region within the CLD. This mode of action, represents a mechanism within T1SS and answers the question, how a large and unfolded substrate is protected inside the cells during secretion.

Functional dissection of SiiE, a giant non-fimbrial adhesin of Salmonella enterica

Salmonella enterica deploys the giant non-fimbrial adhesin SiiE to adhere to the apical side of polarized epithelial cells. The establishment of close contact is a prerequisite for subsequent invasion mediated by translocation of effector proteins of the Salmonella Pathogenicity Island 1 (SPI1)-encoded type III secretion system (T3SS). Although SiiE is secreted into the culture medium, the adhesin is retained on the bacterial envelope in the phase of highest bacterial invasiveness. To dissect the structural requirements for secretion, retention and adhesive properties, comprehensive deletional and functional analyses of various domains of SiiE were performed. We observed that β-sheet and coiled-coil domains in the N-terminal moiety of SiiE are required for the control of SiiE retention on the surface and co-ordinated release. These results indicate a novel molecular mechanism for the control of surface display of a T1SS-secreted adhesin that acts cooperatively with the SPI1-T3SS.

Calcium-induced folding and stabilization of the intrinsically disordered RTX domain of the CyaA toxin

The adenylate cyclase toxin (CyaA) is one of the major virulence factors of Bordetella pertussis, the causative agent of whooping cough. Its C-terminal region, the receptor-binding domain (RD), contains ∼40 calcium-binding Repeat in ToXin (RTX) motifs, which are characteristic of many virulence factors of pathogenic bacteria. We previously showed that RD is intrinsically disordered in the absence of calcium and acquires its functional three-dimensional structure upon calcium binding. To gain further insight into the physicochemical properties of RD, we characterized its calcium-induced conformational and stability changes by combining spectroscopic approaches. We show that RD, in the absence of calcium, adopts premolten globule conformations, due in part to the strong internal electrostatic repulsions between the negative charges of the aspartate-rich polypeptide sequence. Accordingly, sodium is able to screen these electrostatic repulsions, allowing a partial compaction of the polypeptide, whereas calcium triggers a strong compaction as well as the acquisition of secondary and tertiary structures in a highly cooperative manner. The differential sensitivity of the calcium-loaded state to guanidinium- and urea-induced denaturations provides further evidence that electrostatic interactions play a critical role in the folding and stability of RD. These results provide new insights into the folding/function relationship of the RTX motifs.