Role of N-terminal His6-Tags in binding and efficient translocation of polypeptides into cells using anthrax protective antigen (PA).

It is of interest to define bacterial toxin biochemical properties to use them as molecular-syringe devices in order to deliver enzymatic activities into host cells. Binary toxins of the AB(7/8)-type are among the most potent and specialized bacterial protein toxins. The B subunits oligomerize to form a pore that binds with high affinity host cell receptors and the enzymatic A subunit. This allows the endocytosis of the complex and subsequent injection of the A subunit into the cytosol of the host cells. Here we report that the addition of an N-terminal His(6)-tag to different proteins increased their binding affinity to the protective antigen (PA) PA(63)-channels, irrespective if they are related (C2I) or unrelated (gpJ, EDIN) to the AB(7/8)-family of toxins. His(6)-EDIN exhibited voltage-dependent increase of the stability constant for binding by a factor of about 25 when the trans-side corresponding to the cell interior was set to -70 mV. Surprisingly, the C. botulinum toxin C2II-channel did not share this feature of PA(63). Cell-based experiments demonstrated that addition of an N-terminal His(6)-tag promoted also intoxication of endothelial cells by C2I or EDIN via PA(63). Our results revealed that addition of His(6)-tags to several factors increase their binding properties to PA(63) and enhance the property to intoxicate cells.

Cross-reactivity of anthrax and C2 toxin: protective antigen promotes the uptake of botulinum C2I toxin into human endothelial cells.

Binary toxins are among the most potent bacterial protein toxins performing a cooperative mode of translocation and exhibit fatal enzymatic activities in eukaryotic cells. Anthrax and C2 toxin are the most prominent examples for the AB(7/8) type of toxins. The B subunits bind both host cell receptors and the enzymatic A polypeptides to trigger their internalization and translocation into the host cell cytosol. C2 toxin is composed of an actin ADP-ribosyltransferase (C2I) and C2II binding subunits. Anthrax toxin is composed of adenylate cyclase (EF) and MAPKK protease (LF) enzymatic components associated to protective antigen (PA) binding subunit. The binding and translocation components anthrax protective antigen (PA(63)) and C2II of C2 toxin share a sequence homology of about 35%, suggesting that they might substitute for each other. Here we show by conducting in vitro measurements that PA(63) binds C2I and that C2II can bind both EF and LF. Anthrax edema factor (EF) and lethal factor (LF) have higher affinities to bind to channels formed by C2II than C2 toxin's C2I binds to anthrax protective antigen (PA(63)). Furthermore, we could demonstrate that PA in high concentration has the ability to transport the enzymatic moiety C2I into target cells, causing actin modification and cell rounding. In contrast, C2II does not show significant capacity to promote cell intoxication by EF and LF. Together, our data unveiled the remarkable flexibility of PA in promoting C2I heterologous polypeptide translocation into cells.

Escherichia coli producing CNF1 toxin hijacks Tollip to trigger Rac1-dependent cell invasion.

Rho GTPases, which are master regulators of both the actin cytoskeleton and membrane trafficking, are often hijacked by pathogens to enable their invasion of host cells. Here we report that the cytotoxic necrotizing factor-1 (CNF1) toxin of uropathogenic Escherichia coli (UPEC) promotes Rac1-dependent entry of bacteria into host cells. Our screen for proteins involved in Rac1-dependent UPEC entry identifies the Toll-interacting protein (Tollip) as a new interacting protein of Rac1 and its ubiquitinated forms. We show that knockdown of Tollip reduces CNF1-induced Rac1-dependent UPEC entry. Tollip depletion also reduces the Rac1-dependent entry of Listeria monocytogenes expressing InlB invasion protein. Moreover, knockdown of Tollip, Tom1 and clathrin, decreases CNF1 and Rac1-dependent internalization of UPEC. Finally, we show that Tollip, Tom1 and clathrin associate with Rac1 and localize at the site of bacterial entry. Collectively, these findings reveal a new link between Rac1 and Tollip, Tom1 and clathrin membrane trafficking components hijacked by pathogenic bacteria to allow their efficient invasion of host cells.

Transcriptome dysregulation by anthrax lethal toxin plays a key role in induction of human endothelial cell cytotoxicity.

We have investigated how Bacillus anthracis lethal toxin (LT) triggers caspase-3 activation and the formation of thick actin cables in human endothelial cells. By DNA array analysis we show that LT has a major impact on the cell transcriptome and we identify key host genes involved in LT cytotoxic effects. Indeed, upregulation of TRAIL and downregulation of XIAP both participate in LT-induced caspase-3 activation. LT induces a downregulation of the immediate early gene and master regulator of transcription egr1. Importantly, its re-expression in LT-intoxicated cells blocks caspase-3 activation. In parallel, we found that the formation of actin cables induced by LT occurs in the absence of direct activation of RhoA/ROCK signalling. We show that knock-down of cortactin and rhophilin-2 under conditions of calponin-1 expression defines the minimal set of genes regulated by LT for actin cable formation. Together our data establish that the modulation of the cell transcriptome by LT plays a key role in triggering human endothelial cell toxicity.

Injection of Staphylococcus aureus EDIN by the Bacillus anthracis protective antigen machinery induces vascular permeability.

Systemic injection of Bacillus anthracis lethal toxin (LT) produces vascular leakage and animal death. Recent studies suggest that LT triggers direct endothelial cell cytotoxicity that is responsible for the vascular leakage. LT is composed of heptamers of protective antigen (PA), which drives the endocytosis and translocation into host cells of the lethal factor (LF), a mitogen-activated protein kinase kinase protease. Here we investigated the consequences of injection of an endothelium-permeabilizing factor using LT as a "molecular syringe." To this end, we generated the chimeric factor LE, corresponding to the PA-binding domain of LF (LF(1-254)) fused to EDIN exoenzyme. EDIN ADP ribosylates RhoA, leading to actin cable disruption and formation of transcellular tunnels in endothelial cells. We report that systemic injection of LET (LE plus PA) triggers a PA-dependent increase in the pulmonary endothelium permeability. We also report that native LT induces a progressive loss of endothelium barrier function. We established that there is a direct correlation between the extent of endothelium permeability induced by LT and the cytotoxic activity of LT. This suggests new ways to design therapeutic drugs against anthrax directed toward vascular permeability.

Structural basis for the NAD-hydrolysis mechanism and the ARTT-loop plasticity of C3 exoenzymes.

C3-like exoenzymes are ADP-ribosyltransferases that specifically modify some Rho GTPase proteins, leading to their sequestration in the cytoplasm, and thus inhibiting their regulatory activity on the actin cytoskeleton. This modification process goes through three sequential steps involving NAD-hydrolysis, Rho recognition, and binding, leading to Rho ADP-ribosylation. Independently, three distinct residues within the ARTT loop of the C3 exoenzymes are critical for each of these steps. Supporting the critical role of the ARTT loop, we have shown previously that it adopts a distinct conformation upon NAD binding. Here, we present seven wild-type and ARTT loop-mutant structures of C3 exoenzyme of Clostridium botulinum free and bound to its true substrate, NAD, and to its NAD-hydrolysis product, nicotinamide. Altogether, these structures expand our understanding of the conformational diversity of the C3 exoenzyme, mainly within the ARTT loop.

Isoform-specific interaction of C-RAF with mitochondria.

The proteins of the RAF family (A-RAF, B-RAF, and C-RAF) are serine/threonine kinases that play important roles in development, mature cell regulation, and cancer. Although it is widely held that their localization on membranes is an important aspect of their function, there are few data that address this aspect of their mode of action. Here, we report that each member of the RAF family exhibits a specific distribution at the level of cellular membranes and that C-RAF is the only isoform that directly targets mitochondria. We found that the RAF kinases exhibit intrinsic differences in terms of mitochondrial affinity and that C-RAF is the only isoform that binds this organelle efficiently. This affinity is conferred by the C-RAF amino-terminal domain and does not depend on the presence of RAS GTPases on the surface of mitochondria. Finally, we analyzed the consequences of C-RAF activation on mitochondria and observed that this event dramatically changes their morphology and their subcellular distribution. Our observations indicate that: (i) RAF kinases exhibit different localizations at the level of cellular membranes; (ii) C-RAF is the only isoform that directly binds mitochondria; and (iii) through its functional coupling with MEK, C-RAF regulates the shape and the cellular distribution of mitochondria.

Intranasal immunization with tetanus toxoid and CNF1 as a new mucosal adjuvant protects BALB/c mice against lethal challenge.

Although often requiring the development of efficient adjuvants, needle-free mucosal delivery of vaccine is of major interest as a strategy of mass immunization against infectious diseases. We report that mucosal immunization against tetanus toxoid through nasal route, together with active cytotoxic necrotizing factor 1 (CNF1), elicits a specific and long lasting anti-tetanus toxin response, comprising seric IgG and IgA, as well as mucosal IgA. Immunized mice were protected against a challenge with lethal doses of tetanus toxin (10 x LD(50)). The Rho GTPase activating toxin CNF1 is thus an attractive mucosal adjuvant candidate for nasal vaccines.

Rho/Rho-associated kinase-II signaling mediates disassembly of epithelial apical junctions.

Apical junctional complex (AJC) plays a vital role in regulation of epithelial barrier function. Disassembly of the AJC is observed in diverse physiological and pathological states; however, mechanisms governing this process are not well understood. We previously reported that the AJC disassembly is driven by the formation of apical contractile acto-myosin rings. In the present study, we analyzed the signaling pathways regulating acto-myosin-dependent disruption of AJC by using a model of extracellular calcium depletion. Pharmacological inhibition analysis revealed a critical role of Rho-associated kinase (ROCK) in AJC disassembly in calcium-depleted epithelial cells. Furthermore, small interfering RNA (siRNA)-mediated knockdown of ROCK-II, but not ROCK-I, attenuated the disruption of the AJC. Interestingly, AJC disassembly was not dependent on myosin light chain kinase and myosin phosphatase. Calcium depletion resulted in activation of Rho GTPase and transient colocalization of Rho with internalized AJC proteins. Pharmacological inhibition of Rho prevented AJC disassembly. Additionally, Rho guanine nucleotide exchange factor (GEF)-H1 translocated to contractile F-actin rings after calcium depletion, and siRNA-mediated depletion of GEF-H1 inhibited AJC disassembly. Thus, our findings demonstrate a central role of the GEF-H1/Rho/ROCK-II signaling pathway in the disassembly of AJC in epithelial cells.

Bacteria and the ubiquitin pathway.

Ubiquitylation participates in a repertoire of reversible post-translational modifications that modulate the function, localization and half-life of proteins by regulating their association with various ubiquitin-binding proteins. In response to pathogen infection, bacterial effectors impact ubiquitin and ubiquitin-like modifications of key proteins in immune and anti-apoptotic signaling cascades. Certain bacteria corrupt the ubiquitylation machinery in order to regulate their virulence factors spatially and temporally or to trigger internalization of bacteria into host cells. Several new examples of how bacterial factors target ubiquitin and ubiquitin-like regulation emphasize the importance of modulating ubiquitin signaling to establish either long-lasting or devastating relationships of bacteria with their hosts.

Induction of transient macroapertures in endothelial cells through RhoA inhibition by Staphylococcus aureus factors.

The GTPase RhoA is a major regulator of the assembly of actin stress fibers and the contractility of the actomyosin cytoskeleton. The epidermal cell differentiation inhibitor (EDIN) and EDIN-like ADP-ribosyltransferases of Staphylococcus aureus catalyze the inactivation of RhoA, producing actin cable disruption. We report that purified recombinant EDIN and EDIN-producing S. aureus provoke large transcellular tunnels in endothelial cells that we have named macroapertures (MAs). These structures open transiently, followed by the appearance of actin-containing membrane waves extending over the aperture. Disruption of actin cables, either directly or indirectly, through rhoA RNAi knockdown also triggers the formation of MAs. Intoxication of endothelial monolayers by EDIN produces a loss of barrier function and provides direct access of the endothelium basement membrane to S. aureus.

CNF1-induced ubiquitylation and proteasome destruction of activated RhoA is impaired in Smurf1-/- cells.

Ubiquitylation of RhoA has emerged as an important aspect of both the virulence of Escherichia coli producing cytotoxic necrotizing factor (CNF) 1 toxin and the establishment of the polarity of eukaryotic cells. Owing to the molecular activity of CNF1, we have investigated the relationship between permanent activation of RhoA catalyzed by CNF1 and subsequent ubiquitylation of RhoA by Smurf1. Using Smurf1-deficient cells and by RNA interference (RNAi)-mediated Smurf1 knockdown, we demonstrate that Smurf1 is a rate-limiting and specific factor of the ubiquitin-mediated proteasomal degradation of activated RhoA. We further show that the cancer cell lines HEp-2, human embryonic kidney 293 and Vero are specifically deficient in ubiquitylation of either activated Rac, Cdc42, or Rho, respectively. In contrast, CNF1 produced the cellular depletion of all three isoforms of Rho proteins in the primary human cell types we have tested. We demonstrate that ectopic expression of Smurf1 in Vero cells, deficient for RhoA ubiquitylation, restores ubiquitylation of the activated forms of RhoA. We conclude here that Smurf1 ubiquitylates activated RhoA and that, in contrast to human primary cell types, some cancer cell lines have a lower ubiquitylation capacity of specific Rho proteins. Thus, both CNF1 and transforming growth factor-beta trigger activated RhoA ubiquitylation through Smurf1 ubiquitin-ligase.

The Rho GTPase activators CNF1 and DNT bacterial toxins have mucosal adjuvant properties.

Cytotoxic necrotizing factor 1 (CNF1) from uropathogenic Escherichia coli belongs to a family of factors activating Rho GTPases. We report the in vivo effects of CNF1 in mice co-fed toxin and the soluble protein antigen ovalbumin (OVA). Similar to cholera toxin, CNF1 elicits adjuvanticity anti-OVA responses, both systemic and mucosal. In contrast, the catalytic inactive mutant CNF1-C866S demonstrated no effects. Using dermonecrotic toxin (DNT), a closely related Rho activating toxin from Bordetella, we discovered that the adjuvant property is within the DNT catalytic domain. Manipulation of Rho proteins thus provides a possible new approach for the development of effective mucosal immunoadjuvants.

E. coli CNF1 toxin: a two-in-one system for host-cell invasion.

The cytotoxic necrotizing factor-1 (CNF1), a bacterial toxin of uropathogenic bacteria (UPEC), hijacks cellular Rho proteins of the Ras GTPase super-family. Recently, we have made three important findings. First, we have established that, following Rho protein activation by deamidation, these cellular proteins are ubiquitylated and degraded by the proteasome. Second, the low level of activated Rho proteins which results from the dual molecular mechanism of action of CNF1 (Rho protein activation followed by their degradation), confers high invasive properties to UPECs. Finally, we have reported that ubiquitylation and degradation of Rac is lost in HEp-2 carcinoma cells, thereby suggesting a possible link between Rho protein ubiquitylation and tumor progression.

Activation and proteasomal degradation of rho GTPases by cytotoxic necrotizing factor-1 elicit a controlled inflammatory response.

The CNF1 toxin is produced by uropathogenic and meningitis-causing Escherichia coli. CNF1 penetrates autonomously into cells and confers phagocytic properties to epithelial and endothelial cells. CNF1 acts at the molecular level by constitutively activating Rho GTPases attenuated by their cellular ubiquitin-mediated proteasomal degradation. Here we report the relationship between the ubiquitin-mediated proteasomal degradation of activated Rho and the endothelial cell response to the toxin. The type of cellular response to CNF1 intoxication, first screened by DNA microarray analysis, revealed the launching of a program oriented toward an inflammatory response. Parallel to Rho protein activation by CNF1, we also established the kinetics of production of monocyte chemotactic protein-1 (MCP-1), interleukin-8 (IL-8), IL-6, monocyte inflammatory protein-3alpha (MIP-3alpha) and E-selectin. Both the mutation of the catalytic domain of the toxin (CNF1-C866S) and the inhibition of Rho proteins abrogate the CNF1-induced production of the immunomodulators MIP-3alpha, MCP-1, and IL-8. These immunomodulators are also produced upon activation of Cdc42 and Rac preferentially. Our results indicate that, in addition to pathogen molecular pattern recognition by host-receptors, a direct activation of Rho proteins by the CNF1 virulence factor efficiently triggers a cellular reaction of host alert. Consistently, we assume that the CNF1-induced ubiquitin-mediated proteasomal degradation of activated Rho proteins may limit the amplitude of the host cell immune responses.

CNF1 exploits the ubiquitin-proteasome machinery to restrict Rho GTPase activation for bacterial host cell invasion.

CNF1 toxin is a virulence factor produced by uropathogenic Escherichia coli. Upon cell binding and introduction into the cytosol, CNF1 deamidates glutamine 63 of RhoA (or 61 of Rac and Cdc42), rendering constitutively active these GTPases. Unexpectedly, we measured in bladder cells a transient CNF1-induced activation of Rho GTPases, maximal for Rac. Deactivation of Rac correlated with the increased susceptibility of its deamidated form to ubiquitin/proteasome-mediated degradation. Sensitivity to ubiquitylation could be generalized to other permanent-activated forms of Rac and to its sustained activation by Dbl. Degradation of the toxin-activated Rac allowed both host cell motility and efficient cell invasion by uropathogenic bacteria. CNF1 toxicity thus results from a restricted activation of Rho GTPases through hijacking the host cell proteasomal machinery.

NAD binding induces conformational changes in Rho ADP-ribosylating clostridium botulinum C3 exoenzyme.

We have solved the crystal structures of Clostridium botulinum C3 exoenzyme free and complexed to NAD in the same crystal form, at 2.7 and 1.95 A, respectively. The asymmetric unit contains four molecules, which, in the free form, share the same conformation. Upon NAD binding, C3 underwent various conformational changes, whose amplitudes were differentially limited in the four molecules of the crystal unit. A major rearrangement concerns the loop that contains the functionally important ARTT motif (ADP-ribosyltransferase toxin turn-turn). The ARTT loop undergoes an ample swinging motion to adopt a conformation that covers the nicotinamide moiety of NAD. In particular, Gln-212, which belongs to the ARTT motif, flips over from a solvent-exposed environment to a buried conformation in the NAD binding pocket. Mutational experiments showed that Gln-212 is neither involved in NAD binding nor in the NAD-glycohydrolase activity of C3, whereas it plays a critical role in the ADP-ribosyl transfer to the substrate Rho. We observed additional NAD-induced movements, including a crab-claw motion of a subdomain that closes the NAD binding pocket. The data emphasized a remarkable NAD-induced plasticity of the C3 binding pocket and suggest that the NAD-induced ARTT loop conformation may be favored by the C3-NAD complex to bind to the substrate Rho. Our structural observations, together with a number of mutational experiments suggest that the mechanisms of Rho ADP-ribosylation by C3-NAD may be more complex than initially anticipated.

Strategies for the structural determination of the catalytic domain of Escherichia coli cytotoxic necrotizing factor 1.

Cytotoxic necrotizing factor 1 (CNF1), a 114 kDa toxin produced by certain pathogenic strains of Escherichia coli, constitutively activates members of the Rho GTPase family, leading to cytopathic effects. The toxin inhibits GTP turnover by Rho proteins through site-specific deamidation of a Rho glutamine required for GTP hydrolysis. To understand the basis for catalytic activity and target specificity of CNF1, the structure of a catalytically active fragment of CNF1 was sought. Here, strategies that led to successful expression of a soluble 33 kDa active fragment, growth and improvement in the quality of the crystals and determination of phases using a quadruple methionine-substitution mutant of the fragment are presented.