Influence of oligomerization state on the structural properties of invasion plasmid antigen B from Shigella flexneri in the presence and absence of phospholipid membranes

Phosphomimetic Tyrosine Mutations in Spa47 Inhibit Type Three Secretion ATPase Activity and Shigella Virulence Phenotype

Shigella is a highly infectious human pathogen responsible for 269 million infections and 200,000 deaths per year. Shigella virulence is absolutely reliant on the injection of effector proteins into the host cell cytoplasm via its type three secretion system (T3SS). The protein Spa47 is a T3SS ATPase whose activity is essential for the proper function of the Shigella T3SS needle-like apparatus through which effectors are secreted. A phosphoproteomics study recently found several Shigella T3SS proteins, including Spa47, to be tyrosine phosphorylated, suggesting a means of regulating Spa47 enzymatic activity, T3SS function, and overall Shigella virulence. The work presented here employs phosphomimetic mutations in Spa47 to probe the effects of phosphorylation at these targeted tyrosines through in vitro radiometric ATPase assays and circular dichroism as well as in vivo characterization of T3SS secretion activity, erythrocyte hemolysis, and cellular invasion. Results presented here demonstrate a direct correlation between Spa47 tyrosine phosphorylation state, Spa47 ATPase activity, T3SS function, and Shigella virulence. Together, these findings provide a strong foundation that leads the way to uncovering the specific pathway(s) that Shigella employ to mitigate wasteful ATP hydrolysis and effector protein secretion when not required as well as T3SS activation in preparation for host infection and immune evasion.

Topology and Contribution to the Pore Channel Lining of Plasma Membrane-Embedded Shigella flexneri Type 3 Secretion Translocase IpaB

Shigella spp. are human bacterial pathogens that cause bacillary dysentery. Virulence depends on a type 3 secretion system (T3SS), a highly conserved structure present in multiple important human and plant pathogens. Upon host cell contact, the T3SS translocon is delivered to the host membrane, facilitates bacterial docking to the membrane, and enables delivery of effector proteins into the host cytosol. The Shigella translocon is composed of two proteins, IpaB and IpaC, which together form this multimeric structure within host plasma membranes. Upon interaction of IpaC with host intermediate filaments, the translocon undergoes a conformational change that allows for bacterial docking onto the translocon and, together with host actin polymerization, enables subsequent effector translocation through the translocon pore. To generate additional insights into the translocon, we mapped the topology of IpaB in plasma membrane-embedded pores using cysteine substitution mutagenesis coupled with site-directed labeling and proximity-enabled cross-linking by membrane-permeant sulfhydryl reactants. We demonstrate that IpaB function is dependent on posttranslational modification by a plasmid-encoded acyl carrier protein. We show that the first transmembrane domain of IpaB lines the interior of the translocon pore channel such that the IpaB portion of the channel forms a funnel-like shape leading into the host cytosol. In addition, we identify regions of IpaB within its cytosolic domain that protrude into and are closely associated with the pore channel. Taken together, these results provide a framework for how IpaB is arranged within translocons natively delivered by Shigella during infection.

A frog-derived bionic peptide with discriminative inhibition of tumors based on integrin αvβ3 identification

Aureins, natural active peptides extracted from skin secretions of Australian bell frogs, have become a research focus due to the antitumor effects caused by lysing cell membranes. However, clinical translation of Aureins is still limited by non-selective toxicity between normal and cancer cells. Herein, by structure-activity relationship analysis and rational linker design, a dual-function fusion peptide RA3 is designed by tactically fusing Aurein peptide A1 with strong anticancer activity, with a tri-peptide with integrin αvβ3-binding ability which was screened in our previous work. Rational design and selection of fusion linkers ensures α-helical conformation and active functions of this novel fusion peptide, inducing effective membrane rupture and selective apoptosis of cancer cells. The integrin binding and tumor recognition ability of the fusion peptide is further validated by fluorescence imaging in cell and mouse models, in comparison with the non-selective A1 peptide. Meanwhile, increased stability and superior therapeutic efficacy are achieved in vivo for the RA3 fusion peptide. Our study highlights that aided by computational simulation technologies, the biomimetic fusion RA3 peptide has been successfully designed, surmounting the poor tumor-selectivity of the natural defensive peptide, serving as a promising therapeutic agent for cancer treatment.

Novel Noncompetitive Type Three Secretion System ATPase Inhibitors Shut Down Shigella Effector Secretion

Shigella is the causative agent of bacillary dysentery and is responsible for an estimated 165 million infections and 600,000 deaths annually. Like many Gram-negative pathogens, Shigella relies on a type three secretion system (T3SS) to initiate and sustain infection by directly injecting effector proteins into host cells. Protein secretion through the needle-like injectisome and overall Shigella virulence rely on the T3SS ATPase Spa47, making it a likely means for T3SS regulation and an attractive target for therapeutic small molecule inhibitors. Here, we utilize a recently solved 2.15 Å crystal structure of Spa47 to computationally screen 7.6 million drug-like compounds for candidates which avoid the highly conserved active site by targeting a distal, but critical, interface between adjacent protomers of the Spa47 homohexamer. Ten of the top inhibitor candidates were characterized, identifying novel Spa47 inhibitors that reduce in vitro ATPase activity by as much as 87.9 ± 10.5% with IC₅₀'s as low as 25 ± 20 μM and reduce in vivo Shigella T3SS protein secretion by as much as 94.7 ± 3.0%. Kinetic analyses show that the inhibitors operate through a noncompetitive mechanism that likely supports the inhibitors' low cytotoxicity, as they avoid off-target ATPases involved in either Shigella or mammalian cell metabolism. Interestingly, the inhibitors display nearly identical inhibition profiles for Spa47 and the T3SS ATPases EscN from E. coli and FliI from Salmonella. Together, the results of this study provide much-needed insight into T3SS ATPase inhibition mechanisms and a strong platform for developing broadly effective cross-pathogen T3SS ATPase inhibitors.

Dominant negative effects by inactive Spa47 mutants inhibit T3SS function and Shigella virulence

Type three secretion systems (T3SS) are complex nano-machines that evolved to inject bacterial effector proteins directly into the cytoplasm of eukaryotic cells. Many high-priority human pathogens rely on one or more T3SSs to cause disease and evade host immune responses, underscoring the need to better understand the mechanisms through which T3SSs function and their role(s) in supporting pathogen virulence. We recently identified the Shigella protein Spa47 as an oligomerization-activated T3SS ATPase that fuels the T3SS and supports overall Shigella virulence. Here, we provide both in vitro and in vivo characterization of Spa47 oligomerization and activation in the presence and absence of engineered ATPase-inactive Spa47 mutants. The findings describe mechanistic details of Spa47-catalyzed ATP hydrolysis and uncover critical distinctions between oligomerization mechanisms capable of supporting ATP hydrolysis in vitro and those that support T3SS function in vivo. Concentration-dependent ATPase kinetics and experiments combining wild-type and engineered ATPase inactive Spa47 mutants found that monomeric Spa47 species isolated from recombinant preparations exhibit low-level ATPase activity by forming short-lived oligomers with active site contributions from at least two protomers. In contrast, isolated Spa47 oligomers exhibit enhanced ATP hydrolysis rates that likely result from multiple preformed active sites within the oligomeric complex, as is predicted to occur within the context of the type three secretion system injectisome. High-resolution fluorescence microscopy, T3SS activity, and virulence phenotype analyses of Shigella strains co-expressing wild-type Spa47 and the ATPase inactive Spa47 mutants demonstrate that the N-terminus of Spa47, not ATPase activity, is responsible for incorporation into the injectisome where the mutant strains exhibit a dominant negative effect on T3SS function and Shigella virulence. Together, the findings presented here help to close a significant gap in our understanding of how T3SS ATPases are activated and define restraints with respect to how ATP hydrolysis is ultimately coupled to T3SS function in vivo.

Interfacial amino acids support Spa47 oligomerization and shigella type three secretion system activation

Like many Gram-negative pathogens, Shigella rely on a type three secretion system (T3SS) for injection of effector proteins directly into eukaryotic host cells to initiate and sustain infection. Protein secretion through the needle-like type three secretion apparatus (T3SA) requires ATP hydrolysis by the T3SS ATPase Spa47, making it a likely target for in vivo regulation of T3SS activity and an attractive target for small molecule therapeutics against shigellosis. Here, we developed a model of an activated Spa47 homo-hexamer, identifying two distinct regions at each protomer interface that we hypothesized to provide intermolecular interactions supporting Spa47 oligomerization and enzymatic activation. Mutational analysis and a series of high-resolution crystal structures confirm the importance of these residues, as many of the engineered mutants are unable to form oligomers and efficiently hydrolyze ATP in vitro. Furthermore, in vivo evaluation of Shigella virulence phenotype uncovered a strong correlation between T3SS effector protein secretion, host cell membrane disruption, and cellular invasion by the tested mutant strains, suggesting that perturbation of the identified interfacial residues/interactions influences Spa47 activity through preventing oligomer formation, which in turn regulates Shigella virulence. The most impactful mutations are observed within the conserved Site 2 interface where the native residues support oligomerization and likely contribute to a complex hydrogen bonding network that organizes the active site and supports catalysis. The critical reliance on these conserved residues suggests that aspects of T3SS regulation may also be conserved, providing promise for the development of a cross-species therapeutic that broadly targets T3SS ATPase oligomerization and activation.

The Tip Complex: From Host Cell Sensing to Translocon Formation

Type III secretion systems are used by some Gram-negative bacteria to inject effector proteins into targeted eukaryotic cells for the benefit of the bacterium. The type III secretion injectisome is a complex nanomachine comprised of four main substructures including a cytoplasmic sorting platform, an envelope-spanning basal body, an extracellular needle and an exposed needle tip complex. Upon contact with a host cell, secretion is induced, resulting in the formation of a translocon pore in the host membrane. Translocon formation completes the conduit needed for effector secretion into the host cell. Control of type III secretion occurs in response to environmental signals, with the final signal being host cell contact. Secretion control occurs primarily at two sites-the cytoplasmic sorting platform, which determines secretion hierarchy, and the needle tip complex, which is critical for sensing and responding to environmental signals. The best-characterized injectisomes are those from Yersinia, Shigella and Salmonella species where there is a wealth of information on the tip complex and the two translocator proteins. Of these systems, the best characterized from a secretion regulation standpoint is Shigella. In the Shigella system, the tip complex and the first secreted translocon both contribute to secretion control and, thus, both are considered components of the tip complex. In this review, all three of these type III secretion systems are described with discussion focused on the structure and formation of the injectisome tip complex and what is known of the transition from nascent tip complex to assembled translocon pore.

Using disruptive insertional mutagenesis to identify the in situ structure-function landscape of the Shigella translocator protein IpaB

Bacterial type III secretion systems (T3SS) are used to inject proteins into mammalian cells to subvert cellular functions. The Shigella T3SS apparatus (T3SA) is comprised of a basal body, cytoplasmic sorting platform and exposed needle with needle "tip complex" (TC). TC maturation occurs when the translocator protein IpaB is recruited to the needle tip where both IpaD and IpaB control secretion induction. IpaB insertion into the host membrane is the first step of translocon pore formation and secretion induction. We employed disruptive insertional mutagenesis, using bacteriophage T4 lysozyme (T4L), within predicted IpaB loops to show how topological features affect TC functions (secretion control, translocon formation and effector secretion). Insertions within the N-terminal half of IpaB were most likely to result in a loss of steady-state secretion control, however, all but the two that were not recognized by the T3SA retained nearly wild-type hemolysis (translocon formation) and invasiveness levels (effector secretion). In contrast, all but one insertion in the C-terminal half of IpaB maintained secretion control but were impaired for hemolysis and invasion. These nature of the data suggest the latter mutants are defective in a post-secretion event, most likely due to impaired interactions with the second translocator protein IpaC. Intriguingly, only two insertion mutants displayed readily detectable T4L on the bacterial surface. The data create a picture in which the makeup and structure of a functional T3SA TC is highly amenable to physical perturbation, indicating that the tertiary structure of IpaB within the TC is more plastic than previously realized.

Invasion of HEp-2 cells by Shigella spp. isolated from acute pediatric diarrhea

Aim:Shigella infection is an important global health problem in developing countries where hygiene is poor and hence shigellosis is a main cause of diarrhoea-associated mortality and morbidity, particularly in children under the age of five. The bacterial entry into colon and rectal epithelial cells has been named ‘bacterium-directed phagocytosis’. This term highlights that the bacteria actively stimulate their own uptake into non-professional phagocytes. The aim of this study was to demonstrate the invasion of HEp-2 cells by Shigella spp. isolated from acute pediatric diarrhea in Tehran, Iran.

Methods: Three-hundred and ten non-duplicative diarrheal stool samples were collected from the children admitted to Children’s Medical Center in Tehran, Iran. Samples were cultured and suspected colonies were identified by routine microbiological and biochemical tests. The invasion of the two isolated Shigella spp. to HEp-2 cells was studied.

Results: Of 310 stool samples, 16 (5.2%) Shigella spp. were isolated, including seven (43.7%) S. sonnei and nine (56.3%) S. flexneri. Four (44.4%) S. sonnei and seven (42.8%) S. flexneri showed invasive phenotype to HEp-2.

Conclusion:Shigella sonnei and S. flexneri are reported as the most prevalent Shigella spp. in nature which infect humans. Invasion of various cell lines gives the chance of survival to Shigella spp. This ability causes more virulent infections in the host. Despite costly and time consuming cell culture techniques, the current method described in this paper is reliable for detecting invasive behavior of Shigella spp. Results have also shown that not all the Shigella spp. are able to invade intestinal epithelial cells.

Characterization of Type Three Secretion System Translocator Interactions with Phospholipid Membranes

In vitro characterization of type III secretion system (T3SS) translocator proteins has proven challenging due to complex purification schemes and their hydrophobic nature that often requires detergents to provide protein solubility and stability. Here, we provide experimental details for several techniques that overcome these hurdles, allowing for the direct characterization of the Shigella translocator protein IpaB with respect to phospholipid membrane interaction. The techniques specifically discussed in this chapter include membrane interaction/liposome flotation, liposome sensitive fluorescence quenching, and protein-mediated liposome disruption assays. These assays have provided valuable insight into the role of IpaB in T3SS-mediated phospholipid membrane interactions by Shigella and should readily extend to other members of this important class of proteins.

Bacterial Control of Pores Induced by the Type III Secretion System: Mind the Gap

Type III secretion systems (T3SSs) are specialized secretion apparatus involved in the virulence of many Gram-negative pathogens, enabling the injection of bacterial type III effectors into host cells. The T3SS-dependent injection of effectors requires the insertion into host cell membranes of a pore-forming "translocon," whose effects on cell responses remain ill-defined. As opposed to pore-forming toxins that damage host cell plasma membranes and induce cell survival mechanisms, T3SS-dependent pore formation is transient, being regulated by cell membrane repair mechanisms or bacterial effectors. Here, we review host cell responses to pore formation induced by T3SSs associated with the loss of plasma membrane integrity and regulation of innate immunity. We will particularly focus on recent advances in mechanisms controlling pore formation and the activity of the T3SS linked to type III effectors or bacterial proteases. The implications of the regulation of the T3SS translocon activity during the infectious process will be discussed.

The Many Faces of IpaB

The type III secretion system (T3SS) is Shigella's most important virulence factor. The T3SS apparatus (T3SA) is comprised of an envelope-spanning basal body and an external needle topped by a tip complex protein called IpaD. This nanomachine is used to deliver effector proteins into host cells to promote pathogen entry. A key component of the matured T3SS needle tip complex is the translocator protein IpaB. IpaB can exist in multiple states when prepared as a recombinant protein, however, it has also been described as having additional roles in Shigella pathogenesis. This mini-review will briefly describe some of the features of IpaB as a T3SS needle tip protein, as a pore-forming translocator protein and as an effector protein. Reflection on the potential importance of the different in vitro states of IpaB on its function and importance in serotype-independent vaccines is also provided.

Type 3 Secretion Translocators Spontaneously Assemble a Hexadecameric Transmembrane Complex

A type 3 secretion system is used by many bacterial pathogens to inject proteins into eukaryotic cells. Pathogens insert a translocon complex into the target eukaryotic membrane by secreting two proteins known as translocators. How these translocators form a translocon in the lipid bilayer and why both proteins are required remains elusive. Pseudomonas aeruginosa translocators PopB and PopD insert pores into membranes forming homo- or hetero-complexes of undetermined stoichiometry. Single-molecule fluorescence photobleaching experiments revealed that PopD formed mostly hexameric structures in membranes, whereas PopB displayed a bi-modal distribution with 6 and 12 subunits peaks. However, individually the proteins are not functional for effector translocation. We have found that when added together, the translocators formed distinct hetero-complexes containing 8 PopB and 8 PopD molecules. Thus, the interaction between PopB and PopD guide the assembly of a unique hetero-oligomer in membranes.

Chlamydia Outer Protein (Cop) B from Chlamydia pneumoniae possesses characteristic features of a type III secretion (T3S) translocator protein

Background

Chlamydia spp. are believed to use a conserved virulence factor called type III secretion (T3S) to facilitate the delivery of effector proteins from the bacterial pathogen to the host cell. Important early effector proteins of the type III secretion system (T3SS) are a class of proteins called the translocators. The translocator proteins insert into the host cell membrane to form a pore, allowing the injectisome to dock onto the host cell to facilitate translocation of effectors. CopB is a predicted hydrophobic translocator protein within the chlamydial T3SS.

Results

In this study, we identified a novel interaction between the hydrophobic translocator, CopB, and the putative filament protein, CdsF. Furthermore, we identified a conserved PxLxxP motif in CopB (amino acid residues 166–171), which is required for interaction with its cognate chaperone, LcrH_1. Using a synthetic peptide derived from the chaperone binding motif of CopB, we were able to block the LcrH_1 interaction with either CopB or CopD; this CopB peptide was capable of inhibiting C. pneumoniae infection of HeLa cells at micromolar concentrations. An antibody raised against the N-terminus of CopB was able to inhibit C. pneumoniae infection of HeLa cells.

Conclusion

The inhibition of the LcrH_1:CopB interaction with a cognate peptide and subsequent inhibition of host cell infection provides strong evidence that T3S is an essential virulence factor for chlamydial infection and pathogenesis. Together, these results support that CopB plays the role of a hydrophobic translocator.