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Neutralization of activity of effector protein by monoclonal antibody: formulation of antibody dose-dependence of neutralization for an equilibrium system of antibody, effector, and its cellular receptor.

Binding of an antibody to an effector protein will not necessarily inactivate it totally. In order to incorporate this into mathematical expressions of neutralization reaction between a monoclonal antibody and an effector which acts on cells through receptor binding, a static equilibrium system was considered that consists of effector, receptor and antibody, allowing formation of the ternary complex. A formula was derived for the antibody dose-dependence of neutralization, which was qualitatively similar to that found experimentally for monoclonal antibodies to interferon. According to the formula, the extent of neutralization, N, will approach a saturation value N infinity at high antibody concentrations, instead of increasing indefinitely, as is the case with conventional antibodies; the value N infinity is determined by the degree of reduction, upon antibody binding, of the effector's affinity to the receptor (and/or by the ability of antibody to inhibit the effector action subsequent to receptor binding). On the other hand, experimental data in the region of small N's will allow one to estimate the effector-antibody association constant.

Antibodies, Monoclonal↗

AopP, a type III effector protein of Aeromonas salmonicida, inhibits the NF-kappaB signalling pathway.

Aeromonas salmonicida subsp. salmonicida contains a functional type III secretion system that is responsible for the secretion of the ADP-ribosylating toxin AexT. In this study, the authors identified AopP as a second effector protein secreted by this system. The aopP gene was detected in both typical and atypical A. salmonicida isolates and was found to be encoded on a small plasmid of approximately 6.4 kb. Sequence analysis indicates that AopP is a member of the YopJ family of effector proteins, a group of proteins that interfere with mitogen-activated protein kinase (MAPK) and/or nuclear factor kappa B (NF-kappaB) signalling pathways. AopP inhibits the NF-kappaB pathway downstream of IkappaB kinase (IKK) activation, while a catalytically inactivated mutant, AopPC177A, does not possess this inhibitory effect. Unlike other effectors of the YopJ family, such as YopJ and VopA, AopP does not inhibit the MAPK signalling pathway.

Aeromonas salmonicida↗

NopL, an effector protein of Rhizobium sp. NGR234, thwarts activation of plant defense reactions.

Bacterial effector proteins delivered into eukaryotic cells via bacterial type III secretion systems are important virulence factors in plant-pathogen interactions. Type III secretion systems have been found in Rhizobium species that form symbiotic, nitrogen-fixing associations with legumes. One such bacterium, Rhizobium sp. NGR234, secretes a number of type III effectors, including nodulation outer protein L (NopL, formerly y4xL). Here, we show that expression of nopL in tobacco (Nicotiana tabacum) prevents full induction of pathogenesis-related (PR) defense proteins. Transgenic tobacco plants that express nopL and were infected with potato virus Y (necrotic strain 605) exhibited only very low levels of chitinase (class I) and beta-1,3-glucanase (classes I and III) proteins. Northern-blot analysis indicated that expression of nopL in plant cells suppresses transcription of PR genes. Treatment with ethylene counteracted the effect of NopL on chitinase (class I). Transgenic Lotus japonicus plants that expressed nopL exhibited delayed development and low chitinase levels. In vitro experiments showed that NopL is a substrate for plant protein kinases. Together, these data suggest that NopL, when delivered into the plant cell, modulates the activity of signal transduction pathways that culminate in activation of PR proteins.

Bacterial Proteins↗

Getting across--bacterial type III effector proteins on their way to the plant cell.

Pathogenicity of most Gram-negative bacterial plant pathogens depends on hrp (hypersensitive response and pathogenicity) genes, which control the ability to cause disease and to elicit specific defense responses in resistant plants. hrp genes encode a specialized type III secretion (TTS) system that mediates the vectorial delivery of bacterial effector proteins across both bacterial membranes as well as across the eukaryotic plasma membrane into the host cell cytosol. One well-studied effector protein is AvrBs3 from Xanthomonas campestris pv. vesicatoria, the causal agent of bacterial spot in pepper and tomato. AvrBs3 induces hypertrophy symptoms in susceptible plants and triggers a resistance gene-specific cell death reaction in resistant plants. Intriguingly, AvrBs3 has characteristic features of eukaryotic transcription factors, suggesting that it modulates the host's transcriptome. Here, we discuss the TTS system of X.campestris pv. vesicatoria in the light of current knowledge on type III-dependent protein secretion in plant pathogenic bacteria.

Bacteria↗

IQGAP2 functions as a GTP-dependent effector protein in thrombin-induced platelet cytoskeletal reorganization.

Human blood platelets are anucleate cells whose response to extracellular stimuli results in actin cytoskeleton rearrangements, thereby providing the critical initial step in the regulation of hemostasis. The serine protease alpha-thrombin, known to activate platelets by cleavage of a family of protease-activated receptors (PARs), is the most potent physiologic activator of human platelets, though downstream effector proteins uniquely linked to platelet cytoskeletal actin polymerization remain largely uncharacterized. The gene encoding the putative rac1/cdc42 effector protein IQGAP2 was identified within the PAR gene cluster at 5q13, flanked telomeric by PAR1 and encompassing PAR3. Immunofluorescence microscopy demonstrated IQGAP2 expression in filopodial extensions of activated platelets and colocalized with F-actin in lamellipodia and filopodia of IQGAP2-transfected COS1 cells. Platelet activation by alpha-thrombin, but not saturating concentrations of fibrillar collagen or adenosine 5'-diphosphate, uniquely assemble an IQGAP2/arp2/3-actin cytoplasmic complex, an association regulated by guanosine triphosphate rac1 ([GTP]rac1) but not by [GTP]cdc42. Likewise, only thrombin-activated platelets resulted in rapid translocation of IQGAP2 to the platelet cytoskeleton. These observations identify a physiologic scaffolding function for IQGAP2 and establish the presence of a functional genomic unit in humans uniquely evolved to regulate thrombin-induced platelet cytoskeletal actin reorganization.

Actin-Related Protein 2↗

Effector proteins encoded by Salmonella pathogenicity island 2 interfere with the microtubule cytoskeleton after translocation into host cells.

The facultative intracellular pathogen Salmonella enterica has evolved strategies to modify its fate inside host cells. One key virulence factor for the intracellular pathogenesis is the type III secretion system encoded by Salmonella Pathogenicity Island 2 (SPI2). We have previously described SPI2-encoded SseF and SseG as effector proteins that are translocated by intracellular Salmonella. Detailed analysis of the subcellular localization of SseF and SseG within the host cell indicated that these effector proteins are associated with endosomal membranes as well as with microtubules. Specific association with microtubules was observed after translocation by intracellular Salmonella as well as after expression by transfection vectors. In epithelial cells infected with Salmonella, both SseF and SseG are required for the aggregation of endosomal compartments along microtubules and to induce the formation of massive bundles of microtubules. These observations demonstrate that SPI2 effectors interfere with the microtubule cytoskeleton and suggest that microtubule-dependent host cell functions such as vesicle transport or organelle positioning are altered by intracellular Salmonella.

Bacterial Proteins↗

Symbiotic phenotypes and translocated effector proteins of the Mesorhizobium loti strain R7A VirB/D4 type IV secretion system.

The symbiosis island of Mesorhizobium loti strain R7A contains genes with strong similarity to the structural vir genes (virB1-11; virD4) of Agrobacterium tumefaciens that encode the type IV secretion system (T4SS) required for T-DNA transfer to plants. In contrast, M. loti strain MAFF303099 lacks these genes but contains genes not present in strain R7A that encode a type III secretion system (T3SS). Here we show by hybridization analysis that most M. loti strains contain the VirB/D4 T4SS and not the T3SS. Strikingly, strain R7A vir gene mutants formed large nodules containing bacteroids on Leucaena leucocephala in contrast to the wild-type strain that formed only uninfected tumour-like structures. A rhcJ T3SS mutant of strain MAFF303099 also nodulated L. leucocephala, unlike the wild type. On Lotus corniculatus, the vir mutants were delayed in nodulation and were less competitive compared with the wild type. Two strain R7A genes, msi059 and msi061, were identified through their mutant phenotypes as possibly encoding translocated effector proteins. Both Msi059 and Msi061 were translocated through the A. tumefaciens VirB/D4 system into Saccharomyces cerevisiae and Arabidopsis thaliana, as shown using the Cre recombinase Reporter Assay for Translocation (CRAfT). Taken together, these results suggest that the VirB/D4 T4SS of M. loti R7A plays an analogous symbiotic role to that of T3SS found in other rhizobia. The heterologous translocation of rhizobial proteins by the Agrobacterium VirB/D4 T4SS is the first demonstration that rhizobial effector proteins are translocated into plant cells and confirms functional conservation between the M. loti and A. tumefaciens T4SS.

Agrobacterium tumefaciens↗

Eukaryotic fatty acylation drives plasma membrane targeting and enhances function of several type III effector proteins from Pseudomonas syringae.

Bacterial pathogens of plants and animals utilize conserved type III delivery systems to traffic effector proteins into host cells. Plant innate immune systems evolved disease resistance (R) genes to recognize some type III effectors, termed avirulence (Avr) proteins. On disease-susceptible (r) plants, Avr proteins can contribute to pathogen virulence. We demonstrate that several type III effectors from Pseudomonas syringae are targeted to the host plasma membrane and that efficient membrane association enhances function. Efficient localization of three Avr proteins requires consensus myristoylation sites, and Avr proteins can be myristoylated inside the host cell. These prokaryotic type III effectors thus utilize a eukaryote-specific posttranslational modification to access the subcellular compartment where they function.

Acylation↗

Dimerization of the bacterial effector protein AvrBs3 in the plant cell cytoplasm prior to nuclear import.

The effector protein AvrBs3 from the bacterial phytopathogen Xanthomonas campestris pv. vesicatoria is translocated into the plant cell where it specifically induces hypertrophy symptoms or the hypersensitive reaction. Activity of AvrBs3 depends on nuclear localization signals (NLSs) and an acidic activation domain, suggesting a role in regulation of plant transcription. Here, we show that AvrBs3 dimerizes in the plant cell prior to its nuclear import. AvrBs3 deletion derivatives were tested in the yeast two-hybrid system revealing that the repeat region, which confers specific recognition in resistant plants and is crucial for virulence function, is also essential for the self-interaction. GST pull-down assays showed that the AvrBs3-AvrBs3 interaction occurs independent of plant proteins. Coexpression of two different inactive mutant AvrBs3 derivatives in Bs3-resistant pepper plants resulted in 'trans-complementation', i.e., the induction of a hypersensitive reaction. This clearly indicates that AvrBs3-dimerization occurs in planta. Interestingly, 'trans-complementation' was not observed in susceptible plants suggesting that wild-type homodimers are needed for the AvrBs3 virulence function in plants. Furthermore, a green fluorescent protein (GFP) fusion of AvrBs3 deleted in the NLSs (AvrBs3DeltaNLS-GFP), normally localized in the cytoplasm, was imported into the nucleus upon coexpression with wild-type AvrBs3 in Nicotiana benthamiana. Thus, AvrBs3 dimerization takes place in the cytoplasm of the plant cell prior to nuclear import. Given the fact that dimerization is a common feature of transcriptional regulators, our data are consistent with the idea that AvrBs3 manipulates expression of plant genes involved in the establishment of compatible and incompatible interactions.

Bacterial Proteins↗

Wake of the flood: ascribing functions to the wave of type III effector proteins of phytopathogenic bacteria.

Plant pathogenic bacteria use waves of type III effector proteins, delivered into the eukaryotic host cell, to modulate the host cell for the pathogen's benefit. This is evidenced by the flood of effector genes that have recently been uncovered from the genome sequence of several plant pathogenic bacteria. However, pathogens are unwilling to easily reveal the mechanisms by which these effectors function. Nevertheless, persistent scrutiny has led to the successful characterization of a handful of effectors and it is beginning to provide insights into how phytopathogenic bacteria cause disease on their hosts.

Host-Parasite Interactions↗

Specific binding of phosphatidylinositol 4,5-bisphosphate to calcium-dependent activator protein for secretion (CAPS), a potential phosphoinositide effector protein for regulated exocytosis.

The calcium-dependent activator protein for secretion (CAPS) is a novel neural/endocrine-specific cytosolic and peripheral membrane protein required for the Ca2+-regulated exocytosis of secretory vesicles. CAPS acts at a stage in exocytosis that follows ATP-dependent priming, which involves the essential synthesis of phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2). In the present studies, CAPS is shown to bind liposomes that contain acidic phospholipids and binding was markedly enhanced by inclusion of PtdIns(4,5)P2 but not other phosphoinositides in the absence of Ca2+. PtdIns(4,5)P2, but not other phosphoinositides including PtdIns(3, 4)P2 and PtdIns(3,4,5)P3, altered the susceptibility of CAPS to proteolysis by trypsin and proteinase K, suggesting that phosphoinositide binding promoted a conformational change. Photoaffinity labeling studies with a photoactivatable benzoylcinnimidyl acyl chain derivative of PtdIns(4,5)P2 confirmed the phosphoinositide-binding properties of CAPS and suggested a hydrophobic aspect of the interaction. CAPS, as one of very few characterized proteins with a binding specificity for 4-, 5-phosphorylated inositides over 3-phosphorylated inositides, may function in regulated exocytosis as an effector of PtdIns(4,5)P2.

Animals↗

Rab27a negatively regulates CFTR chloride channel function in colonic epithelia: involvement of the effector proteins in the regulatory mechanism.

Cystic fibrosis, an autosomal recessive disorder, is caused by the disruption of biosynthesis or function of CFTR. CFTR regulatory mechanisms include channel transport to plasma membrane and protein-protein interactions. Rab proteins are small GTPases involved in vesicle transport, docking, and fusion. The colorectal epithelial HT-29 cells natively express CFTR and respond to cAMP with an increase in CFTR-mediated currents. DPC-inhibited currents could be completely eliminated with CFTR-specific SiRNA. Over-expression of Rab27a inhibited, while isoform specific SiRNA and Rab27a antibody stimulated CFTR-mediated currents in HT-29 cells. CFTR activity is inhibited both by Rab27a (Q78L) (constitutive active GTP-bound form of Rab27a) and Rab27a (T23N) (constitutive negative form that mimics the GDP-bound form). Rab27a mediated effects could be reversed by Rab27a-binding proteins, the synaptotagmin-like protein (SLP-5) and Munc13-4 accessory protein (a putative priming factor for exocytosis). The SLP reversal of Rab27a effect was restricted to C2A/C2B domains while the SHD motif imparted little more inhibition. The CFTR-mediated currents remain unaffected by Rab3 though SLP-5 appears to weakly bind it. The immunoprecipitation experiments suggest protein-protein interactions between Rab27a and CFTR. Rab27a appears to impair CFTR appearance at the cell surface by trapping CFTR in the intracellular compartments. Munc13-4 and SLP-5, on the other hand, limit Rab27a availability to CFTR, thus minimizing its effect on channel function. These observations decisively prove that Rab27a is involved in CFTR channel regulation through protein-protein interactions involving Munc13-4 and SLP-5 effector proteins, and thus could be a potential target for cystic fibrosis therapy.

Carrier Proteins↗

The conserved Xanthomonas campestris pv. vesicatoria effector protein XopX is a virulence factor and suppresses host defense in Nicotiana benthamiana.

Nicotiana benthamiana leaves display a visible plant cell death response when infiltrated with a high titer inoculum of the non-host pathogen, Xanthomonas campestris pv. vesicatoria (Xcv). This visual phenotype was used to identify overlapping cosmid clones from a genomic cosmid library constructed from the Xcv strain, GM98-38. Individual cosmid clones from the Xcv library were conjugated into X. campestris pv. campestris (Xcc) and exconjugants were scored for an altered visual high titer inoculation response in N. benthamiana. The molecular characterization of the cosmid clones revealed that they contained a novel gene, xopX, that encodes a 74-kDa type III secretion system (TTSS) effector protein. Agrobacterium-mediated transient expression of XopX in N. benthamiana did not elicit the plant cell death response although detectable XopX protein was produced. Interestingly, the plant cell death response occurred when the xopX Agrobacterium-mediated transient expression construct was co-inoculated with strains of either XcvDeltaxopX or Xcc, both lacking xopX. The co-inoculation complementation of the plant cell death response also depends on whether the Xanthomonas strains contain an active TTSS. Transgenic 35S-xopX-expressing N. benthamiana plants also have the visible plant cell death response when inoculated with the non-xopX-expressing strains XcvDeltaxopX and Xcc. Unexpectedly, transgenic 35S-xopX N. benthamiana plants displayed enhanced susceptibility to bacterial growth of Xcc as well as other non-xopX-expressing Xanthomonas and Pseudomonas strains. This result is also consistent with the increase in bacterial growth on wild type N. benthamiana plants observed for Xcc when XopX is expressed in trans. Furthermore, XopX contributes to the virulence of Xcv on host pepper (Capsicum annuum) and tomato (Lycopersicum esculentum) plants. We propose that the XopX bacterial effector protein targets basic innate immunity in plants, resulting in enhanced plant disease susceptibility.

Amino Acid Sequence↗

Multiple approaches to a complete inventory of Pseudomonas syringae pv. tomato DC3000 type III secretion system effector proteins.

Pseudomonas syringae pv. tomato DC3000 is a pathogen of tomato and Arabidopsis that translocates virulence effector proteins into host cells via a type III secretion system (T3SS). Many effector-encoding hypersensitive response and pathogenicity (Hrp) outer protein (hop) genes have been identified previously in DC3000 using bioinformatic methods based on Hrp promoter sequences and characteristic N-terminal amino acid patterns that are associated with T3SS substrates. To approach completion of the Hop/effector inventory in DC3000, 44 additional candidates were tested by the Bordetella pertussis calmodulin-dependent adenylate cyclase (Cya) translocation reporter assay; 10 of the high-probability candidates were confirmed as T3SS substrates. Several previously predicted hop genes were tested for their ability to be expressed in an HrpL-dependent manner in culture or to be expressed in planta. The data indicate that DC3000 harbors 53 hop/avr genes and pseudogenes (encoding both injected effectors and T3SS substrates that probably are released to the apoplast); 33 of these genes are likely functional in DC3000, 12 are nonfunctional members of valid Hop families, and 8 are less certain regarding their production at functional levels. Growth of DC3000 in tomato and Arabidopsis Col-0 was not impaired by constitutive expression of repaired versions of two hops that were disrupted naturally by transposable elements or of hop genes that are naturally cryptic. In summary, DC3000 carries a complex mixture of active and inactive hop genes, and the hop genes in P. syringae can be identified efficiently by bioinformatic methods; however, a precise inventory of the subset of Hops that are important in pathogenesis awaits more knowledge based on mutant phenotypes and functions within plants.

Adenylate Cyclase Toxin↗

Effector proteins from P450(cam) and methane monooxygenase: lessons in tuning nature's powerful reagents.

Effector proteins alter the kinetic or catalytic course of many oxygenase reactions. One of the first oxygenase effectors to be described was putidaredoxin, which serves to gate electron transfer into oxy-P450(cam). In the nonheme, methane monooxygenase (MMO) system, the B-component (MMOB) serves a distinct effector function by gating substrate and oxygen into the active site of the hydroxylase component (MMOH). Here the binding parameters and binding surfaces of the MMOB-MMOH complex are determined by site-specific labeling, fluorescence titrations, chemical cross-linking, and MALDI-TOF peptide identification. Based on these data, a model for the bimolecular complex is described and a hypothesis for the structural basis for the effector function is elaborated. The bearing on the putidaredoxin effector function is discussed.

Binding Sites↗

Residues in Cdc42 that specify binding to individual CRIB effector proteins.

Cdc42 is a member of the Rho family of small G proteins. Signal transduction events emanating from Cdc42 lead to cytoskeletal rearrangements, cell proliferation, and cell differentiation. Many effector proteins have been identified for Cdc42; however, it is not clear how certain effectors specifically recognize and bind to Cdc42, as opposed to Rac or Rho, or in many cases, which effector controls what cellular events. Mutations were introduced into Cdc42 at residues: Met1, Val8, Phe28, Tyr32, Val33, Thr35, Val36, Phe37, Asp38, Tyr40, Val42, Met45, Ile46, Glu127, Ala130, Asn132, Gln134, Lys135, and Leu174. Measurements were made of their equilibrium binding constants to the Cdc42 binding domains of the CRIB effectors ACK, PAK, and WASP and to the GTPase-activating protein Rho GAP. Generally, mutations in the effector loop have an equally deleterious effect on binding to all CRIB proteins tested, though the F37A mutation resulted in significant selectivity. Residues outside the effector loop were found to be important for binding of Cdc42 to CRIB containing proteins and also to contribute to selectivity. Mutations such as V42A and L174A resulted in large, selective changes in binding to specific CRIB effectors. Neither mutation resulted in alteration in PAK binding, whereas both severely disrupt binding to ACK and only L174A disrupted binding to WASP. These mutations are interpreted using the structures of the Cdc42/ACK and Cdc42/WASP complexes to give insight into how effectors can specifically recognize Cdc42. Those mutations in Cdc42 that inhibit certain interactions, while retaining others, should aid investigations of the role of specific effectors in Cdc42 signaling in vivo.

Amino Acid Motifs↗

Functional dissection of SseF, a type III effector protein involved in positioning the salmonella-containing vacuole.

Intracellular replication of Salmonella enterica requires the formation of a unique organelle termed Salmonella-containing vacuole (SCV). The type III secretion system (T3SS) encoded by Salmonella Pathogenicity Island 2 (SPI2-T3SS) has a crucial role in the formation and maintenance of the SCV. The SPI2-T3SS translocates a large number of effector proteins that interfere with host cell functions such as microtubule-dependent transport. We investigated the function of the effector SseF and observed that this protein is required to maintain the SCV in a juxtanuclear position in infected epithelial cells. The formation of juxtanuclear clusters of replicating Salmonella required the recruitment of dynein to the SCV but SseF-deficient strains were highly reduced in dynein recruitment to the SCV. We performed a functional dissection of SseF and defined domains that were important for translocation and the specific effector functions of this protein. Of particular importance was a hydrophobic domain in the C-terminal half that contains three putative transmembrane (TM) helices. Deletion of one of these TM helices ablated the effector functions of SseF. We observed that this domain was essential for the proper intracellular positioning of the SCV to a juxtanuclear, Golgi-associated localization. These data show that SseF, in concert with the effector proteins SifA and SseG mediate the precise positioning of the SCV by differentially modulating the recruitment of microtubule motor proteins to the SCV.

Bacterial Proteins↗

Alternative splicing in the first alpha-helical region of the Rab-binding domain of Rim regulates Rab3A binding activity: is Rim a Rab3 effector protein during evolution?

Rim1 and Rim2 were originally described as specific Rab3A-effector proteins involved in the regulation of secretory vesicle exocytosis. The putative Rab3A-binding domain (RBD) of Rim consists of two alpha-helical regions (named RBD1 and RBD2) separated by two zinc finger motifs. Although alternative splicing in the RBD1 of Rim is known to produce long and short forms of RBD (named Rim1 and Rim1Delta56-105, and Rim2(+40A) and Rim2, respectively), with the long form of Rim1 and short form of Rim2 being dominant in mouse brain, the physiological significance of the alternative splicing in RBD1 has never been elucidated. In the present study I discovered that alternative splicing in Rim RBD1 alters Rab3A binding affinity to Rims, and found that insertion of 40 amino acids into the RBD1 of Rim2 (i.e. Rim2(+40A)) dramatically reduced its Rab3A binding activity (more than a 50-fold decrease in affinity). Similarly, Rim1Delta56-105 exhibited higher affinity binding to Rab3A than the long form of Rim1. Expression of the short forms of the Rim RBD in PC12 cells co-localized well with endogenous Rab3A, whereas expression of the long forms of the Rim RBD in PC12 cells resulted in cytoplasimc and nuclear localization. Moreover, I found that Caenorhabditis elegans Rim/UNC-10 (ce-Rim) and Drosophila Rim (dm-Rim) do not interact with ce-Rab3 and dm-Rab3, respectively, indicating that the Rab3-effector function of Rim has not been retained during evolution. Based on these findings, I propose that the Rab3A-effector function of Rim during secretory vesicle exocytosis is limited to the short form of the mammalian Rim RBD alone.

Alternative Splicing↗