Search

Ricin, a highly lethal plant-derived toxin, is a potential biological threat agent due to its high availability, ease of production and the lack of approved medical countermeasures for post-exposure treatment. To date, no specific ricin receptors were identified. Here we show for the first time, that the low density lipoprotein receptor-related protein-1 (LRP1) is a major target molecule for binding of ricin. Pretreating HEK293 acetylcholinesterase-producer cells with either anti-LRP1 antibodies or with Receptor-Associated Protein (a natural LRP1 antagonist), or using siRNA to knock-down LRP1 expression resulted in a marked reduction in their sensitivity towards ricin. Binding assays further demonstrated that ricin bound exclusively to the cluster II binding domain of LRP1, via the ricin B subunit. Ricin binding to the cluster II binding domain of LRP1 was significantly reduced by an anti-ricin monoclonal antibody, which confers high-level protection to ricin pulmonary-exposed mice. Finally, we tested the contribution of LRP1 receptor to ricin intoxication of lung cells derived from mice. Treating these cells with anti-LRP1 antibody prior to ricin exposure, prevented their intoxication. Taken together, our findings clearly demonstrate that the LRP1 receptor plays an important role in ricin-induced pulmonary intoxications.

Cellular stress and proinflammatory cytokines induce phosphorylation of insulin receptor substrate (IRS) proteins at Ser sites that inhibit insulin and IGF-I signaling. We therefore examined the effects of mutation of five “inhibitory” Ser phosphorylation sites on IRS2 function in transgenic mice that overexpress, selectively in pancreatic β-cells, either wild-type (WT) or a mutated IRS2 protein (IRS25A). Islets size, number, and mRNA levels of catalase and superoxide dismutase were increased, whereas those of nitric oxide synthase were decreased, in 7- to 10-week-old IRS25A-β mice compared with IRS2WT-β mice. However, glucose homeostasis and insulin secretion in IRS25A-β mice were impaired when compared with IRS2WT-β mice or to nontransgenic mice. This was associated with reduced mRNA levels of Glut2 and islet β-cell transcription factors such as Nkx6.1 and MafA. Similarly, components mediating the unfolded protein response were decreased in islets of IRS25A-β mice in accordance with their decreased insulin secretion. The beneficial effects of IRS25A on β-cell proliferation and β-cell transcription factors were evident only in 5- to 8-day-old mice. These findings suggest that elimination of inhibitory Ser phosphorylation sites of IRS2 exerts short-term beneficial effects in vivo; however, their sustained elimination leads to impaired β-cell function.

Abrin and ricin are potent AB toxins, which are considered biological threats. To date, there are no approved treatments against abrin or ricin intoxications. Previously, we showed that the administration of polyclonal anti-abrin antibodies to mice that were intranasally exposed to abrin, even very late post-exposure, conferred an exceedingly high-level of protection, while following ricin intoxication, similar treatment with anti-ricin antibodies resulted in negligible survival rates. To probe this unexpected difference in protection ability, we first examined whether the efficient anti-abrin-induced protection was due to neutralization of the A-subunit responsible for the catalytic effect, or of the B-subunit, which enables binding/internalization, by evaluating the protection conferred by antibodies directed against one of the two subunits. To this end, we generated and immunized rabbits with chimeric toxins containing a single abrin subunit, AabrinBricin in which abrin A-subunit was linked to ricin B-subunit, and AricinBabrin in which ricin A-subunit is linked to abrin B-subunit. Here, we show that antibodies raised against either AabrinBricin or AricinBabrin conferred exceptionally high protection levels to mice following intranasal exposure to a a lethal dose of abrin, suggesting that the high level of protection conferred by anti-abrin antibodies is not related to the neutralization of a particular subunit.

The seven-transmembrane superfamily member 3 protein (TM7SF3) is a p53-regulated homeostatic factor that attenuates the development of cellular stress and unfolded protein response. Here we show that TM7SF3 localizes to nuclear speckles; eukaryotic nuclear bodies enriched in splicing factors. This rather unexpected location of a membranal protein enables it to form stable complexes with regulators of pre-mRNA splicing including DHX15, LARP7, HNRNPU and RBM14 that co-precipitate with TM7SF3. Indeed, TM7SF3 modulates alternative splicing of >400 genes, mainly splicing at the 3’end of introns. These effects are observed both in primary human pancreatic islets and in a number of cell lines, under basal conditions and under stress. Accordingly, silencing of TM7SF3 resulted in differential expression of 1465 genes (about 7% of the human genome); with 844 and 621 genes being up- or down-regulated, respectively. Our findings implicate TM7SF3, as a resident of nuclear speckles and suggest a role for seven-transmembrane proteins as regulators of alternative splicing.

Ricin, a plant-derived toxin extracted from the seeds of Ricinus communis (castor bean plant), is one of the most toxic proteins known. Ricin's high toxicity, widespread availability, and ease of its extraction make it a potential agent for bioterrorist attacks. Most ricin detection methods are based on immunoassays. These methods may suffer from low efficiency in matrices containing interfering substances, or from false positive results due to antibody cross reactivity, with highly homologous proteins. In this study, we have developed a simple, rapid, sensitive, and selective mass spectrometry assay, for the identification of ricin in complex environmental samples. This assay involves three main stages: (a) Ricin affinity capture by commercial lactamyl-agarose (LA) beads. (b) Tryptic digestion. (c) LC-MS/MS (MRM) analysis of tryptic fragments. The assay was validated using 60 diverse environmental samples such as soil, asphalt, and vegetation, taken from various geographic regions. The assay's selectivity was established in the presence of high concentrations of competing lectin interferences. Based on our findings, we have defined strict criteria for unambiguous identification of ricin. Our novel method, which combines affinity capture beads followed by MRM-based analysis, enabled the identification of 1 ppb ricin spiked into complex environmental matrices. This methodology has the potential to be extended for the identification of ricin in body fluids from individuals exposed (deliberately or accidentally) to the toxin, contaminated food or for the detection of the entire family of RIP-II toxins, by applying multiplex format.

Earlier reported small interfering RNA (siRNA) high-throughput screens, identified seven-transmembrane superfamily member 3 (TM7SF3) as a novel inhibitor of pancreatic β-cell death. Here we show that TM7SF3 maintains protein homeostasis and promotes cell survival through attenuation of ER stress. Overexpression of TM7SF3 inhibits caspase 3/7 activation. In contrast, siRNA-mediated silencing of TM7SF3 accelerates ER stress and activation of the unfolded protein response (UPR). This involves inhibitory phosphorylation of eukaryotic translation initiation factor 2α activity and increased expression of activating transcription factor-3 (ATF3), ATF4 and C/EBP homologous protein, followed by induction of apoptosis. This process is observed both in human pancreatic islets and in a number of cell lines. Some of the effects of TM7SF3 silencing are evident both under basal conditions, in otherwise untreated cells, as well as under different stress conditions induced by thapsigargin, tunicamycin or a mixture of pro-inflammatory cytokines (tumor necrosis factor alpha, interleukin-1 beta and interferon gamma). Notably, TM7SF3 is a downstream target of p53: activation of p53 by Nutlin increases TM7SF3 expression in a time-dependent manner, although silencing of p53 abrogates this effect. Furthermore, p53 is found in physical association with the TM7SF3 promoter. Interestingly, silencing of TM7SF3 promotes p53 activity, suggesting the existence of a negative-feedback loop, whereby p53 promotes expression of TM7SF3 that acts to restrict p53 activity. Our findings implicate TM7SF3 as a novel p53-regulated pro-survival homeostatic factor that attenuates the development of cellular stress and the subsequent induction of the UPR.

The enteropathogenicYersiniastrains are known to downregulate signaling pathways in macrophages by effectors of the type III secretion system, in which YopJ/YopP plays a crucial role. The adverse effects ofYersinia pestis, the causative agent of plague, were examined by infecting J774A.1 cells, RAW264.7 cells, and primary murine macrophages with the EV76 strain and with the fully virulent Kimberley53 strain.Y. pestisexerts YopJ-dependent suppression of tumor necrosis factor alpha secretion and phosphorylation of mitogen-activated protein kinases and thus resembles enteropathogenicYersinia. However,Y. pestisis less able to activate caspases, to suppress NF-κB activation, and to induce apoptosis in macrophages than the high-virulenceY. enterocoliticaWA O:8 strain. These differences appear to be related to lower efficiency of YopJ effector translocation byY. pestis. The efficiencies of effector translocation and of apoptosis induction can be enhanced either by using a high bacterial load in a synchronized infection or by overexpressing exogenous YopJ inY. pestis. Replacing YopJ with the homologousY. enterocoliticaeffector YopP can further enhance these effects. Overexpression of YopP in ayopJ-deletedY. pestisbackground leads to rapid and effective translocation into target cells, providingY. pestiswith the high cytotoxic potential ofY. enterocoliticaWA O:8. We suggest that the relative inferiority ofY. pestisin triggering cell death in macrophages may be advantageous for its in vivo propagation in the early stages of infection.

Bacillus anthracis is the causative agent of anthrax disease. Improvement of existing anthrax vaccines, which are currently based on the administration of Protective Antigen (the highly immunogenic nontoxic subunit of the bacterial toxin) may entail other bacterial immunogenic elements, part of which are predicted to reside on the surface of bacterial cells. In the present study, membranal proteins extracted from a stationary-phase culture of a nonvirulent B. anthracis strain, devoid of the native virulence plasmids pXO1 and pXO2, were separated by two-dimensional electrophoresis (2-DE) and a characteristic protein map was defined. The proteomic analysis allowed matrix-assisted laser desorption/ionization-time of flight mass spectrometry-assisted identification of 86 protein spots which represent the product of 30 individual open reading frames (ORF). Among these, a prevalent class of proteins was the S-layer proteins (which were found to represent more than 75% of the B. anthracis membranal fraction) and proteins containing S-layer homology (SLH)-membranal localization domains. Five novel SLH proteins, previously inferred only from bioinformatic ORF analysis (draft genome sequence), were identified and one was shown to be a highly abundant membranal protein. Western blots of the 2-DE gels were probed with sera from convalescent rabbits and guinea pigs infected with virulent B. anthracis (Vollum strain). This analysis revealed that B. anthracis immune animals exhibit antibodies against at least 14 distinct membranal proteins present in the 2-DE map, establishing that these proteins are expressed in vivo and are able to elicit an immune response. The identification of the protein components of the B. anthracis membranal fraction, as well as the establishment of their potential immunogenicity, underscore the strength of the proteomic approach for identifying molecules which may serve for further analysis of immune and protective abilities.

Bacillus anthracis (Ames strain) chromosome-derived open reading frames (ORFs), predicted to code for surface exposed or virulence related proteins, were selected as B. anthracis -specific vaccine candidates by a multistep computational screen of the entire draft chromosome sequence (February 2001 version, 460 contigs, The Institute for Genomic Research, Rockville, Md.). The selection procedure combined preliminary annotation (sequence similarity searches and domain assignments), prediction of cellular localization, taxonomical and functional screen and additional filtering criteria (size, number of paralogs). The reductive strategy, combined with manual curation, resulted in selection of 240 candidate ORFs encoding proteins with putative known function, as well as 280 proteins of unknown function. Proteomic analysis of two-dimensional gels of a B. anthracis membrane fraction, verified the expression of some gene products. Matrix-assisted laser desorption ionization-time-of-flight mass spectrometry analyses allowed identification of 38 spots cross-reacting with sera from B. anthracis immunized animals. These spots were found to represent eight in vivo immunogens, comprising of EA1, Sap, and 6 proteins whose expression and immunogenicity was not reported before. Five of these 8 immunogens were preselected by the bioinformatic analysis (EA1, Sap, 2 novel SLH proteins and peroxiredoxin/AhpC), as vaccine candidates. This study demonstrates that a combination of the bioinformatic and proteomic strategies may be useful in promoting the development of next generation anthrax vaccine.

The high-molecular-weight neurofilament protein (NF-H) is highly phosphorylated in vivo, with estimates as high as 16-51 mol of Pi/mol of protein. Most of the phosphorylation sites are thought to be located on Ser residues in multiple KSP repeats, in the carboxy-terminal tail region of the molecule. Because the extent and site-specific patterns of tail domain phosphorylation are believed to modulate neurofilament structure and function, it becomes essential to identify the endogenous sites of phosphorylation. In this study, we have used selective proteolytic cleavage procedures, Pi determinations, microsequencing, and mass-spectral analysis to determine the endogenously phosphorylated sites in the NF-H tail isolated from rat spinal cord. Twenty Ser residues in NF-H carboxy-terminal tail were analyzed; nine of these, all located in KSP repeats, were phosphorylated. No detectable phosphorylation could be identified in any of the 11 "non-KSP" Ser residues that were examined. KSPXKX, KSPXXX, and KSPXXK motifs were found to be phosphorylated. In addition, a 27-kDa KSP-rich domain, containing 43 virtually uninterrupted KSPXXX repeats, was isolated from the tail domain and found to contain between 30 and 35 mol of Pi/mol of protein. This domain appeared to be highly resistant to endoproteinase Glu-C digestion, although it contains a large number of glutamate residues. It could be proteolyzed, however, after dephosphorylation. This suggests that phosphorylation of the tail domain may contribute to neurofilament stability in vivo. A neuronal-derived protein kinase that specifically phosphorylates only KSPXKX motifs in neurofilaments has been reported. The presence of extensively phosphorylated KSPXXX repeats in NF-H in vivo suggests the existence of yet another, unidentified kinase(s) with specificity for KSPXXX motifs.

Analysis by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) was applied for the characterization of Bacillus anthracis spore biomarkers. B. anthracis spores were extracted under a simple procedure, followed by linear mode analysis, using sinapinic acid as the matrix. Several markers with a mass range of 4-7 kDa were detected in three B. anthracis strains: Vollum, Sterne and V770-NP1-R. Similar spectra were also obtained for spore extracts of two members of the B. cereus group: B. thuringiensis and B. cereus, but not for B. mycoides, B. subtilis or B. licheniformis, suggesting that these markers are specific to closely related members of the B. cereus group. When alpha-cyano-4-hydroxycinnamic acid was used as the matrix, at least four additional new markers within a mass range of 2-4 kDa could be detected in all B. anthracis spore extracts. These markers, corresponding to a molecular weight of 2528.3, 2792.4, 3077.4, and 3590.7 Da, have not been observed in extracts of the three closely related Bacillus species - B. cereus, B. thuringiensis and B. mycoides. These unique B. anthracis biomarkers, which were isotopically resolved and reproducibly detected in the highly accurate MALDI-TOFMS reflectron mode, may be useful as a basis for rapid and specific identification of B. anthracis strains.

The tetrameric form of native serum-derived bovine acetylcholinesterase is retained in the circulation for much longer periods (mean residence time, MRT = 1390 min) than recombinant bovine acetylcholinesterase (rBoAChE) produced in the HEK-293 cell system (MRT = 57 min). Extensive matrix-assisted laser desorption ionization-time of flight analyses established that the basic structures of the N-glycans associated with the native and recombinant enzymes are similar (the major species (50–60%) are of the biantennary fucosylated type and 20–30% are of the triantennary type), yet the glycan termini of the native enzyme are mostly capped with sialic acid (82%) and α-galactose (12%), whereas glycans of the recombinant enzyme exhibit a high level of exposed β-galactose residues (50%) and a lack of α-galactose. Glycan termini of both fetal bovine serum and rBoAChE were altered in vitro using exoglycosidases and sialyltransferase or in vivo by a HEK-293 cell line developed specifically to allow efficient sialic acid capping of β-galactose-exposed termini. In addition, the dimeric and monomeric forms of rBoAChE were quantitatively converted to tetramers by complexation with a synthetic peptide representing the human ColQ-derived proline-rich attachment domain. Thus by controlling both the level and nature ofN-glycan capping and subunit assembly, we generated and characterized 9 distinct bovine AChE glycoforms displaying a 400-fold difference in their circulatory lifetimes (MRT = 3.5–1390 min). This revealed some general rules and a hierarchy of post-translation factors determining the circulatory profile of glycoproteins. Accordingly, an rBoAChE was generated that displayed a circulatory profile indistinguishable from the native form.

The etiological agent of anthrax disease in animals and humans is the spore-forming bacterium Bacillus anthracis. The major factors of virulence of B. anthracis are located on two plasmids, pXO1 and pXO2. pXO2 encodes a poly-d-glutamic acid capsule (19, 41), while pXO1 encodes two binary exotoxins, the lethal toxin (LT) and the edema toxin (ET) (43, 46, 61). These two toxins are composed of three different proteins: protective antigen (PA), edema factor (EF), and lethal factor (LF) (for a review, see reference 36). PA is the common receptor binding domain of the toxins and can interact with the two different effector domains, EF and LF, to mediate their entry into target cells (14). EF is a calmodulin-dependent adenylate cyclase (37) responsible for the edema seen at the site of infection in experimental animals (17). The LF is a metalloprotease (34) recently shown to cleave the amino termini of the mitogen-activated protein kinase kinases 1 and 2, which results in their inactivation (13). It remains to be determined whether these are the main physiological substrates for the LT activity in vivo (5, 22). Two types of anthrax vaccines are licensed for use in humans: the spores of the toxigenic, nonencapsulated B. anthracis STI-1 strain (55) and the cell-free PA-based vaccines consisting of aluminum hydroxide-adsorbed supernatant material from cultures of the toxigenic, nonencapsulated B. anthracis strain V770-NPI-R (49) or alum-precipitated culture filtrate from the Sterne strain (6). The use of the live attenuated STI-1 occasionally results in general and local adverse responses, observed both after primary application and revaccination, and the frequency of responses increases with the number of vaccinations (58). Furthermore, it was reported that the STI-1 vaccine has a relatively low immunogenicity (reviewed by Stepanov et al. in reference 58). To increase the immunogenicity, a combined vaccine of live STI-1 supplemented with cell-free PA formulation was evaluated and proposed for veterinary use (1). While the cell-free PA-based vaccines appear to be safer, they require numerous boosters (8) and were shown to have reduced ability to protect laboratory animals against certain virulent strains of B. anthracis (39, 60). In addition, these vaccines contain variable amounts of PA, as well as undefined quantities of LF and EF, adsorbed to aluminum hydroxide (4, 21, 49, 59). It appears, therefore, that there is a need for a safe and more efficient vaccine which could generate stable and prolonged immunity in humans (59). These conclusions led to the evaluation of various adjuvants with purified PA (2, 16, 29, 59) and to the creation of two types of live vaccines: vaccines based on nonvirulent B. anthracis (pXO1+) mutated strains (31, 47) and vaccines expressing PA from a cloned pagA gene using heterologous hosts such as the vaccinia virus, Bacillus subtilis, Salmonella typhimurium (10, 27, 28, 30, 31, 64), or a nontoxinogenic strain of B. anthracis (4). These pioneering studies suggest that recombinant B. anthracis live vaccines may have potential as a future anthrax vaccine. We report here the construction of several recombinant, nonencapsulated, and nontoxinogenic B. anthracis spore-forming strains expressing different levels of PA. We demonstrate that one of these strains, containing the pagA gene under a potent heterologous constitutive promoter, can be safely used to provide efficacious long-lasting immunity in experimental animals following a single immunization dose.

Tumor necrosis factor alpha (TNFalpha) or chronic hyperinsulinemia that induce insulin resistance trigger increased Ser/Thr phosphorylation of the insulin receptor (IR) and of its major insulin receptor substrates, IRS-1 and IRS-2. To unravel the molecular basis for this uncoupling in insulin signaling, we undertook to study the interaction of Ser/Thr-phosphorylated IRS-1 and IRS-2 with the insulin receptor. We could demonstrate that, similar to IRS-1, IRS-2 also interacts with the juxtamembrane (JM) domain (amino acids 943-984) but not with the carboxyl-terminal region (amino acids 1245-1331) of IR expressed in bacteria as His6 fusion peptides. Moreover, incubation of rat hepatoma Fao cells with TNFalpha, bacterial sphingomyelinase, or other Ser(P)/Thr(P)-elevating agents reduced insulin-induced Tyr phosphorylation of IRS-1 and IRS-2, markedly elevated their Ser(P)/Thr(P) levels, and significantly reduced their ability to interact with the JM region of IR. Withdrawal of TNFalpha for periods as short as 30 min reversed its inhibitory effects on IR-IRS interactions. Similar inhibitory effects were obtained when Fao cells were subjected to prolonged (20-60 min) pretreatment with insulin. Incubation of the cell extracts with alkaline phosphatase reversed the inhibitory effects of insulin. These findings suggest that insulin resistance is associated with enhanced Ser/Thr phosphorylation of IRS-1 and IRS-2, which impairs their interaction with the JM region of IR. Such impaired interactions abolish the ability of IRS-1 and IRS-2 to undergo insulin-induced Tyr phosphorylation and further propagate the insulin receptor signal. Moreover, the reversibility of the TNFalpha effects and the ability to mimic its action by exogenously added sphingomyelinase argue against the involvement of a proteolytic cascade in mediating the acute inhibitory effects of TNFalpha on insulin action.

The well-characterized virulence factors of Yersinia pestis the causative agent of plague are those encoded by the virulence plasmids. Previously we have isolated an attenuated Y. pestis transposon insertion mutant in which the chromosomal pcm gene was disrupted. The pcm gene is located within a putative stress response locus that includes the surE, nlpD, and rpoS genes. In this study, we investigated the expression and the role of pcm locus genes in Y. pestis pathogenesis by constructing a set of isogenic surE, pcm, nlpD and rpoS mutants in the fully virulent Kimberley53 strain. We show that the NlpD lipoprotein is the only factor encoded from the pcm locus that is essential for Y. pestis virulence. A chromosomal deletion of the nlpD gene sequence resulted in a drastic reduction in virulence to an LD50 of at least 107 cfu for subcutaneous and airway routes of infection. The mutant was unable to colonize mouse organs following infection. The unsegmented morphology of the nlpD mutant indicates that NlpD is involved in cell separation; however, deletion of nlpD did not affect in vitro growth rate. Trans-complementation experiments with the Y. pestis nlpD gene restored virulence and all other phenotypic defects. Finally, we demonstrate that the nlpD mutant could be used as a very potent live vaccine against bubonic and pneumonic plague.

Yersinia pestis is the causative agent of plague. Previously we have isolated an attenuated Y. pestis transposon insertion mutant in which the pcm gene was disrupted. In the present study, we investigated the expression and the role of pcm locus genes in Y. pestis pathogenesis using a set of isogenic surE, pcm, nlpD and rpoS mutants of the fully virulent Kimberley53 strain. We show that in Y. pestis, nlpD expression is controlled from elements residing within the upstream genes surE and pcm. The NlpD lipoprotein is the only factor encoded from the pcm locus that is essential for Y. pestis virulence. A chromosomal deletion of the nlpD gene sequence resulted in a drastic reduction in virulence to an LD(50) of at least 10(7) cfu for subcutaneous and airway routes of infection. The mutant was unable to colonize mouse organs following infection. The filamented morphology of the nlpD mutant indicates that NlpD is involved in cell separation; however, deletion of nlpD did not affect in vitro growth rate. Trans-complementation experiments with the Y. pestis nlpD gene restored virulence and all other phenotypic defects. Finally, we demonstrated that subcutaneous administration of the nlpD mutant could protect animals against bubonic and primary pneumonic plague. Taken together, these results demonstrate that Y. pestis NlpD is a novel virulence factor essential for the development of bubonic and pneumonic plague. Further, the nlpD mutant is superior to the EV76 prototype live vaccine strain in immunogenicity and in conferring effective protective immunity. Thus it could serve as a basis for a very potent live vaccine against bubonic and pneumonic plague.

Protective antigen (PA) is a central virulence factor of Bacillus anthracis and a key component in anthrax vaccines. PA binds to target cell receptors, is cleaved by the furin protease, self-aggregates to heptamers, and finally internalizes as a complex with either lethal or edema factors. Under mild room temperature storage conditions, PA cytotoxicity decreased (t(1/2) approximately 7 days) concomitant with the generation of new acidic isoforms, probably through deamidation of Asn residues. Ranking all 68 Asn residues in PA based on their predicted deamidation rates revealed five residues with half-lives of60 days, and these residues were further analyzed: Asn10 in the 20-kDa region, Asn162 at P6 vicinal to the furin cleavage site, Asn306 in the pro-pore translocation loop, and both Asn713 and Asn719 in the receptor-binding domain. We found that PA underwent spontaneous deamidation at Asn162 upon storage concomitant with decreased susceptibility to furin. A panel of model synthetic furin substrates was used to demonstrate that Asn162 deamidation led to a 20-fold decrease in the bimolecular rate constant (k(cat)/Km) of proteolysis due to the new negatively charged residue at P6 in the furin recognition sequence. Furthermore, reduced PA cytotoxicity correlated with a decrease in PA cell binding and also with deamidation of Asn713 and Asn719. On the other hand, neither deamidation of Asn10 or Asn306 nor impairment of heptamerization could be observed upon prolonged PA storage. We suggest that PA inactivation during storage is associated with susceptible deamidation sites, which are intimately involved in both mechanisms of PA cleavage by furin and PA-receptor binding.

Understanding reaction pathways of phosphylation, reactivation, and "aging" of AChE with toxic organophosphate compounds is both a biochemical and a pharmacological challenge. Here we describe experiments which allowed to resolve some of the less well understood reaction pathways of phosphylation and "aging" of acetylcholinesterase (AChE) involving phosphoroamidates (P-N agents) such as tabun or the widely used pesticide methamidophos. Tryptic digests of phosphylated AChEs (from human and Torpedo californica), ZipTip peptide fractionation and matrix-assisted laser desorption ionization mass spectrometry (MALDI-TOF/MS) enabled reproducible signal enrichment of the isotopically resolved peaks of organophosphoroamidate conjugates of the AChE active site Ser peptides. For tabun and its hexadeuterio analogue, we find, as expected, that the two phosphoramidate adducts of the active site peptide differ by 6.05 mass units but following aging we find that the two corresponding phospho-peptides have identical molecular weights. We further show that the aging product of paraoxon-AChE adduct is identical to the aging product of the tabun-AChE conjugate. These results unequivocally demonstrate that the pathway of aging of tabun adducts of the human or the Torpedo californica AChEs proceeds through P-N bond scission. For methamidophos, we show that phosphylation of AChE involves elimination of the thiomethyl moiety and that the spontaneous reactivation of the resulting organophosphate adduct generates the phosphorus free AChE active site Ser-peptide.