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R John Collier

Publications and source records attributed to R John Collier.

At least 19 recordsLinked to original sources
PubMed2004

Acid-induced unfolding of the amino-terminal domains of the lethal and edema factors of anthrax toxin.

The two enzymatic components of anthrax toxin, lethal factor (LF) and edema factor (EF), are transported to the cytosol of mammalian cells by the third component, protective antigen (PA). A heptameric form of PA binds LF and/or EF and, under the acidic conditions encountered in endosomes, generates a membrane-spanning pore that is thought to serve as a passageway for these enzymes to enter the cytosol. The pore contains a 14-stranded transmembrane beta-barrel that is too narrow to accommodate a fully folded protein, necessitating that LF and EF unfold, at least partly, in order to pass. Here, we describe the pH-dependence of the unfolding of LF(N) and EF(N), the 30kDa N-terminal PA-binding domains, and minimal translocatable units, of LF and EF. Equilibrium chemical denaturation studies using fluorescence and circular dichroism spectroscopy show that each protein unfolds via a four-state mechanism: N<-->I<-->J<-->U. The acid-induced N-->I transition occurs within the pH range of the endosome (pH 5-6). The I state predominates at lower pH values, and the J and U states are populated significantly only in the presence of denaturant. The I state is compact and has characteristics of a molten globule, as shown by its retention of significant secondary structure and its ability to bind an apolar fluorophore. The N-->I transition leads to an overall 60% increase in buried surface area exposure. The J state is expanded significantly and has diminished secondary structure content. We analyze the different protonation states of LF(N) and EF(N) in terms of a linked equilibrium proton binding model and discuss the implications of our findings for the mechanism of acidic pH-induced translocation of anthrax toxin. Finally, analysis of the structure of the transmembrane beta-barrel of PA shows that it can accommodate alpha-helix, and we suggest that the steric constraints and composition of the lumen may promote alpha-helix formation.

Acids↗
PubMed2004

Evidence that translocation of anthrax toxin's lethal factor is initiated by entry of its N terminus into the protective antigen channel.

Entry of the enzymatic components of anthrax toxin [lethal factor (LF) and edema factor] into the cytosol of mammalian cells depends on the ability of the activated protective antigen (PA63) component to form a channel (pore) in the membrane of an acidic intracellular compartment. To investigate the mechanism of translocation, we characterized N-terminally truncated forms of the PA63-binding domain of LF (LFN). Deleting 27 or 36 residues strongly inhibited acid-triggered translocation of LFN across the plasma membrane of CHO-K1 cells and ablated the protein's ability to block PA63 channels in planar lipid bilayers at a small positive voltage (+20 mV). Fusing a H6-tag to the N terminus of the truncated proteins restored both translocation and channel-blocking activities. At +20 mV, N-terminal H6 and biotin tags were accessible to Ni2+ and streptavidin, respectively, added to the trans compartment of a planar bilayer. On the basis of these findings, we propose that the N terminus of PA63-bound LF or edema factor enters the PA63-channel under the influence of acidic pH and a positive transmembrane potential and initiates translocation in an N- to C-terminal direction.

Amino Acid Sequence↗
PubMed2004

Conformational changes in BID, a pro-apoptotic BCL-2 family member, upon membrane binding. A site-directed spin labeling study.

The BCL-2 family proteins constitute a critical control point in apoptosis. BCL-2 family proteins display structural homology to channel-forming bacterial toxins, such as colicins, transmembrane domain of diphtheria toxin, and the N-terminal domain of delta-endotoxin. By analogy, it has been hypothesized the BCL-2 family proteins would unfold and insert into the lipid bilayer upon membrane association. We applied the site-directed spin labeling method of electron paramagnetic resonance spectroscopy to the pro-apoptotic member BID. Here we show that helices 6-8 maintain an alpha-helical conformation in membranes with a lipid composition resembling mitochondrial outer membrane contact sites. However, unlike colicins and the transmembrane domain of diphtheria toxin, these helices of BID are bound to the lipid bilayer without adopting a transmembrane orientation. Our study presents a more detailed model for the reorganization of the structure of tBID on membranes.

Amino Acid Sequence↗
PubMed2004

Protein translocation through anthrax toxin channels formed in planar lipid bilayers.

The 63-kDa fragment of the protective antigen (PA) component of anthrax toxin forms a heptameric channel, (PA63)7, in acidic endosomal membranes that leads to the translocation of edema factor (EF) and lethal factor (LF) to the cytosol. It also forms a channel in planar phospholipid bilayer membranes. What role does this channel play in the translocation of EF and LF? We report that after the 263-residue N-terminal piece of LF (LFN) binds to its receptor on the (PA63)7 channel and its N-terminal end enters the channel at small positive voltages to block it, LFN is translocated through the channel to the opposite side at large positive voltages, thereby unblocking it. Thus, all of the translocation machinery is contained in the (PA63)7 channel, and translocation does not require any cellular proteins. The kinetics of this translocation are S-shaped, voltage-dependent, and occur on a timescale of seconds. We suggest that the translocation process might be explained simply by electrophoresis of unfolded LFN through the channel, but the refolding of the N-terminal half of LFN as it emerges from the channel may also provide energy for moving the rest of the molecule through the channel.

Antigens, Bacterial↗
PubMed2004

Anthrax toxin complexes: heptameric protective antigen can bind lethal factor and edema factor simultaneously.

The 83 kDa protective antigen (PA(83)) component of anthrax toxin, after proteolytic activation, self-associates to form ring-shaped heptamers ([PA(63)](7)) that bind and aid delivery of the Edema Factor (EF) and Lethal Factor (LF) components to the cytosol. Here we show using fluorescence (Förster) resonance energy transfer that a molecule of [PA(63)](7) can bind EF and LF simultaneously. We labeled EF and LF with an appropriate donor/acceptor pair and found quenching of the donor and an increase in sensitized emission of the acceptor when, and only when, a mixture of the labeled proteins was combined with [PA(63)](7). Addition of unlabeled PA(63)-binding domain of LF to the mixture competitively displaced labeled EF and LF, causing a loss of energy transfer. In view of the known maximum occupancy of 3 ligand molecules per [PA(63)](7), these findings indicate that PA, EF, and LF can form mixtures of liganded toxin complexes containing both EF and LF.

Antigens, Bacterial↗
PubMed2004

Structure of heptameric protective antigen bound to an anthrax toxin receptor: a role for receptor in pH-dependent pore formation.

After binding to cellular receptors and proteolytic activation, the protective antigen component of anthrax toxin forms a heptameric prepore. The prepore later undergoes pH-dependent conversion to a pore, mediating translocation of the edema and lethal factors to the cytosol. We describe structures of the prepore (3.6 A) and a prepore:receptor complex (4.3 A) that reveal the location of pore-forming loops and an unexpected interaction of the receptor with the pore-forming domain. Lower pH is required for prepore-to-pore conversion in the presence of the receptor, indicating that this interaction regulates pH-dependent pore formation. We present an example of a receptor negatively regulating pH-dependent membrane insertion.

Antigens, Bacterial↗
PubMed2004

Reversible refolding of the diphtheria toxin T-domain on lipid membranes.

The catalytic domain of diphtheria toxin (DT) is translocated across endosomal membranes by the T-domain (DTT) in response to acidification. Understanding the energetics of translocation, besides clarifying the mechanism of translocation, should provide insights into general principles of membrane protein stability and assembly. As a first step, we have evaluated the energetics of DTT binding to lipid vesicles using three single-cysteine mutants (L350C, Q369C, and Y280C) labeled with a 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD) fluorophore sensitive to polarity changes. Remarkably strong association with the vesicles was detected for all mutants, even at pH 7 at which DTT is believed to be in a fully folded membrane-incompetent state. Lowering the pH in the presence of anionic membranes resulted in a strong but reversible increase in emission of NBD-labeled mutants, consistent with reversible membrane insertion. This reversibility permitted free energies of DTT interactions with vesicles to be determined for the first time. Free energy values for the three mutants ranged from -8 to -10 kcal mol(-1) at pH 4.3 and from -7 to -8 kcal mol(-1) at pH 7. Insights into the disposition of DTT on membranes were obtained using a novel hydropathy analysis that considers the relative free energies of transmembrane and interfacial interactions as a function of pH. This analysis suggests that interactions at the membrane interface dominate pH-triggered insertion of DTT, implying that the folding pathway involves interfacial intermediates.

Diphtheria Toxin↗
PubMed2004

Crystal structure of the von Willebrand factor A domain of human capillary morphogenesis protein 2: an anthrax toxin receptor.

Anthrax toxin is released from Bacillus anthracis as three monomeric proteins, which assemble into toxic complexes at the surface of receptor-bearing host cells. One of the proteins, protective antigen (PA), binds to receptors and orchestrates the delivery of the other two (the lethal and edema factors) into the cytosol. PA has been shown to bind to two cellular receptors: anthrax toxin receptor/tumor endothelial marker 8 and capillary morphogenesis protein 2 (CMG2). Both are type 1 membrane proteins that include an approximately 200-aa extracellular von Willebrand factor A (VWA) domain with a metal ion-dependent adhesion site (MIDAS) motif. The anthrax toxin receptor/tumor endothelial marker 8 and CMG2 VWA domains share approximately 60% amino acid identity and bind PA directly in a metal-dependent manner. Here, we report the crystal structure of the CMG2 VWA domain, with and without its intramolecular disulfide bond, to 1.5 and 1.8 A, respectively. Both structures contain a carboxylate ligand-mimetic bound at the MIDAS and appear as open conformations when compared with the VWA domains from alpha-integrins. The CMG2 structures provide a template to begin probing the high-affinity CMG2-PA interaction (200 pM) and may facilitate understanding of toxin assembly/internalization and the development of new anthrax treatments. The structural data also allow molecular interpretation of known CMG2 VWA domain mutations linked to the genetic disorders, juvenile hyaline fibromatosis, and infantile systemic hyalinosis.

Amino Acid Sequence↗
PubMed2004

Binding stoichiometry and kinetics of the interaction of a human anthrax toxin receptor, CMG2, with protective antigen.

The protective antigen (PA) moiety of anthrax toxin binds to cellular receptors and mediates entry of the two enzymatic moieties of the toxin into the cytosol. Two PA receptors, anthrax toxin receptor (ATR)/tumor endothelial marker 8 (TEM8) and capillary morphogenesis protein 2 (CMG2), have been identified. We expressed and purified the von Willebrand A (VWA) domain of CMG2 and examined its interactions with monomeric and heptameric forms of PA. Monomeric PA bound a stoichiometric equivalent of CMG2, whereas the heptameric prepore form bound 7 eq. The Kd of the VWA domain-PA interaction is 170 pm when liganded by Mg2+, reflecting a 1000-fold tighter interaction than most VWA domains with their endogenous ligands. The dissociation rate constant is extremely slow, indicating a 30-h lifetime for the CMG2.PA monomer complex. CMG2 metal ion-dependent adhesion site (MIDAS) was studied kinetically and thermodynamically. The association rate constant (approximately 10(5) m(-1) s(-1)) is virtually identical in the presence or absence of Mg2+ or Ca2+ , but the dissociation rate of metal ion liganded complex is up to 4 orders of magnitude slower than metal ion free complex. Residual affinity (Kd approximately 960 nm) in the absence of divalent metal ions allowed the free energy for the contribution of the metal ion to be calculated as 5 kcal mol(-1), demonstrating that the metal ion-dependent adhesion site is directly coordinated by CMG2 and PA in the binding interface. The high affinity of the VWA domain for PA supports its potency in neutralizing anthrax toxin, demonstrating its potential utility as a novel therapeutic for anthrax.

Antigens, Bacterial↗
PubMed2004

Effects of dynamin inactivation on pathways of anthrax toxin uptake.

Internalization and traffic to acidic endosomes of anthrax lethal factor (LF) and protective antigen (PA), bound to the anthrax toxin receptor (ATR), is required for LF translocation into the cytosol, where it can elicit its toxic effects. Dynamin is required for clathrin-mediated endocytosis, and long-term disruption of dynamin function blocks internalization of PA. We have used LFn-DTA, a surrogate of LF consisting of the N-terminal domain of LF fused to the catalytic subunit of diphtheria toxin, to differentiate the effects of acute and long-term block of dynamin function on LFn-DTA toxicity. Both forms of interference reduce LFn-DTA toxicity only partially, consistent with alternative routes for LFn-DTA endocytosis. In contrast, a long-term block of dynamin activity results in a further interference with LFn-DTA toxicity that is consistent with an altered endosomal environment, probably an increase in endosomal pH.

Animals↗
PubMed2004

Use of phage display and polyvalency to design inhibitors of protein-protein interactions.

We describe the synthesis of an inhibitor that interferes with critical protein-protein interactions occurring during the assembly of anthrax toxin. Using a phage display selection strategy, we isolated a peptide directed against the cell binding moiety of the toxin that was able to interfere with binding of the enzymatic moieties. Because the cell binding moiety of the toxin is a heptamer, the peptide can potentially bind up to seven equivalent sites. We synthesized a polyvalent molecule displaying multiple copies of this peptide and showed that it is a much more potent inhibitor than the free peptide. Because little structural knowledge of the interacting proteins was required to synthesize this inhibitor, we believe that this approach may prove useful in the design of inhibitors of protein-protein interactions in other systems.

Amino Acid Sequence↗
PubMed2003

The structural basis for substrate and inhibitor selectivity of the anthrax lethal factor.

Recent events have created an urgent need for new therapeutic strategies to treat anthrax. We have applied a mixture-based peptide library approach to rapidly determine the optimal peptide substrate for the anthrax lethal factor (LF), a metalloproteinase with an important role in the pathogenesis of the disease. Using this approach we have identified peptide analogs that inhibit the enzyme in vitro and that protect cultured macrophages from LF-mediated cytolysis. The crystal structures of LF bound to an optimized peptide substrate and to peptide-based inhibitors provide a rationale for the observed selectivity and may be exploited in the design of future generations of LF inhibitors.

Amino Acid Motifs↗
PubMed2003

Mapping dominant-negative mutations of anthrax protective antigen by scanning mutagenesis.

The protective antigen (PA) moiety of anthrax toxin transports edema factor and lethal factor to the cytosol of mammalian cells by a mechanism that depends on its ability to oligomerize and form pores in the endosomal membrane. Previously, some mutated forms of PA, designated dominant negative (DN), were found to coassemble with wild-type PA and generate defective heptameric pore-precursors (prepores). Prepores containing DN-PA are impaired in pore formation and in translocating edema factor and lethal factor across the endosomal membrane. To create a more comprehensive map of sites within PA where a single amino acid replacement can give a DN phenotype, we used automated systems to generate a Cys-replacement mutation for each of the 568 residues of PA63, the active 63-kDa proteolytic fragment of PA. Thirty-three mutations that reduced PA's ability to mediate toxicity at least 100-fold were identified in all four domains of PA63. A majority (22) were in domain 2, the pore-forming domain. Seven of the domain-2 mutations, located in or adjacent to the 2beta6 strand, the 2beta7 strand, and the 2beta10-2beta11 loop, gave the DN phenotype. This study demonstrates the feasibility of high-throughput scanning mutagenesis of a moderate sized protein. The results show that DN mutations cluster in a single domain and implicate 2beta6 and 2beta7 strands and the 2beta10-2beta11 loop in the conformational rearrangement of the prepore to the pore. They also add to the repertoire of mutations available for structure-function studies and for designing new antitoxic agents for treatment of anthrax.

Animals↗
PubMed2003

A dually active anthrax vaccine that confers protection against both bacilli and toxins.

Systemic anthrax is caused by unimpeded bacillar replication and toxin secretion. We developed a dually active anthrax vaccine (DAAV) that confers simultaneous protection against both bacilli and toxins. DAAV was constructed by conjugating capsular poly-gamma-d-glutamic acid (PGA) to protective antigen (PA), converting the weakly immunogenic PGA to a potent immunogen, and synergistically enhancing the humoral response to PA. PGA-specific antibodies bound to encapsulated bacilli and promoted the killing of bacilli by complement. PA-specific antibodies neutralized toxin activity and protected immunized mice against lethal challenge with anthrax toxin. Thus, DAAV combines both antibacterial and antitoxic components in a single vaccine against anthrax. DAAV introduces a vaccine design that may be widely applicable against infectious diseases and provides additional tools in medicine and biodefense.

Animals↗
PubMed2003

Fluorescence resonance energy transfer studies on anthrax lethal toxin.

Anthrax lethal toxin is a binary bacterial toxin consisting of two proteins, protective antigen (PA) and lethal factor (LF), that self-assemble on receptor-bearing eukaryotic cells to form toxic, non-covalent complexes. PA(63), a proteolytically activated form of PA, spontaneously oligomerizes to form ring-shaped heptamers that bind LF and translocate it into the cell. Site-directed mutagenesis was used to substitute cysteine for each of three residues (N209, E614 and E733) at various levels on the lateral face of the PA(63) heptamer and for one residue (E126) on LF(N), the 30 kDa N-terminal PA binding domain of LF. Cysteine residues in PA were labeled with IAEDANS and that in LF(N) was labeled with Alexa 488 maleimide. The mutagenesis and labeling did not significantly affect function. Time-resolved fluorescence methods were used to study fluorescence resonance energy transfer (FRET) between the AEDANS and Alexa 488 probes after the complex assembled in solution. The results clearly indicate energy transfer between AEDANS labeled at residue N209C on PA and the Alexa 488-labeled LF(N), whereas transfer from residue E614C on PA was slight, and none was observed from residue E733C. These results support a model in which LF(N) binds near the top of the ring-shaped (PA(63))(7) heptamer.

Animals↗
PubMed2003

Exchange characteristics of calcium ions bound to anthrax protective antigen.

Protective antigen (PA), the receptor-binding moiety of anthrax toxin, contains two calcium atoms buried within domain 1(') (amino acid residues 168-258). We showed that these ions are stably bound and exchange with free 45Ca(2+) only slowly (t(1/2) approximately 4.0 h). Dissociation is the rate-limiting step. PA(63), the heptameric prepore form of PA, showed a slightly higher exchange rate than the monomeric intact protein. Exchange by this form was retarded by binding of the enzymatic moieties of the toxin, but was unaffected by reducing the pH to 5.0, a condition known to trigger conversion of the prepore to the pore form. These results are consistent with the hypothesis that bound Ca(2+) within PA plays primarily a structural role, maintaining domain 1(') in a conformation that allows PA(63) to oligomerize and bind the enzymatic moieties of the toxin.

Antigens, Bacterial↗
PubMed2003

Anthrax toxin.

Anthrax toxin consists of three nontoxic proteins that associate in binary or ternary combinations to form toxic complexes at the surface of mammalian cells. One of these proteins, protective antigen (PA), transports the other two, edema factor (EF) and lethal factor (LF), to the cytosol. LF is a Zn2+-protease that cleaves certain MAP kinase kinases, leading to death of the host via a poorly defined sequence of events. EF, a calmodulin- and Ca2+-dependent adenylate cyclase, is responsible for the edema seen in the disease. Both enzymes are believed to benefit the bacteria by inhibiting cells of the host's innate immune system. Assembly of toxic complexes begins after PA binds to cellular receptors and is cleaved into two fragments by furin proteases. The smaller fragment dissociates, allowing the receptor-bound fragment, PA63 (63 kDa), to self-associate and form a ring-shaped, heptameric pore precursor (prepore). The prepore binds up to three molecules of EF and/or LF, and the resulting complexes are endocytosed and trafficked to an acidic compartment. There, the prepore converts to a transmembrane pore, mediating translocation of EF and LF to the cytosol. Recent studies have revealed (a) the identity of receptors; (b) crystallographic structures of the three toxin proteins and the heptameric PA63 prepore; and (c) information about toxin assembly, entry, and action within the cytosol. Knowledge of the structure and mode of action of the toxin has unveiled potential applications in medicine, including approaches to treating anthrax infections.

Adenylyl Cyclases↗