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Membrane insertion by anthrax protective antigen in cultured cells.

ABSTRACT: The enzymatic moieties of anthrax toxin enter the cytosol of mammalian cells via a pore in the endosomal membrane formed by the protective antigen (PA) moiety. Pore formation involves an acidic pH-induced conformational rearrangement of a heptameric precursor (the prepore), in which the seven 2beta2-2beta3 loops interact to generate a 14-strand transmembrane beta-barrel. To investigate this model in vivo, we labeled PA with the fluorophore 7-nitrobenz-2-oxa-1,3-diazole (NBD) at cysteine residues introduced into the 2beta2-2beta3 loop. Each labeled PA was bound to CHO cells, and NBD fluorescence was monitored over time in stirred cell suspensions or by confocal microscopy. A strong increase was observed with NBD at positions 305, 307, 309, and 311, sites where side chains are predicted to face the bilayer, and little change was seen at residues 304, 306, 308, 310, and 312, sites where side chains are predicted to face the pore lumen. The increase at position 305 was inhibited by membrane-restricted quenchers, low temperature, or various reagents known to affect toxin action. Of the 24 NBD attachment sites examined, all but three gave results qualitatively consistent with the beta-barrel model. Besides supporting the beta-barrel model of membrane insertion, our results describe the time course of insertion and identify PA residues where NBD gives a strong signal upon membrane insertion in vivo.

SUBMITTER: Qa'dan M

PROVIDER: S-EPMC1156972 | biostudies-literature |

REPOSITORIES: biostudies-literature

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The Disulfide Bond Cys255-Cys279 in the Immunoglobulin-Like Domain of Anthrax Toxin Receptor 2 Is Required for Membrane Insertion of Anthrax Protective Antigen Pore.

Project description:Anthrax toxin receptors act as molecular clamps or switches that control anthrax toxin entry, pH-dependent pore formation, and translocation of enzymatic moieties across the endosomal membranes. We previously reported that reduction of the disulfide bonds in the immunoglobulin-like (Ig) domain of the anthrax toxin receptor 2 (ANTXR2) inhibited the function of the protective antigen (PA) pore. In the present study, the disulfide linkage in the Ig domain was identified as Cys255-Cys279 and Cys230-Cys315. Specific disulfide bond deletion mutants were achieved by replacing Cys residues with Ala residues. Deletion of the disulfide bond C255-C279, but not C230-C315, inhibited the PA pore-induced release of the fluorescence dyes from the liposomes, suggesting that C255-C279 is essential for PA pore function. Furthermore, we found that deletion of C255-C279 did not affect PA prepore-to-pore conversion, but inhibited PA pore membrane insertion by trapping the PA membrane-inserting loops in proteinaceous hydrophobic pockets. Fluorescence spectra of Trp59, a residue adjacent to the PA-binding motif in von Willebrand factor A (VWA) domain of ANTXR2, showed that deletion of C255-C279 resulted in a significant conformational change on the receptor ectodomain. The disulfide deletion-induced conformational change on the VWA domain was further confirmed by single-particle 3D reconstruction of the negatively stained PA-receptor heptameric complexes. Together, the biochemical and structural data obtained in this study provides a mechanistic insight into the role of the receptor disulfide bond C255-C279 in anthrax toxin action. Manipulation of the redox states of the receptor, specifically targeting to C255-C279, may become a novel strategy to treat anthrax.

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Project description:Anthrax toxin, comprising protective antigen, lethal factor, and oedema factor, is the major virulence factor of Bacillus anthracis, an agent that causes high mortality in humans and animals. Protective antigen forms oligomeric prepores that undergo conversion to membrane-spanning pores by endosomal acidification, and these pores translocate the enzymes lethal factor and oedema factor into the cytosol of target cells. Protective antigen is not only a vaccine component and therapeutic target for anthrax infections but also an excellent model system for understanding the mechanism of protein translocation. On the basis of biochemical and electrophysiological results, researchers have proposed that a phi (Φ)-clamp composed of phenylalanine (Phe)427 residues of protective antigen catalyses protein translocation via a charge-state-dependent Brownian ratchet. Although atomic structures of protective antigen prepores are available, how protective antigen senses low pH, converts to active pore, and translocates lethal factor and oedema factor are not well defined without an atomic model of its pore. Here, by cryo-electron microscopy with direct electron counting, we determine the protective antigen pore structure at 2.9-Å resolution. The structure reveals the long-sought-after catalytic Φ-clamp and the membrane-spanning translocation channel, and supports the Brownian ratchet model for protein translocation. Comparisons of four structures reveal conformational changes in prepore to pore conversion that support a multi-step mechanism by which low pH is sensed and the membrane-spanning channel is formed.

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Plant-based vaccine: mice immunized with chloroplast-derived anthrax protective antigen survive anthrax lethal toxin challenge.

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