Crystal structure of the anthrax toxin protective antigen

• Published in: Nature (London) Vol. 385; no. 6619; pp. 833 - 838
• Main Authors: Petosa, Carlo, Collier, R. John, Klimpel, Kurt R, Leppla, Stephen H, Liddington, Robert C
• Format: Journal Article
• Language: English
• Published: London Nature Publishing 27-02-1997; Nature Publishing Group
• Subjects: Amino Acid Sequence; Antigens, Bacterial; Bacillus anthracis; Bacillus anthracis - chemistry; Bacteria; Bacterial Toxins - chemistry; Bacterial Toxins - toxicity; Bacteriology; Binding Sites; Biochemistry; Biological and medical sciences; Cell Membrane; Crystallography, X-Ray; Crystals; Disease; Fundamental and applied biological sciences. Psychology; Membranes; Microbiology; Models, Biological; Models, Molecular; Molecular Sequence Data; Pathogenicity, virulence, toxins, bacteriocins, pyrogens, host-bacteria relations, miscellaneous strains; Protein Conformation; Proteins; Toxins; Bacillus anthracis; Toxin; Bacillales; Molecular structure; Bacteria; Bacillaceae; Structural analysis; Crystalline structure
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/385833a0

Protective antigen (PA) is the central component of the three-part protein toxin secreted by Bacillus anthracis, the organism responsible for anthrax. After proteolytic activation on the host cell surface, PA forms a membrane-inserting heptamer that translocates the toxic enzymes, oedema factor and lethal factor, into the cytosol. PA, which has a relative molecular mass of 83,000 (M(r) 83K), can also translocate heterologous proteins, and is being evaluated for use as a general protein delivery system. Here we report the crystal structure of monomeric PA at 2.1 A resolution and the water-soluble heptamer at 4.5 A resolution. The monomer is organized mainly into antiparallel beta-sheets and has four domains: an amino-terminal domain (domain 1) containing two calcium ions and the cleavage site for activating proteases; a heptamerization domain (domain 2) containing a large flexible loop implicated in membrane insertion; a small domain of unknown function (domain 3); and a carboxy-terminal receptor-binding domain (domain 4). Removal of a 20K amino-terminal fragment from domain 1 allows the assembly of the heptamer, a ring-shaped structure with a negatively charged lumen, and exposes a large hydrophobic surface for binding the toxic enzymes. We propose a model of pH-dependent membrane insertion involving the formation of a porin-like, membrane-spanning beta-barrel.