Anthrax toxin protective antigen protein (PA, 83 kDa) binds to receptors on the surface of mammalian cells, is cleaved by the cell surface protease furin, and then captures either of the two other toxin proteins, lethal factor (LF, 90 kDa) or edema factor (EF, 89 kDa). The PA-LF and PA-EF complexes enter cells by endocytosis and pass to an acidic vesicle from which LF and EF escape to the cytosol. EF is a calcium- and calmodulin-dependent adenylyl cyclase that causes large and unregulated increases in intracellular cAMP concentrations. LF is a metalloprotease that cleaves several mitogen-activated protein kinase kinases (MEKs). Our work exploits unique features of these proteins to construct therapeutic agents. Mutagenesis of the small loop of domain 4 of PA confirmed the importance of residues surrounding N682 for interaction with the cellular receptor. Single amino acid substitutions were made in each residue in the small loop and several substitutions were identified that decrease toxicity by more than 100-fold. Identification of PA mutants such as this provides the basis for construction of fusion proteins targeted to new receptors. PA proteins were constructed that are redirected to the GMCSF receptor and to the folate receptor. As an alternative method of achieving cell-type specificity, the furin site in PA was replaced by sequences cleaved by cell surface proteases that are enriched on tumor cells, specifically matrix metalloproteinases and plasminogen activators. These altered PA proteins were shown to be highly specific for cultured tumor cells while sparing normal cells in tissue culture systems. Studies in knockout mice confirmed that all components of the plasminogen activator system are needed to activate these proteins in vivo. These constructs will be tested for their ability to block tumor growth and metastasis in mice. PA is the critical immunogen in the current and all proposed second-generation human anthrax vaccines. Methods for production of anthrax toxin components were further improved, so as to provide large amounts for use as improved anthrax vaccines. Anthrax toxin LF and a catalytically inactive LF mutant made in large amounts were used by a collaborator to solve the X-ray crystal structure.
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