The action of anthrax toxin relies in part upon the ability of the protective antigen (PA) moiety to form a heptameric pore in the endosomal membrane, providing a portal for entry of the enzymic moieties of the toxin into the cytosol. Pore formation is dependent on a conformational change in the heptameric prepore that occurs in the neutral to mildly acidic pH range, and it has been hypothesized that protonation of one or more histidine residues triggers this transition. To test this hypothesis, we used biosynthetic methods to incorporate the unnatural amino acid analogue 2-fluorohistidine (2-FHis) into PA. 2-FHis is isosteric with histidine but resists protonation at physiological pH values due to a dramatically reduced side-chain pKa ( approximately 1). We found that 2-FHis-labeled PA was biologically inactive, as judged by its inability to deliver a model intracellular effector, LFN-DTA, to the cytosol of CHO-K1 cells. However, whereas 2-FHis blocked a conformational transition in the full-length PA83 protein in the pH 5-6 range, the pH dependence of prepore-to-pore conversion of (PA63)7 was unchanged from the wild-type protein, implying that this conversion is not dependent on His protonation. Consistent with this result, the labeled, trypsin-activated PA was able to permeabilize liposomes to K+ and retained pore-forming activity in planar phospholipid bilayers. The pores in planar bilayers were incapable, however, of translocating a model ligand in response to a transmembrane pH gradient or elevated voltage. The results indicate that protonation of residues other than His, presumably Glu and/or Asp side chains, triggers pore formation in vitro, but His residues are nonetheless important for PA functioning in vivo.

The protective antigen (PA) component of the anthrax toxin forms pores within the low pH environment of host endosomes, through mechanisms that are poorly understood. It has been proposed that pore formation is dependent on histidine protonation. In previous work, we biosynthetically incorporated 2-fluorohistidine (2-FHis), an isosteric analog of histidine with a significantly reduced pKa (~1), into PA, and showed that the pH-dependent conversion from the soluble prepore to a pore was unchanged. However, we also observed that 2-FHisPA was nonfunctional in the ability to mediate cytotoxicity of CHO-K1 cells by LF N -DTA, and was defective in translocation through planar lipid bilayers. Here, we show that the defect in cytotoxicity is due to both a defect in translocation and, when bound to the host cellular receptor, an inability to undergo low pH-induced pore formation. Combining X-ray crystallography with hydrogendeuterium (H-D) exchange mass spectrometry, our studies lead to a model in which hydrogen bonds to the histidine ring are strengthened by receptor binding. The combination of both fluorination and receptor binding is sufficient to block low pH-induced pore formation.Anthrax toxin is an AB toxin secreted by Bacillus anthracis and is required for disease pathogenesis (1). The B component, the protective antigen (PA), is an 83 kDa, four domain protein that binds to the extracellular von Willebrand factor A (vWA) integrin-like I domain 1 To whom correspondence should be addressed: Tel: (316) Fax: (316) 978-3431; Jim.Bann@wichita.edu. 2 Current address: Department of Biological Sciences, University of Texas at El Paso, El Paso, Texas 79968-0519, USA 3 Current address: Department of Chemistry, University of Missouri-Columbia, 125 Chemistry Building, 601 S. College Avenue, Columbia, MO 65211-7600. Supporting informationSupplementary figures of translocation of LFN-DTA, a simulated-annealing omit map for the β1-β2 and β3-β4 loops, a figure showing the crystal contact between R344 and E224 and mass spectra of peptides used in the H-D exchange experiments. This material is free of charge via the Internet at http://pubs.acs.org. NIH Public Access Author ManuscriptBiochemistry. Author manuscript; available in PMC 2011 August 24. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript of either one of two identified host cell receptors, anthrax toxin receptor 1/tumor endothelial marker 8 (ANTXR1/TEM8) or anthrax toxin receptor 2/capillary morphogenesis protein 2 (ANTXR2/CMG2) (2,3). Binding of PA to the receptor vWA domain is followed by cleavage of PA into two fragments of 63 kDa and 20 kDa, by a cell-surface furin protease (4), leading to the spontaneous formation of a ring-shaped heptameric (PA 63 ) 7 structure termed the prepore (5). Formation of the prepore creates binding sites for the enzymatic A components, edema factor (EF) and lethal factor (LF), which bind with a stoichiometry of three EF or LF's per prepore (6). In addition, PA has been shown recently to form functional octa...

A series of 3-amino-2-phenylpropene (APP) derivatives have been synthesized and characterized as novel competitive inhibitors, with K(i) values in the microM range, for the bovine chromaffin granule membrane monoamine transporter(s) (bVMAT). Although, these inhibitors are structurally similar to the bVMAT substrate tyramine, none of them were measurably transported into the granule. Structure-activity studies have revealed that, while the 3'- or 4'-OH groups on the aromatic ring enhance the inhibition potency, Me or OMe groups in these positions reduce the inhibition potency. Halogen substitution on the 4'-position of the aromatic ring causes gradual increase of the inhibition potency parallel to the electron donor ability of the halogen. Substituents on the NH(2) as well as on the 3-position of the alkyl chain reduce the inhibition potency. Comparative structure-activity analyses of APP derivatives with tyramine and the neurotoxin 1-methyl-4-phenylpyridinium suggest that the flexibility of the side chain and the relative orientation of the NH(2) group may be critical for the efficient transport of the substrate through the bVMAT. Comparable bVMAT affinities of these inhibitors to that of DA and other pharmacologically active amines suggest that they are suitable for the structure-activity and mechanistic studies of monoamine transporters and may also be useful in modeling the mechanism of action of amphetamine-related derivatives.

The active metabolite of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), N-methyl-4-phenylpyridinium (MPP(+)), selectively destroys the dopaminergic neurons and induces the symptoms of Parkinson's disease. Inhibition of mitochondrial complex I and/or the perturbation of dopamine metabolism through cellular and granular accumulation have been proposed as some of the major causes of neurotoxicity. In the present study we have synthesized and characterized a number of MPTP and MPP(+) derivatives that are suitable for the comparative neurotoxicity and complex I inhibition versus dopamine metabolism perturbation studies. Structure-activity studies with bovine chromaffin granule ghosts show that 3'-hydroxy-MPP(+) is one of the best known substrates for the vesicular monoamine transporter (VMAT). A series of compounds that combine the structural features of MPP(+) and a previously characterized VMAT inhibitor, 3-amino-2-phenyl-propene, have been identified as the most effective VMAT inhibitors. These derivatives have been used to define the structural requirements of the VMAT substrate and inhibitor activities.