A tale of two toxins: Anthrax protective antigen and Staphylococcus aureus alpha hemolysin

Abstract

The protective antigen protein of anthrax toxin (PA) mediates entry of the toxins enzymatic components into the cell. Proteolytically-cleaved PA (nPA) also makes homoheptameric pores in cells, liposomes, and black lipid membranes. In this work, I explore the topology of the channels made by protective antigen in planar bilayers, and expand on previous topological studies, which have used Substituted-Cysteine-Accessibility Mutagenesis (SCAM) to identify pore-lining residues in the membrane-spanning region of the PA63 channel (Benson et al., 1998). SCAM entails single-cysteine substitution of residues in a protein, which is subsequently assayed for reaction with a cysteine-specific reagent; in this case, the channels formed by mutant nPA are probed with methyltrimethylammonium ion (MTS) species. From analogy to the heptameric channels of Staphylococcus aureus alpha-hemolysin toxin, previous investigators had speculated that a disordered loop in PA (2beta2--2beta3) would rearrange itself into the bilayer and form a 14-stranded beta-barrel (Song et al. , 1996), comprised of seven beta hairpin loops conferred by each protomer. Bensons experiments confirmed this hypothesis, and delineated the borders of the membrane-spanning beta-barrel. This thesis extends that work to explore the full extent of the stem of the channel.