Protein Unfolding During Anthrax Toxin Translocation

Harvard University (Medical School), Boston MA

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Abstract

DESCRIPTION (provided by applicant): Bacterial pathogens have evolved a general strategy of infecting host organisms by means of toxin protein complexes that pattern the AIB mechanism. The active, enzymatic, A moiety binds the receptor, B moiety; the receptor moiety then acts as a vehicle that delivers the A moiety to the cytoplasm from an isolated compartment separated by a membrane bilayer. The enzymatic effector, by means of its specific catalytic activity, then disables the normal physiology of the cell. The tripartite toxin of Bacillus anthracis relies on a "molecular syringe" B moiety, Protective Antigen (PA), which forms a heptad ring structure with a narrow central cavity at neutral extracellular pH. Two different A moiety effectors, Lethal Factor (LF) and Edema Factor (EF), bind PA heptamer. Immediately following endocytosis of membrane bound PA/LF/EF complexes, endosomes acidify, triggering the synchronized formation of a membrane piercing pore form of PA heptamer that presumably injects LF and EF into the cytosol. The narrow central cavity of PA heptamer is too small to accommodate fully native LF or EF, invoking the possibility that translocation is mitigated by protein unfolding. Indeed, stabilized fusions of LF validate this type of mechanism. Modern advances in a variety of fluorescence spectroscopy and imaging methods have enabled the detailed elucidation of protein folding pathways even at the single molecule level. A thorough structural, energetic, and kinetic understanding of the sequence of events leading up to and immediately following translocation using fluorescence methods will enable improved discourse toward the design and implementation of potential therapies for intoxication, including designed protein inhibitors and drugs.