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A Phenylalanine Clamp Catalyzes Protein Translocation Through the Anthrax Toxin Pore

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Science  29 Jul 2005:
Vol. 309, Issue 5735, pp. 777-781
DOI: 10.1126/science.1113380

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Abstract

The protective antigen component of anthrax toxin forms a homoheptameric pore in the endosomal membrane, creating a narrow passageway for the enzymatic components of the toxin to enter the cytosol. We found that, during conversion of the heptameric precursor to the pore, the seven phenylalanine-427 residues converged within the lumen, generating a radially symmetric heptad of solvent-exposed aromatic rings. This “φ-clamp” structure was required for protein translocation and comprised the major conductance-blocking site for hydrophobic drugs and model cations. We conclude that the φ clamp serves a chaperone-like function, interacting with hydrophobic sequences presented by the protein substrate as it unfolds during translocation.

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