A Phenylalanine Clamp Catalyzes Protein Translocation Through the Anthrax Toxin Pore.

Phenylalanine; Amino acids; Low-phenylalanine diet; Aspartame; Chlorophenylalanine; Dopa

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. [ABSTRACT FROM AUTHOR]

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