An anthrax bacterium secretes three nontoxic proteins that assemble into a toxic complex on the surface of the host cell to set off a chain of events leading to cell toxicity and death. Protective antigen (PA) is one of these proteins, and after binding to the cell, seven copies of it assemble into a specific complex that is capable of forming a pore in a cellular membrane. The pore permits the other two proteins to enter the cell interior, where the factors interfere with metabolic processes and can kill the infected individual.

The scientists demonstrated this role by investigating the channel’s chemical make-up. Using cysteine-scanning mutagenesis, they identified the hydrophobic (“greasy”) amino acid phenylalanine in protective antigen’s pore-forming domain. Seven of these amino acids project into the lumen of the pore and form a collection of greasy residues, nicknamed “the phi- clamp” by the scientists. Because the water-filled lumen of the membrane pore is smaller than the folded lethal factor and edema factor, these proteins must first unfold before being actively translocated through the heptameric channel. The researchers demonstrated that the phi-clamp was critical to infection by mutating the region and blocking translocation of the toxin proteins.

R. John Collier, professor of microbiology and molecular genetics at HMS, and his colleagues found that the phi-clamp composes the main conductance-blocking site for hydrophobic drugs.