Anthrax toxin protective antigen: low‐pH‐induced hydrophobicity and channel formation in liposomes

Abstract

To probe the role of the protective antigen (PA) component of anthrax toxin in toxin entry into animals cells, we examined the membrane channel-forming properties and hydrophobicity of intact and trypsin-cleaved forms of the protein at various pH values. At neutral pH neither form caused release of entrapped K⁺ from unilamellar lipid vesicles. At pH values below 6.0, however, K⁺ was rapidly released upon addition of either the nicked PA (PA_N) or the 63kDa tryptic fragment of PA (PA₆₃), which has been implicated in the toxin entry process. Under the same conditions intact PA exhibited only weak channel-forming activity, and PA₂₀, the complementary tryptic fragment, showed no such activity. Both PA and PA₆₃ exhibited enhanced hydrophobicity at acidic pH values, but the enhancement was greater and the pH threshold higher with PA₆₃. Our findings indicate that proteolytic removal of PA₂₀ from intact PA enables the residual protein, Pasb₆₃, to adopt a conformation at mildly acidic pH values that permits it to insert readily and form channels in membranes. Thus acidic conditions within endocytic vesicles may trigger membrane insertion of PA₆₃, which in turn promotes translocation of ligated effector moieties, edema factor or lethal factor, across the vesicle membrane into the cytosol.