Distinct steps in the penetration of adenylate cyclase toxin of Bordetella pertussis into sheep erythrocytes: Translocation of the toxin across the membrane

Adenylate cyclase (AC) toxin from Bordetella pertussis penetrates eukaryotic cells and upon activation by calmodulin generates unregulated levels of intracellular cAMP. The process of toxin penetration into sheep erythrocytes was resolved into three consecutive steps including insertion, translocation, and intracellular cleavage. Insertion of the toxin into the cell membrane occurred over a wide temperature range (4-36°C). In contrast, translocation of the toxin, i.e. transfer of the NH₂-terminal catalytically active fragment across the membrane, occurred only above 20°C and was highly temperature-dependent. While a single exposure of the toxin to Ca²⁺ was sufficient for its insertion into the plasma membrane, toxin translocation required exogenous Ca²⁺ at mM concentrations. Translocation was not affected by pretreatment of cells with trypsin, N-ethylmaleimide, and sodium carbonate at alkaline pH. The NH₂-terminal fragment of the toxin was cleaved in the cell releasing the 45-kDa active AC into the cytosol. The cleavage was blocked by treatment of cells with N-ethylmaleimide. It is hypothesized that the COOH-terminal portion of the toxin creates in the membrane a channel through which the NH₂-terminal fragment is translocated.