Abstract
The adenylate cyclase (CyaA) toxin, one of the virulence factors secreted by Bordetella pertussis, the pathogenic bacteria responsible for whooping cough, plays a critical role in the early stages of respiratory tract colonization by this bacterium. The CyaA toxin is able to invade eukaryotic cells by translocating its N-terminal catalytic domain directly across the plasma membrane of the target cells, where, activated by endogenous calmodulin, it produces supraphysiological levels of cAMP. How the catalytic domain is transferred from the hydrophilic extracellular medium into the hydrophobic environment of the membrane and then to the cell cytoplasm remains an unsolved question. In this report, we have characterized the membrane-interacting properties of the CyaA catalytic domain. We showed that a protein covering the catalytic domain (AC384, encompassing residues 1-384 of CyaA) displayed no membrane association propensity. However, a longer polypeptide (AC489), encompassing residues 1-489 of CyaA, exhibited the intrinsic property to bind to membranes and to induce lipid bilayer destabilization. We further showed that deletion of residues 375-485 within CyaA totally abrogated the toxin's ability to increase intracellular cAMP in target cells. These results indicate that, whereas the calmodulin dependent enzymatic domain is restricted to the amino-terminal residues 1-384 of CyaA, the membrane-interacting, translocation-competent domain extends up to residue 489. This thus suggests an important role of the region adjacent to the catalytic domain of CyaA in promoting its interaction with and its translocation across the plasma membrane of target cells.
Highlights
Translocation of the CyaA toxin across plasma membrane is still poorly understood
After solubilization in 20 mM Hepes, 8 M urea, pH 7.4, the proteins were purified to near homogeneity and readily refolded upon urea removal. Both purified AC proteins were able to interact with calmodulin with high affinity (KD(AC384) ϭ 0.06 nM and KD(AC489) ϭ 0.20 nM, as determined from concentrations giving half-maximal activation) and exhibited a high catalytic turnover (Ͼ2000 sϪ1) in the presence of the activator
AC384 and AC489 were further characterized in solution to check their solubility, stability, folding, and hydrodynamic properties using a combination of biophysical approaches as described previously [31, 35,36,37,38]
Summary
Translocation of the CyaA toxin across plasma membrane is still poorly understood. Results: The region 375– 485 is involved in membrane destabilization in vitro and required for cell intoxication. The CyaA toxin is able to invade eukaryotic cells by translocating its N-terminal catalytic domain directly across the plasma membrane of the target cells, where, activated by endogenous calmodulin, it produces supraphysiological levels of cAMP. We further showed that deletion of residues 375– 485 within CyaA totally abrogated the toxin’s ability to increase intracellular cAMP in target cells These results indicate that, whereas the calmodulin dependent enzymatic domain is restricted to the amino-terminal residues 1–384 of CyaA, the membrane-interacting, translocation-competent domain extends up to residue 489. This suggests an important role of the region adjacent to the catalytic domain of CyaA in promoting its interaction with and its translocation across the plasma membrane of target cells
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