Adenylate cyclase toxin (ACT) is secreted by Bordetella pertussis, the bacterium causing whooping cough. ACT
is a member of the RTX (repeats in toxin) family of toxins, and like other members in the family, it may bind
cell membranes and cause disruption of the permeability barrier, leading to efflux of cell contents. The present
paper summarizes studies performed on cell and model membranes with the aim of understanding the
mechanism of toxin insertion and membrane restructuring leading to release of contents. ACT does not
necessarily require a protein receptor to bind the membrane bilayer, and this may explain its broad range of
host cell types. In fact, red blood cells and liposomes (large unilamellar vesicles) display similar sensitivities
to ACT. A varying liposomal bilayer composition leads to significant changes in ACT-induced membrane lysis,
measured as efflux of fluorescent vesicle contents. Phosphatidylethanolamine (PE), a lipid that favors formation
of nonlamellar (inverted hexagonal) phases, stimulated ACT-promoted efflux. Conversely, lysophosphatidylcholine,
a micelle-forming lipid that opposes the formation of inverted nonlamellar phases, inhibited
ACT-induced efflux in a dose-dependent manner and neutralized the stimulatory effect of PE. These results
strongly suggest that ACT-induced efflux is mediated by transient inverted nonlamellar lipid structures.
Cholesterol, a lipid that favors inverted nonlamellar phase formation and also increases the static order of
phospholipid hydrocarbon chains, among other effects, also enhanced ACT-induced liposomal efflux. Moreover,
the use of a recently developed fluorescence assay technique allowed the detection of trans-bilayer
(flip-flop) lipid motion simultaneous with efflux. Lipid flip-flop further confirms the formation of transient
nonlamellar lipid structures as a result of ACT insertion in bilayers.
This work was supported in part by European Union contract
QLK2-1999-0.0556 and by grant A502907 from the Grant Agency of
the Czech Academy of Sciences.
Peer reviewed