Quartz crystal microbalance investigation of the interaction of bacterial toxins with ganglioside containing solid supported membranes

The binding of cholera toxin, tetanus toxin and pertussis toxin to ganglioside containing solid supported membranes has been investigated by quartz crystal microbalance measurements. The bilayers were prepared by fusion of phospholipid-vesicles on a hydrophobic monolayer of octanethiol chemisorbed on one gold electrode placed on the 5 MHz AT-cut quartz crystal. The ability of the gangliosides G(M1), G(M3), G(D1a), G(D1b), G(T1b) and asialo-G(M1) to act as suitable receptors for the different toxins was tested by measuring the changes of quartz resonance frequencies. To obtain the binding constants of each ligand-receptor-couple Langmuir-isotherms were successfully fitted to the experimental adsorption isotherms. Cholera toxin shows a high affinity for G(M1) (K-a=1.8 . 10(8)M(-1)), a lower one for asialo-G(M1) (K-a=1.0.10(7) M-1) and no affinity for G(M3). The C-fragment of tetanus toxin binds to ganglioside G(D1a), G(D1b) and G(T1b) containing membranes with similar affinity (K-a similar to 10(6) M-1), while no binding was observed with G(M3). Pertussis toxin binds to membranes containing the ganglioside G(D1a) with a binding constant of K-a = 1.6.10(6) M-1, but only if large amounts (40 mol%) of G(D1a) are present. The maximum frequency shift caused by the protein adsorption depends strongly on the molecular structure of the receptor. This is clearly demonstrated by an observed maximum frequency decrease of 99 Hz for the adsorption of the C-fragment of tetanus toxin to G(D1b) In contrast to this large frequency decrease, which was unexpectedly high with respect to Sauerbrey's equation, implying pure mass loading, a maximum shift of only 28 Hz was detected after adsorption of the C-fragment of tetanus toxin to G(D1a).