Direct measurement of recognition forces between proteins and membrane receptors

The interactions between the protein, cholera toxin B subunit attached to an atomic force microscope, AFM, cantilever, CTB and its receptor the ganglioside, GM1 have been measured in a dilute electrolyte solution, pH 5.5. Although there is variation in the force separation data obtained, particularly on approach of the AFM tip to the GM1 surface where usually, but not always an attraction is noted, an adhesion is always noted on separation of the surfaces. The strength of this adhesion varies from experiment to experiment, but appears to be quantised at a value of around 90 pN. Addition of cholera toxin to the aqueous electrolyte solution completely removes the attractive interaction and adhesion. This gives us confidence that in the earlier experiments, a specific interaction between the CTB and GM1 was measured.