Studies on the protein-receptor interaction by atomic force microscopy

The specific interaction between a model biomolecular pair, cholera toxin B oligomer (CTB) versus its receptor ganglioside GM1, was investigated using an atomic force microscope. Force versus separation curves in approach/retraction cycles between a GM1-coated tip and CTB were collected. On the basis of a statistically significant number of force curves measured at different points on the sample surface, attractive forces during retraction were frequently observed. As confirmed by the results of control experiments, the attractive force was correlated to the rupture force of the CTB-GM1 specific bond, a quantitative measure of the specific interaction. Using a GM1-coated tip, force maps were generated on the CTB-containing surface to illustrate the CTB distribution. By varying the approach/retraction rate at 0.5, 1.0, and 3.0 Hz, the mean adhesive forces of 0.49, 0.17, and 0.03 nN were measured. It allows the quantification of the characteristic interaction time needed for strong CTB-GM1 binding in the range of 7-20 ms when CTB and GM1 were separated at a distance shorter than 10 nm. A shorter interaction time dramatically decreased the strength of the binding. We also observed a strong influence of the ionic strength on the binding time. The specific bond formation was significantly delayed in the presence of electrolyte in the media. This is ascribed to the influence of the ionic strength on both long-range and short-range interactions.