DIRECT FORCE MEASUREMENTS OF SPECIFIC AND NONSPECIFIC PROTEIN INTERACTIONS

Streptavidin-biotin (receptor-ligand) interaction forces were measured directly as a function of their intermolecular separation in various salt solutions and at various temperatures with a surface forces apparatus. Electrostatic and van der Waals forces were found to dominate the long-range streptavidin-biotin interaction at >20 Angstrom. At intermediate separations, down to similar to 10 Angstrom, the interaction is governed by repulsive steric and attractive van der Waals and hydrophobic forces. A much stronger short-range attraction giving rise to the strong, specific adhesive binding was measured at molecular separations of less than 5 Angstrom. A decrease in the pH from 7.2 to 6.0 resulted in complete charge reversal on the binding surface of streptavidin (pK similar to 6) from net negative to net positive, while leaving the negatively charged biotin surface (pK similar to 3.0) unchanged, and the long-range interaction switched from repulsive to attractive. This observed behavior can be attributed to the titration of two histidines on the biotin binding surface of streptavidin. These results reveal a strong sensitivity of the long-range interaction forces to the detailed amino acid composition of the biotin binding surface. They also demonstrate the powerful regulatory potential conferred by small changes in local surface ionic conditions on protein interaction forces over different distance regimes. The effects of temperature on receptor-ligand dynamics and on the strength of intermembrane adhesion forces were studied by measuring the long-range force profiles and short-range adhesion forces above and below the chain melting temperature (T-c similar to 30 degrees C) of the lipids in the supporting bilayers. Increased bilayer fluidity due to a temperature increase to 33 degrees C (T > T-c) increased short-range adhesion by 7-fold relative to bilayers in the gel state at 25 degrees C (T < T-c). This effect was attributed to the enhanced rates of lateral diffusion and molecular rearrangements on the more fluid bilayer surfaces, which resulted in greater and more rapid intermembrane bond formation. A change in the rates of molecular rearrangements was also found to affect the repulsive part of the interaction potential at intermediate separations (10-20 Angstrom) via modulation of the steric repulsion between streptavidin and the highly flexible, polymer-like biotin molecules. This is expected to have a large effect on the association rates of receptor-ligand binding, even if it does not change the equilibrium binding energy. These results indicate that the lateral mobility of receptors and the flexibility of protruding ligand groups are also important determinants of the effective strength of specific intermembrane adhesion.