Measuring the forces that control protein interactions.

Although the force fields and interaction energies that control protein behavior can be inferred indirectly from equilibrium and kinetic measurements, recent developments have made it possible to quantify directly (a) the ranges, magnitudes, and time dependence of the interaction energies and forces between biological materials; (b) the mechanical properties of isolated proteins; and (c) the strength of single receptor-ligand bonds. This review describes recent results obtained by using the atomic force microscope, optical tweezers, the surface force apparatus, and micropipette aspiration to quantify short-range protein-ligand interactions and the long-range, nonspecific forces that together control protein behavior. The examples presented illustrate the power of force measurements to quantify directly the force fields and energies that control protein behavior.