We investigated the interaction between GroEL and a denatured protein from a mechanical point of view using an atomic force microscope. Pepsin was bound to an atomic force microscope probe and used at a neutral pH as an example of denatured proteins. To measure a specific and delicate interaction force, we obtained force curves without pressing the probe onto GroEL molecules spread on a mica surface. Approximately 40 pN of tensile force was observed for ∼10 nm while pepsin was pulled away from the chaperonin after a brief contact. This length of force duration corresponding to the circumference of GroEL's interior cavity was shortened by the addition of ATP. The relation between the observed mechanical parameters and the chaperonin's refolding function is discussed.
