We have analyzed the dynamics of the chaperonin (GroEL)-cochaperonin (GroES
) interaction at the single-molecule level. In the presence of ATP and non-
native protein, binding of GroES to the immobilized GroEL occurred at a rat
e that is consistent with bulk kinetics measurements. However, the release
of GroES from GroEL occurred after a lag period (similar to3 s) that was no
t recognized in earlier bulk-phase studies. This observation suggests a new
kinetic intermediate in the GroEL-GroES reaction pathway.