We have analysed the conformational states of barnase that are bound by the molecular chaperones GroEL and SecB. Line broadening in the NMR spectra of barnase in the presence of chaperone indicates binding of the native state of barnase to both GroEL and SecB, with a dissociation constant of > 3 x 10(-4) M for the GroEL-native barnase complex. GroEL and SecB catalyse the hydrogen-deuterium exchange of amide proteins of barnase that require global unfolding for exchange to occur, indicating that both chaperones bind to a fully unfolded state of barnase. Binding of the denatured state was also detected by a reversible lowering of the melting temperature of barnase in the presence of chaperone. The dissociation constant of the complex between denatured barnase and either chaperone is 5 x 10(-8) M. The chaperone-bound fully unfolded state is a minor conformation that would not be seen by direct observation under physiological conditions, as the folding intermediate of barnase is the most populated state in the complex. The rate-limiting step for exchange of buried amide protons of bound barnase is the unfolding of the folding intermediate, which is retarded > 2000-fold in the complex with GroEL. The reverse refolding step is retarded > 1000-fold by GroEL leading to an EX1 mechanism for exchange. In contrast, unfolding of native barnase is catalysed by > 1000-fold. Thus, molecular chaperones GroEL and SecB have the potential to act in vivo and in vitro as: (1) a folding/transport-scaffold to prevent aggregation of partially folded states by binding; (2) as an annealing-machine to generate continuous unfolding of misfolded states until a low-affinity state is formed; and (3) as an unfoldase to catalyse unfolding of the misfolded states.