Toward a mechanism for GroEL center dot GroES chaperone activity: An ATPase-gated and -pulsed folding and annealing cage

Free GroEL binds denatured proteins very tightly: it retards the folding of barnase 400-fold and catalyzes unfolding fluctuations in native barnase and its folding intermediate. GroEL undergoes an allosteric transition from its tight-binding T-state to a weaker binding R-state on the cooperative binding of nucleotides (ATP/ADP) and GroES. The preformed GroEL . GroES . nucleotide complex retards the folding of barnase by only a factor of 4, and the folding rate is much higher than the ATPase activity that releases GroES from the complex. Binding of GroES and nucleotides to a preformed GroEL denatured-barnase complex forms an intermediately fast-folding complex. We propose the following mechanism for the molecular chaperone. Denatured proteins bind to the resting GroEL GroES nucleotide complex. Fast-folding proteins are ejected as native structures before ATP hydrolysis. Slow-folding proteins enter chaperoning cycles of annealing and folding after the initial ATP hydrolysis. This step causes transient release of GroES and formation of the GroEL denatured-protein complexes with higher annealing potential. The intermediately fast-folding complex is formed on subsequent rebinding of GroES. The ATPase activity of GroEL GroES is thus the gatekeeper that selects for initial entry of slow-folding proteins to the chaperone action and then pumps successive transitions from the faster-folding R-states to tie tighter-binding/stronger annealing T-states. The molecular chaperone acts as a combination of folding cage and an annealing machine.