GroES promotes the T to R transition of the GroEL ring distal to GroES in the GroEL-GroES complex.

Curves of initial rates of ATP hydrolysis by GroEL as a function of ATP concentration, in the presence of fixed concentrations of GroES, were found to deviate from sigmoidal kinetics. Instead of the lag phase typical of sigmoidal curves, a linear phase is observed at low ATP concentrations. Consequently, a good fit of the data to the Hill equation could not be achieved. Such curves could be simulated using a linear combination of Hill equations, thus indicating that more than one allosteric transition is taking place in the ATP concentration range studied. The data were fitted to a fractional saturation equation for ATP binding to GroEL based on a partition function that includes both GroES and ATP-liganded states of GroEL. Using this equation, it was possible to estimate in a reliable manner the value of the allosteric constant, L2', for the transition of the ring distal to GroES in the GroEL-GroES complex from the low (T)- to the high (R)-affinity state for ATP. The value of L2' is found to be 4 x 10(-5) whereas the value of the allosteric constant, L2, for the transition of the second ring of GroEL from the T to R state is 2 x 10(-9) [Yifrach, O., & Horovitz, A. (1995) Biochemistry 34, 5303-5308]. Comparison of these values shows that GroES promotes the T to R transition of the ring distal to GroES in the GroEL-GroES complex. Owing to the relatively low affinity of the R conformation for nonfolded proteins, this transition will lead to release of protein substrates from trans ternary complexes of GroEL, GroES, and protein substrate. The role of this release mechanism may be to assist the folding of relatively large proteins that cannot form cis ternary complexes and/or to facilitate degradation of damaged proteins which cannot fold.