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 AT P concentration, in the presence of fixed concentrations of GroES, wer e found to deviate from sigmoidal kinetics. Instead of the lag phase t ypical of sigmoidal curves, a Linear phase is observed at low ATP conc entrations. Consequently, a good fit of the data to the Hill equation could not be achieved. Such curves could be simulated using a linear c ombination of I-Fill equations, thus indicating that more than one all osteric transition is taking place in the ATP concentration range stud ied. The data were fitted to a fractional saturation equation for ATP binding to GroEL based on a partition function that includes both GroE S and ATP-liganded states of GroEL. Using this equation, it was possib le to estimate in a reliable manner the value of the allosteric consta nt, L-2', 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 L-2' is found to be 4 x 10(-5) whereas the value of the allosteric constant, L-2, for the transition of the second ring of Gro EL from the T to R state is 2 x 10(-9) [Yifrach, O., & Horovitz, A. (1 995) Biochemistry 34, 5303-5308]. Comparison of these values shows tha t 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 rel ease of protein substrates from trans ternary complexes of GroEL, GroE S, 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.