Nucleotide-induced transition of GroEL from the high-affinity to the low-affinity state for a target protein: effects of ATP and ADP on the GroEL-affected refolding of alpha-lactalbumin

We studied the refolding kinetics of alpha-lactalbumin in the presence of wild-type GroEL and its ATPase-deficient mutant D398A at various concentrations of nucleotides (ATP and ADP). We evaluated the apparent binding constant between GroEL and the alpha-lactalbumin refolding intermediate quantitatively by numerical simulation analysis of the alpha-lactalbumin refolding curves in the presence and absence of GroEL. The binding constant showed a co-operative decrease with an increase in ATP concentration, whereas the binding constant decreased in a non-co-operative manner with respect to ADP concentration. For the D398A mutant, the ATP-induced decrease in affinity occurred much faster than the steady-state ATP hydrolysis by this mutant, suggesting that ATP binding to GroEL rather than ATP hydrolysis, was responsible for the co-operative decrease in the affinity for the target protein. We thus analyzed the nucleotide-concentration dependence of affinity of GroEL for the target protein using an allosteric Monod-Wyman-Changeux model in which GroEL underwent an ATP-induced co-operative conformational transition between the high-affinity and low-affinity states of the target protein. The transition midpoint of the ATP-induced transition of GroEL has been found to be around 30 microM, in good agreement with the midpoint evaluated in other structural studies of GroEL. The results show that the observed difference between ATP and ADP-induced transitions of GroEL are brought about by a small difference in an allosteric parameter (the ratio of the nucleotide affinities of GroEL in the high-affinity and the low-affinity states), i.e. 4.1 for ATP and 2.6 for ADP.