Allosteric control by ATP of non-folded protein binding to GroEL

Co-operativity in ATP hydrolysis by GroEL can be described by a model in which each ring of GroEL is in equilibrium between a low (T) and high (R) affinity state for ATP. According to this model, the GroEL double-ring is in equilibrium between three states: TT, TR and RR. In order to find out which states bind non-folded proteins, we measured the co-operativity in ATP hydrolysis by GroEL in the absence and presence of non-folded alpha-lactalbumin, under equilibrium conditions between GroEL and the non-folded protein. The non-folded protein is found to bind preferentially the T state of GroEL rings and to stimulate the ATPase activity of GroEL by (1) a direct effect on GroEL rings in the T state and (2) a shift in the equilibrium from the RR state toward the more active TR state. The coupling between co-operativity in ATP hydrolysis by GroEL and protein substrate binding and release by this molecular chaperone is shown.