A monomeric variant of GroEL binds nucleotides but is inactive as a molecular chaperone

Abstract

The heat shock protein GroEL from Escherichia coli is a tetradecameric oligomer that facilitates the refolding of nonnative polypeptides in an ATP- hydrolysis dependent reaction. A mutant in GroEL was prepared in which lysine 3 was substituted with glutamate, which destabilizes the oligomeric structure of GroEL (Horovitz, A., Bochkareva, E.S., and Girshovich, A.S. (1993) J. Biol. Chem. 268, 9957-9959). The highly expressed and purified GroEL(K3E) was judged to be monomeric by sedimentation equilibrium, yielding a molecular weight of 54,500, despite a weak tendency of the mutant to reversibly form higher order aggregates above 4 mg ml-1. The monomeric variant appears to be folded based on the far UV circular dichroism spectrum, which shows significant α-helical content, but with slight differences in conformation relative to wild-type GroEL. The increase in exposed hydrophobic surface of the monomer was probed with the dye 4,4'-bis-1-anilino-3- naphthalenesulfonate (bis-ANS). The fluorescence of bis-ANS increases approximately 150-fold in the presence of the mutant, and about 4 mol of bis- ANS bind per mol of monomer, with a binding constant of 1.6 μM. Adenosine nucleotide binding to monomeric GroEL(K3E) resulted in considerable quenching of bis-ANS fluorescence, correlating with significant structural changes as seen in the far UV circular dichroism, and permitted the measurement of binding isotherms for ATP and ADP. Hyperbolic ATP binding isotherms yield a dissociation constant of 82 μM, about 4-fold weaker than the K0.5 for ATP seen in steady-state kinetics assays of the wild-type GroEL ATPase. A similar difference was seen for ADP binding. These results suggest that the mutation disrupts the native tetradecameric quaternary structure through conformational changes that may also weaken nucleotide binding. The monomeric mutant exhibited no chaperone activity as evidenced by a failure to inhibit or facilitate the refolding of chemically denatured enolase, an inability to refold denatured rhodanese above spontaneous levels, and a lack of binding to α-casein, a competitor in many chaperonin-promoted refolding reactions. Thus, the formation of assembly incompetent monomers by the lysine 3 to glutamate mutation results in a dramatic decrease in the affinity for nonnative polypeptide chains and suggests that the oligomeric nature of GroEL is crucial for its molecular chaperone function.