Mutational Analysis of the Energetics of the GrpEaDnaK Binding Interface: Equilibrium Association Constants by Sedimentation Velocity Analytical Ultracentrifugation

Abstract

DnaK, the prokaryotic Hsp70 molecular chaperone, requires the nucleotide exchange factor and heat shock protein GrpE to release ADP. GrpE and DnaK are tightly associated molecules with an extensive protein–protein interface, and in the absence of ADP, the dissociation constant for GrpE and DnaK is in the low nanomolar range. GrpE reduces the affinity of DnaK for ADP, and the reciprocal linkage is also true: ADP reduces the affinity of DnaK for GrpE. The energetic contributions of GrpE side-chains to GrpE–DnaK binding were probed by alanine-scanning mutagenesis . Sedimentation velocity (SV) analytical ultracentrifugation (AUC) was used to measure the equilibrium constants ( K eq ) for GrpE binding to the ATPase domain of DnaK in the presence of ADP. ADP-bound DnaK is the natural target of GrpE, and the addition of ADP (final concentration of 5 μM) to the preformed GrpE–DnaK ATPase complexes allowed the equilibrium association constants to be brought into an experimentally accessible range. Under these experimental conditions, the substitution of one single GrpE amino acid residue, arginine 183 with alanine , resulted in a GrpE–DnaK ATPase complex that was weakly associated ( K eq =9.4×10 4 M). This residue has been previously shown to be part of a thermodynamic linkage between two structural domains of GrpE: the thermosensing long helices and the C-terminal β-domains. Several other GrpE side-chains were found to have a significant change in the free energy of binding (ΔΔ G ∼1.5 to 1.7 kcal mol −1 ), compared to wild-type GrpE·DnaK ATPase in the same experimental conditions. Overall, the strong interactions between GrpE and DnaK appear to be dominated by electrostatics, not unlike barnase and barstar, another well-characterized protein–protein interaction. GrpE, an inherent thermosensor, exhibits non-Arrhenius behavior with respect to its nucleotide exchange function at bacterial heat shock temperatures, and mutation of several solvent-exposed side-chains located along the thermosensing indicated that these residues are indeed important for GrpE–DnaK interactions.