GrpE accelerates nucleotide exchange of the molecular chaperone DnaK with an associative displacement mechanism.

The ATP hydrolysis and protein-binding and release cycle of the molecular chaperone DnaK is regulated by the accessory proteins GrpE and DnaJ. Here we describe a study of the formation of complexes between the molecular chaperone DnaK, its nucleotide exchange factor GrpE, and the fluorescent ADP analog N8-[4-[(N'-methylanthraniloyl)amino]butyl]-8-aminoadenosine 5'-diphosphate (MABA-ADP) by equilibrium and stopped flow kinetic experiments. The catalytic cycle of the GrpE-stimulated nucleotide exchange involves a ternary DnaK x GrpE x ADP complex as well as the binary DnaK x GrpE and DnaK x ADP complexes. The equilibrium data of the interaction of GrpE with DnaK x ADP and the nucleotide-free DnaK can be described by a simple equilibrium system where GrpE reduces the affinity of ADP for DnaK 200-fold. However, transient kinetic studies revealed that the functional cycle of GrpE in addition includes at least two distinct ternary DnaK x GrpE x ADP complexes. Our data indicate that the initial weak binding of GrpE to DnaK x ADP is followed by an isomerization of the ternary complex which leads to weakening of nucleotide binding and finally to its rapid dissociation. The maximal stimulation for nucleotide exchange brought about by GrpE was found to be 5000-fold. We propose that this kinetically observed isomerization represents a structural change (opening) of the nucleotide binding pocket of DnaK that allows for fast nucleotide exchange.

The second step of ATP binding to DnaK induces peptide release.

The interaction of the nucleotide-free molecular chaperone DnaK (Hsp70) from Escherichia coli with nucleotides was studied under equilibrium and transient kinetic conditions. These studies used the intrinsic fluorescence signal of the single tryptophan residue (Trp102) of DnaK, or of novel fluorescent nucleotide analogs of ADP and ATP, N8-(4-N'-methylanthraniloylaminobutyl)-8-aminoadenosine 5'-di- or triphosphate (MABA-ADP and MABA-ATP) as spectroscopic probes. Titration of MABA-ADP with DnaK resulted in a 2.3-fold increase of the fluorescence signal, from which a binding stoichiometry of 1:1, and a dissociation constant (Kd) of 0.09 microM were derived. The intrinsic rate constant of hydrolysis of ATP or MABA-ATP in single turnover experiments was found to be 1.5 x 10(-3) s-1 and 1.6 x 10(-3) s-1, identical with the catalytic rate constant of 1.5(+/- 0.17) x 10(-3) s-1 obtained under steady-state conditions. The dissociation rate constant of ADP was measured to be 35(+/- 7) x 10(-3) s-1 in the absence or 15(+/- 5) x 10(-3) in the presence of 2 mM inorganic phosphate (Pi) and is therefore 10 to 20 times faster than the rate of hydrolysis. These results demonstrated that processes governing ATP hydrolysis are rate-limiting in the DnaK ATPase reaction cycle. The three observed different fluorescent states of the single tryptophan residue were investigated. The binding of ATP gave a decrease of 15% in fluorescence intensity compared with the nucleotide-free state. Subsequent ATP hydrolysis, or the simultaneous addition of ADP and Pi, increased the fluorescence 7% above the fluorescence intensity of the nucleotide-free protein. Changes in the tryptophan fluorescence could not be detected when ADP, Pi or the non-hydrolyzable nucleotide analogs AMPPNP (Kd = 1.62(+/- 0.1) microM) or ATP gamma S (Kd = 0.044(+/- 0.003) microM) were added. These data suggested that DnaK exists in at least three different conformational states, depending on nucleotide site occupancy. The fluorescence increase of DnaK upon ATP binding was resolved into two steps; a rapid first step (Kd 1 = 7.3 microM) is followed by a second slow step (k+2 = 1.5 s-1 and k-2 < or = 1.5 x 10(-3) s-1) that causes the decrease in the tryptophan fluorescence signal. The addition of ATP also resulted in the release of DnaK-bound peptide substrate with koff = 3.8 s-1, comparable with the rate of the second step of nucleotide binding. AMPPNP or ATP gamma S were not able to change the fluorescence signal nor to release the peptide. We therefore conclude that the second step of ATP binding, and not the 1000-fold slower ATP hydrolysis is coupled to peptide release.

Nucleotide binding to the heat-shock protein DnaK as studied by ESR spectroscopy.

We employed ESR spectroscopy using spin-labeled adenine nucleotides to investigate nucleotide binding to the 70-kDa heat shock protein, DnaK, from Escherichia coli. Binding stoichiometries of 1 mol/ mol for both ATP and ADP to previously nucleotide-depleted protein in the presence of Mg2+ were determined directly and under equilibrium binding conditions. Of the spin-labeled adenine nucleotides available to us, only the derivatives with the spin label attached to the C8 position of the adenine moiety, 8-SL-AdoP3 and 8-SL-AdoP2 [8-(2,2,6,6-tetramethyl-piperidin-4-yl -1-oxyl-)amino-adenosine-5'-triphosphate or diphosphate], were bound sufficiently tightly by the heat-shock protein, resulting in ESR spectra typical for immobilized radicals. In the absence of Mg2+, only approximately 0.5 mol were bound. Subsequent addition of Mg2+, however, led to the previously observed maximum binding of 1 mol/mol. Both 8-SL-AdoP3 and 8-SL-AdoP2 were fully exchangeable upon addition of excess ATP or ADP suggesting that the analogs bound directly to the nucleotide binding sites within the protein. 8-SL-AdoP2 release was also observed in the presence of the co-chaperone GrpE, indicating that the spin-labeled analogs of adenine nucleotides function like the natural nucleotide-substrates of the heat-shock protein. Small differences in the ESR spectra of 8-SL-AdoP3 and 8-SL-AdoP2 in complex with DnaK were observed.

Nucleotide-induced conformational changes in the ATPase and substrate binding domains of the DnaK chaperone provide evidence for interdomain communication.

Interactions of the DnaK (Hsp70) chaperone from Escherichia coli with substrates are controlled by ATP. Nucleotide-induced changes in DnaK conformation were investigated by monitoring changes in tryptic digestion pattern and tryptophan fluorescence. Using nucleotide-free DnaK preparations, not only the known ATP-induced major changes in kinetics and pattern of proteolysis but also minor ADP-induced changes were detected. Similar ATP-induced conformational changes occurred in the DnaK-T199A mutant protein defective in ATPase activity, demonstrating that they result from binding, not hydrolysis, of ATP. N-terminal sequencing and immunological mapping of tryptic fragments of DnaK identified cleavage sites that, upon ATP addition, appeared within the proposed C-terminal substrate binding region and disappeared in the N-terminal ATPase domain. They hence reflect structural alterations in DnaK correlated to substrate release and indicate ATP-dependent domain interactions. Domain interactions are a prerequisite for efficient tryptic degradation as fragments of DnaK comprising the ATPase and C-terminal domains were highly protease-resistant. Fluorescence analysis of the N-terminally located single tryptophan residue of DnaK revealed that the known ATP-induced alteration of the emission spectrum, proposed to result directly from conformational changes in the ATPase domain, requires the presence of the C-terminal domain and therefore mainly results from altered domain interaction. Analyses of the C-terminally truncated DnaK163 mutant protein revealed that nucleotide-dependent interdomain communication requires a 15-kDa segment assumed to constitute the substrate binding site.