Chaperones, Molecular
503
ATP
ADP
ATP
ATP
ADP
J
J
J
E
E
E
K
K
K
K
1
4
3
2
Fig. 4
The proposed DnaK/DnaJ/GrpE reaction cycle. Substrate protein is initially bound to
DnaJ (shown as J), which delivers it to DnaK-ATP (step 1). The substrate is transferred to
DnaK, and the presence of DnaJ stimulates the ATPase of DnaK to form a stable
DnaK-ADP-substrate complex; DnaJ is probably lost at this point (step 2). This complex is
stable until the cochaperone GrpE catalyzes the exchange of ADP for ATP, shown here as a
two-step reaction where GrpE initially displacesboundADP(step3)andisinturndisplacedby
ATP (step 4). This causes DnaK to release the bound peptide, and the cycle can begin again. It
is important to note that several molecules of DnaJ or DnaK may bind one protein. The
reaction cycle is best characterized for the
E. coli
proteins shown; in eukaryotes, many different
cofactors are involved with cycles involving Hsp70 homologs.
coli
proteins DnaK and DnaJ, is thought
to begin with the binding of unfolded
protein to the Hsp40 partner, from where
it is transferred to Hsp70, which is
initially in a low-afFnity binding state
(with ATP bound). The formation of the
Hsp70-Hsp40-unfolded protein complex
stimulates the Hsp70 ATPase, and the
ATP is hydrolyzed to ADP, which in
turn has the effect of increasing the
afFnity of Hsp70 for the unfolded protein,
but weakening it for Hsp40, which then
dissociates. The resultant Hsp70-ADP-
unfolded protein complex is now quite
stable. In order for the unfolded protein
to be discharged from its bound state, the
ADP must be exchanged for ATP to lower
the Hsp70 afFnity for the unfolded protein.
This exchange is a slow reaction but it is
speeded up considerably by a cochaperone,
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