Fig. 1.

Overview of the Hsp70–substrate interaction cycle. Kinetics of the Hsp70–substrate interaction cycle are driven by ATP binding and hydrolysis, followed by exchange of ADP for ATP. Hsp70s have two domains: a nucleotide binding domain (NBD) and a substrate binding domain (SBD). The SBD has two subdomains; one has the substrate binding cleft, the other the lid, which can cover the cleft, trapping substrate (right). When ATP is bound (left), Hsp70 is in what is called the open- or docked-state. Substrate has easy access to the substrate binding cleft in the SBD because both subdomains of the SBD are restrained by interaction with the NBD. Although this conformation allows a high on-rate of substrate binding, the off-rate is also rapid. Binding of the J-domain (J) of a J-protein co-chaperone at the NBD–SBD interface, in concert with substrate in the cleft, stimulates hydrolysis of ATP to ADP. The resulting conformational changes cause the domains to disengage, forming the undocked/closed state and stabilizing substrate interaction by closure of the lid over the cleft. The brackets indicate dynamic transitions between the predominant ATP and ADP conformations. Nucleotide release by nucleotide exchange factors (NEF) and rebinding of ATP completes the cycle