Structural and biochemical studies of ATP-binding cassette (ABC) transporters suggest that an ATP-driven dimerization of the nucleotide-binding domains (NBDs) is an important reaction intermediate of the transport cycle. Moreover, an asymmetric occlusion of ATP at one of the two ATP sites of P-glycoprotein (Pgp) may follow the formation of the symmetric dimer. It has also been postulated that ADP drives the dissociation of the dimer. In this study, we show that the E.S conformation of Pgp (previously demonstrated in the E556Q/E1201Q mutant Pgp) can be obtained with the wild-type protein by use of the nonhydrolyzable ATP analogue ATP-gamma-S. ATP-gamma-S is occluded into the Pgp NBDs at 34 degrees C but not at 4 degrees C, whereas ATP is not occluded at either temperature. Using purified Pgp incorporated into proteoliposomes and ATP-gamma-35S, we demonstrate that the occlusion of ATP-gamma-35S has an Eact of 60 kJ/mol and the stoichiometry of ATP-gamma-35S:Pgp is 1:1 (mol/mol). Additionally, in the conserved Walker B mutant (E556Q/E1201Q) of Pgp, we find occlusion of the nucleoside triphosphate but not the nucleoside diphosphate. Furthermore, Pgp in the occluded nucleotide conformation has reduced affinity for transport substrates. These data provide evidence for the ATP-driven dimerization and ADP-driven dissociation of the NBDs, and although two ATP molecules may initiate dimerization, only one is driven to an occluded pre-hydrolysis intermediate state. Thus, in a full-length ABC transporter like Pgp, it is unlikely that there is complete association and disassociation of NBDs and the occluded nucleotide conformation at one of the NBDs provides the power-stroke at the transport-substrate site.