Conformational changes induced by ATP-hydrolysis in an ABC transporter: a molecular dynamics study of the Sav1866 exporter

ATP-Binding Cassette (ABC) transporters are ubiquitous membrane proteins that use energy from ATP binding or/and hydrolysis to actively transport allocrites across membranes. In this study, we identify ATP-hydrolysis induced conformational changes in a complete ABC exporter (Sav1866) from Staphylococcus aureaus, using molecular dynamics (MD) simulations. By performing MD simulations on the ATP and ADP+IP bound states, we identify the conformational consequences of hydrolysis, showing that the major rearrangements are not restricted to the NBDs, but extend to the transmembrane domains (TMDs) external regions. For the first time, to our knowledge, we see, within the context of a complete transporter, NBD dimer opening in the ADP+IP state in contrast with all ATP-bound states. This opening results from the dissociation of the ABC signature motif from the nucleotide. In addition, in both states, we observe the opening of a gate entrance in the intracellular loop region leading to the exposure of the TMDs internal cavity to the cytoplasm. To see if this opening was large enough to allow allocrite transport, the adiabatic energy profile for doxorubicin passage was determined. For both states, this profile, although an approximation, is overall downhill from the cytoplasmatic to the extracellular side, and the local energy barriers along the TMDs are relatively small, evidencing the exporter nature of Sav1866. The major difference between states is an energy barrier located in the cytoplasmic gate region, which becomes reduced upon hydrolysis, suggesting that allocrite passage is facilitated, and evidencing a possible molecular mechanism for the active transport in these proteins.