Editing ABC transporter (section)

=== Mechanism of transport for importers ===
[[Image:Abc importer.jpg|thumb|Proposed mechanism of transport for ABC importers. This alternating-access model was based on the crystal structures of ModBC-A<ref name=modb/> and HI1470/1.<ref name=hi1471/>]]

The mechanism of transport for importers supports the alternating-access model. The resting state of importers is inward-facing, where the nucleotide binding domain (NBD) dimer interface is held open by the TMDs and facing outward but occluded from the cytoplasm. Upon docking of the closed, substrate-loaded binding protein towards the periplasmic side of the transmembrane domains, ATP binds and the NBD dimer closes. This switches the resting state of transporter into an outward-facing conformation, in which the TMDs have reoriented to receive substrate from the binding protein. After hydrolysis of ATP, the NBD dimer opens and substrate is released into the cytoplasm. Release of ADP and P<sub>i</sub> reverts the transporter into its resting state. The only inconsistency of this mechanism to the ATP-switch model is that the conformation in its resting, nucleotide-free state is different from the expected outward-facing conformation. Although that is the case, the key point is that the NBD does not dimerize unless ATP and binding protein is bound to the transporter.<ref name=davidson/><ref name=davidsonchen/><ref name=rees/><ref name=higgins/><ref name=oldham2008/>