Here's an excerpt from my senior honors thesis (with some editing):

The superfamily of ABC transporters (or traffic ATPases) includes prokaryotic and eukaryotic transport proteins that share a conserved nucleotide-binding domain.  Hence, the name ABC transporters which stands for ATP-binding cassette.  In addition, traffic ATPases share extensive sequence homology and a similar architectural organization.  The four structural elements of membrane-bound complex include two transmembrane domains and two hydrophilic nucleotide-binding domains.  In prokaryotic systems, the various domains usually exist as separate polypeptides, while in eukaryotic systems the various domains are usually fused into a single polypeptide.  The histidine permease of Salmonella typhimurium has been well characterized genetically and biochemically.  It provides a good model system for studying the mechanism of action of ABC transporters in general. 

Histidine permease is a bacterial periplasmic permease which consists of a periplamic substrate-binding receptor (HisJ) and a membrane-bound complex (HisQMP2) composed of two transmembrane subunits (HisQ and HisM) and two identical nucleotide-binding  proteins (two HisPs).  A single cycle of transport through the histidine permease transport system requires the interaction of HisJ with the HisQMP2 complex.  HisJ functions by binding free ligand in the periplasm and shuttling its ligand to HisQMP2 for translocation.  The interaction of HisJ with the transmembrane subunits HisQ and HisM sends a signal through the membrane-bound complex causing conformational changes which results in ATP hydrolysis by the HisP subunits and substrate translocation across the inner membrane.

The periplasmic receptor HisJ is a bilobate protein consisting of two globular domains as determined by x-ray crystallography.  Upon binding of a ligand, HisJ undergoes a conformational change from an open, unliganded form in which the two globular domains or lobes are spatially far apart to a closed, liganded form where the lobes are close to each other. Given that it is the closed, liganded form of HisJ that interacts with the HisQMP2 complex to stimulate ATP hydrolysis by the HisP subunits and substrate translocation, the interaction of HisJ to the HisQMP2 complex was studied using different ligands and mutants in either of the two lobes of HisJ.

One further approach used for studying the interaction of HisJ to the HisQMP2 complex involves the development of a two-plasmid system for in vivo cross-linking experiments.  The data collected suggests that (1) HisJ assumes different conformations upon biding of different ligands, (2) both lobes of HisJ carry an interaction site to the HisQMP2 complex, and (3) the physical interaction of HisJ to the HisQMP2 complex is a key factor in dictating the stimulation of ATP hydrolysis and substrate translocation.