HisJ is the periplasmic substrate-binding protein.  It happily floats around in the periplasmic space, that is that area between the outer membrane (cell wall) and the inner membrane of the bacteria.   Once HisJ encounters a substrate such as L-histidine, it binds its substrate extremely tightly with a dissociation constant or Kd of 10 nM and undergoes a conformational change from the open to the closed form.  HisJ then shuttles its cargo to the HisQMP2 complex.

The HisQMP2 complex is composed of two transmembrane subunits, HisQ and HisM.  The ATP hydrolyzing portion of the complex in composed of two HisP subunits.  It is believed that HisJ interacts primarily with the HisQ subunit to stimulate the complex to hydrolyze ATP and translocate the substrate across the inner membrane.