FIGURE 3. ATP-dependent conformational changes in an NBD dimer. The ATP-free (left) and ATP-bound (right) of MalK the NBD of the maltose importer from E.coli are shown [pdb files 1Q1E and 1Q12, respectively (7)]. The ATP-free conformation of MalK remains dimeric because the additional regulatory domains (colored pink and cyan) attached to their respective NBDs (colored gold and blue) maintains an interface in this state. In the ATP-bound form, the -helical subdomain containing the ABC signature (green spheres) is rotated forward with respect to the core subdomain of the same NBD to form a composite ATP-binding site with the Walker A motif of the second NBD. The Q-loop, which links the two subdomains and also makes contact with the -phosphate of the ATP is though to be instrumental in controlling this conformational change, but the extent of the conformational change is questionable because the TMDs, which would contact the top surface of the NBDs and likely constrain their separation, are absent. Kenneth J. Linton. Structure and Function of ABC Transporters. Physiology 22: 122-130, 2007