FIGURE 2. NBD closed dimer and roles for conserved motifs. A: the NBD closed dimer viewed from the membrane with two ATPs at the dimer interface. The ATP binding pockets are composite sites comprised of the core subdomain of one NBD (darker shade of gold and blue) and the -helical subdomain of the other NBD (paler shade of gold and blue). B: the gold NBD viewed from the blue NBD. The elementally colored ATP is cradled by the Walker A and B, the H-loop, stacking aromatic, and Q-loop of the core subdomain. The Q-loop extends from the top surface of the NBD, where it will be in close proximity to the ICLs of the TMDs, to the -helical subdomain, making it a prime candidate for energy and signal transduction within the complex. The -helical subdomain contains the ABC signature, which contacts the gray-colored ATP. The reciprocal arrangement of the fold of the blue NBD ensures that ATP is bound with high affinity. The D-loop hydrogen bonds with the blue NBD. The NBDs shown are from P-glycoprotein (54) modelled on the dimer arrangement from MJ0796 (52). Kenneth J. Linton. Structure and Function of ABC Transporters. Physiology 22: 122-130, 2007