FIGURE 4. Domain organization of full-size ABC transporters. A: cartoon representations of the three full-size structures for ABC transporters; Sav1866 is crystallized with ADP (pdb 2HYD); BtuCD with tetravanadate (pdb 1L7V); and HI1470/1 in the absence of nucleotide (pdb 2NQ2). The TMDs are shown in dark blue and gray for the homologous BtuCD and HI1470/1, and red and teal for the non-homologous Sav1866. The NBDs, which are homologous throughout, are shown in gold and slate blue. Sav1866 is a “”half transporter”” and functions as a homodimer of a two-domain polypeptide (the red TMD is fused to the gold NBD). In BtuCD and HI1470/1, each domain is on a different polypeptide and the NBDs and TMDs are homodimers. The Walker A motif of the gold NBD and the ABC signature motif of the blue NBD are shown as spheres and colored red and green, respectively. These should be able to come together to coordinate nucleotide between them. B: ribbon model of Sav1866 rotated 90 degrees to the left with respect to that shown in A to illustrate the composite helical bundles in the TMDs. C: surface rendering of the NBD dimers of Sav1866, BtuCD, and HI1470/1 viewed from the membrane and showing the footprint (all residues within 4 Å) made by the ICLs of the red TMD for Sav1866 or blue TMD in the case of BtuCD and HI1470/1. For Sav1866, the footprint of ICL1….. Kenneth J. Linton. Structure and Function of ABC Transporters. Physiology 22: 122-130, 2007