Figure 3: Head-to-tail arrangement of NBD dimer. (a) The standard NBD homodimer found in most ABC transporters with the full consensus of catalytic residues at each NBD. The interfacial composite site binding an ATP is made up of a major "head" region containing the Walker A motif, Walker B motif, and the D-loop together with the minor "tail" containing the LSGGQ signature motif. (b) This contrasts with the NBD heterodimer arrangement of CFTR that contains only a single consensus catalytic site at NBD2. These structural findings are an indication of the function of CFTR, with studies showing ATP catalysis at the catalytically active NBD2 to be sufficient to drive the gating cycle.