Editing ABC transporter (section)

=== Nucleotide-binding domain (NBD) ===
[[Image:Abc domain.jpg|thumb|left|Structure of the NBD of ABC transporters with bound nucleotide ({{PDB|2onj}}). Linear representation of protein sequence above shows the relative positions of the conserved amino acid motifs in the structure (colors match with 3D structure)]]

The ABC domain consists of two domains, the ''catalytic core domain'' similar to [[RecA]]-like motor [[ATPases]] and a smaller, structurally diverse ''α-helical subdomain'' that is unique to ABC transporters. The larger domain typically consists of two β-sheets and six α helices, where the catalytic ''[[Walker motifs|Walker A motif]]'' (GXXGXGKS/T where X is any amino acid) or ''P-loop'' and ''Walker B motif'' (ΦΦΦΦD, of which Φ is a hydrophobic residue) is situated. The helical domain consists of three or four helices and the ''ABC signature motif'', also known as ''LSGGQ motif'', linker peptide or C motif. The ABC domain also has a glutamine residue residing in a flexible loop called ''Q loop'', lid or γ-phosphate switch, that connects the TMD and ABC. The Q loop is presumed to be involved in the interaction of the NBD and TMD, particularly in the coupling of nucleotide [[hydrolysis]] to the conformational changes of the TMD during substrate translocation. The ''H motif'' or switch region contains a highly conserved [[histidine]] residue that is also important in the interaction of the ABC domain with ATP. The name ATP-binding cassette is derived from the diagnostic arrangement of the folds or motifs of this class of proteins upon formation of the ATP sandwich and ATP hydrolysis.<ref name=davidson/><ref name=davidsonchen/><ref name=rees/>