ATP-binding Cassette Transporter - Structure

Structure

The common feature of all ABC transporters is that they consist of two distinct domains, the transmembrane domain (TMD) and the nucleotide-binding domain (NBD). The TMD, also known as membrane-spanning domain (MSD) or integral membrane (IM) domain, consists of alpha helices, embedded in the membrane bilayer. It recognizes a variety of substrates and undergoes conformational changes to transport the substrate across the membrane. The sequence and architecture of TMDs is variable, reflecting the chemical diversity of substrates that can be translocated. The NBD or ATP-binding cassette (ABC) domain, on the other hand, is located in the cytoplasm and has a highly conserved sequence. The NBD is the site for ATP binding. In most exporters, the N-terminal transmembrane domain and the C-terminal ABC domains are fused as a single polypeptide chain, arranged as TMD-NBD-TMD-NBD. An example is the E. coli hemolysin exporter HlyB. Importers have an inverted organization, that is, NBD-TMD-NBD-TMD, where the ABC domain is N-terminal whereas the TMD is C-terminal, such as in the E. coli MacB protein responsible for macrolide resistance.

The structural architecture of ABC transporters consists minimally of two TMDs and two ABCs. Four individual polypeptide chains including two TMD and two NBD subunits, may combine to form a full transporter such as in the E. coli BtuCD importer involved in the uptake of vitamin B₁₂. Most exporters, such as in the multidrug exporter Sav1866 from Staphylococcus aureus, are made up of a homodimer consisting of two half transporters or monomers of a TMD fused to a nucleotide-binding domain (NBD). A full transporter is often required to gain functionality. Some ABC transporters have additional elements that contribute to the regulatory function of this class of proteins. In particular, importers have a high-affinity binding protein (BP) that specifically associates with the substrate in the periplasm for delivery to the appropriate ABC transporter. Exporters do not have the binding protein but have an intracellular domain (ICD) that joins the membrane-spanning helices and the ABC domain. The ICD is believed to be responsible for communication between the TMD and NBD.