This is communicated to the TMs, which shift to re-orientate the translocation pathway. It is known that when an ATP molecule binds to each cassette of an ABC-transporter, it induces a conformational change in which the NBFs interact more closely. The mechanism of ABC transporters has not been fully identified. Every possible organisation for these domains has been observed (Higgins, 1992). For instance many bacterial importers are composed of four polypeptides. However though these are the most common domain organisations, others exist. Half transporters can thus form homodimers if two identical ABC transporters join, and heterodimers if two unlike ABC transporters join. Half transporters consist of only one TM and one NBF and must combine with another half transporter to gain functionability. Full transporters consist of the typical two TMs and NBFs. Many ABC transporters may be classified as half transporters or full transporters. The transmembrane domains and nucleotide-binding folds are often arranged in the order NH3+-TM-NBF-TM-NBF-COO. These folds form the “cassettes” which the protein family is named after. Other conserved motifs include the Q-loop, the his-loop, the pro-loop and the D-loop. Unlike the Walker A and Walker B motifs, which are found in other proteins which hydrolyze ATP, the signature motif is unique to ABC transporters. In addition, there is a third short and highly conserved motif (called LSGGQ motif, C motif, or "signature" motif) located after the Walker B motif. These folds are divided into parts or motifs, called Walker A and Walker B, which are separated by approximately 90-120 amino acids. In between the TMs is a ligand binding-domain, which is on the extracellular side of the protein for importers and on the cytoplasmic side for exporters.Īll ABC proteins also contain either one or two ATP-binding domain(s), (nucleotide-binding folds (NBFs)) and are located on the cytoplasm side of the membrane. These transmembrane domains provide the specificity for the substrate, and prevent unwanted molecules from using the transporter. These helices form between six to eleven (usually six) membrane-spanning regions. Typical ABC-transporters will contain two transmembrane domains (TMs), each of which consists of α-helices which cross the phospholipid bilayer multiple times. The range of transported compounds includes: More recently ABC-transporters have been shown to exist within the placenta, indicating they could play a protective role for the developing fetus against xenobiotics. Within eukaryotes, ABC-transporters mainly transport molecules to the outside of the plasma membrane or into membrane-bound organelles such as the endoplasmic reticulum, mitochondria, etc. ![]() Within bacteria, ABC-transporters mainly pump essential compounds such as sugars, vitamins, and metal ions into the cell. ABC-transporters utilize the energy of ATP hydrolysis to transport various substrates across cellular membranes.
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