Six-transmembrane helical domain (6-TMD) of ATP-binding cassette sub-family B member 9 and similar proteins
ATP-binding cassette sub-family B member 9 is also known as transporter associated with antigen processing, TAP-like protein, TAPL, and ABCB9. It is a half transporter comprises a homodimeric lysosomal peptide transport complex. It belongs to the ABC_6TM_TAP_ABCB8_10_like subgroup of the ABC_6TM exporter family. The ABC_6TM exporter family represents the six transmembrane (TM) helices typically found in the ATP-binding cassette (ABC) transporters that function as exporters, which contain 6 TM helices per subunit (domain), or a total of 12 TM helices for the complete transporter. The ABC exporters are found in both prokaryotes and eukaryotes, where they mediate the cellular secretion of toxic compounds and a various type of lipids. In addition to ABC exporters, ABC transporters include two classes of ABC importers, classified depending on details of their architecture and mechanism. Only the ABC exporters are included in the ABC_6TM exporter family. ABC transporters typically consist of two transmembrane domains (TMDs) and two nucleotide-binding domains (NBDs. The sequences and structures of the TMDs are quite varied between the different type of transporters, suggesting chemical diversity of the translocated substrates, whereas NBDs are conserved among all ABC transporters. The two NBDs together bind and hydrolyze ATP, thereby providing the driving force for transport, while the TMDs participate in substrate recognition and translocation across the lipid membrane. However, some ABC genes are organized as half-transporters, which must form either homodimers or heterodimers to form a functional unit.
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