Blizard Institute of Cell and Molecular Science, Barts and The London School of Medicine and Dentistry, Queen Mary University of London, The Blizard Building, 4 Newark Street, London E1 2AT, U.K.
Essays Biochem. 2011 Sep 7;50(1):43-61. doi: 10.1042/bse0500043.
ABC (ATP-binding cassette) transporters are primary active membrane proteins that translocate solutes (allocrites) across lipid bilayers. The prototypical ABC transporter consists of four domains: two cytoplasmic NBDs (nucleotide-binding domains) and two TMDs (transmembrane domains). The NBDs, whose primary sequence is highly conserved throughout the superfamily, bind and hydrolyse ATP to power the transport cycle. The TMDs, whose primary sequence and protein fold can be quite disparate, form the translocation pathway across the membrane and generally (but not always) determine allocrite specificity. Structure determination of ABC proteins initially took advantage of the relative ease of expression and crystallization of the hydrophilic bacterial NBDs in isolation from the transporter complex, and revealed detailed information on the structural fold of these domains, the amino acids involved in the binding and hydrolysis of nucleotide, and the head-to-tail arrangement of the NBD-NBD dimer interface. More recently, several intact transporters have been crystallized and three types have, so far, been characterized: type I and II ABC importers, and ABC exporters. All three are present in prokaryotes, but only the ABC exporters appear to be present in eukaryotes. Their structural determination has provided insight into the mechanisms of energy and signal transduction between the NBDs and TMDs (i.e. between the ATP- and allocrite-binding sites) and, for some, the nature of the allocrite-binding site(s) within the TMDs. In this chapter, we focus primarily on the ABC exporters and describe the structural, biochemical and biophysical evidence for and against the controversial bellows-like mechanism proposed for allocrite efflux.
ABC（ATP 结合盒）转运蛋白是主要的主动膜蛋白，可将溶质（分配物）跨脂质双层转运。典型的 ABC 转运蛋白由四个结构域组成：两个细胞质 NBD（核苷酸结合结构域）和两个 TMD（跨膜结构域）。NBD 的一级序列在整个超家族中高度保守，结合并水解 ATP 以提供运输循环的动力。TMD 的一级序列和蛋白质折叠可能非常不同，形成跨膜的转运途径，通常（但并非总是）决定分配物的特异性。ABC 蛋白的结构测定最初利用了亲水性细菌 NBD 从转运复合物中分离出来时相对容易表达和结晶的优势，并揭示了这些结构域的结构折叠、参与核苷酸结合和水解的氨基酸以及 NBD-NBD 二聚体界面的头尾排列的详细信息。最近，已经结晶了几种完整的转运蛋白，并且迄今为止已经表征了三种类型：I 型和 II 型 ABC 摄取体和 ABC 外排体。所有三种都存在于原核生物中，但只有 ABC 外排体似乎存在于真核生物中。它们的结构测定为 NBD 和 TMD 之间（即 ATP 和分配物结合位点之间）的能量和信号转导机制提供了深入的了解，并且对于一些，TMD 内的分配物结合位点的性质也提供了深入的了解。在本章中，我们主要关注 ABC 外排体，并描述了结构、生化和生物物理证据，以支持或反对针对分配物外排提出的有争议的风箱样机制。
