ATP-binding cassette (ABC) transporters translocate substrates across cell membranes, using energy harnessed from ATP binding and hydrolysis at their nucleotide-binding domains1,2. ABC exporters are present both in prokaryotes and eukaryotes, with examples implicated in multidrug resistance of pathogens and cancer cells, as well as in many human diseases3,4. TmrAB is a heterodimeric ABC exporter from the thermophilic Gram-negative eubacterium Thermus thermophilus; it is homologous to various multidrug transporters and contains one degenerate site with a non-catalytic residue next to the Walker B motif5. Here we report a subnanometre-resolution structure of detergent-solubilized TmrAB in a nucleotide-free, inward-facing conformation by single-particle electron cryomicroscopy. The reconstructions clearly resolve characteristic features of ABC transporters, including helices in the transmembrane domain and nucleotide-binding domains. A cavity in the transmembrane domain is accessible laterally from the cytoplasmic side of the membrane as well as from the cytoplasm, indicating that the transporter lies in an inward-facing open conformation. The two nucleotide-binding domains remain in contact via their carboxy-terminal helices. Furthermore, comparison between our structure and the crystal structures of other ABC transporters suggests a possible trajectory of conformational changes that involves a sliding and rotating motion between the two nucleotide-binding domains during the transition from the inward-facing to outward-facing conformations.
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This work was supported by grants from the National Institutes of Health (R01GM098672, S10RR026814 and P50GM082250 to Y.C., 1P41CA196276-01 to C.S.C., P50GM073210 to R.M.S. and C.S.C., and R37GM024485 to R.M.S.), the University of California San Francisco Program for Breakthrough Biomedical Research (to Y.C.), and the German Research Foundation (SFB 807, SFB 902 and TA157/7 to R.T.) as well as the European Drug Initiative on Channels and Transporters (EDICT to R.T.) funded by the European Commission Seventh Framework Program.
Extended data figures
Extended data tables
3D reconstruction of TmrAB-AH5 complex at a resolution of 8.2Å.
Slicing through TMDs of TmrAB, showing micelle density and the separation of TM helices.
Morph video shows a conformational change from apo (TmrAB) through TM287/288 to Sav1866. In this orientation, closing of the lateral gate is shown.
Morph video shows the same conformational change from a different orientation to Supplementary video 3. Transition from inward-facing to outward-facing conformations is shown.
About this article
Cellular and Molecular Life Sciences (2017)