Molecules and Cells

  • 제45권8호
  • /
  • Pages.575-587
  • /
  • 2022
  • /
  • 1016-8478(pISSN)
  • /
  • 0219-1032(eISSN)
한국분자세포생물학회 (Korean Society for Molecular and Cellular Biology)
권호 표지
DOI QR코드

DOI QR Code

Structural Insights into Porphyrin Recognition by the Human ATP-Binding Cassette Transporter ABCB6

  • Kim, Songwon (School of Life Sciences, Gwangju Institute of Science and Technology (GIST)) ;
  • Lee, Sang Soo (School of Life Sciences, Gwangju Institute of Science and Technology (GIST)) ;
  • Park, Jun Gyou (School of Life Sciences, Gwangju Institute of Science and Technology (GIST)) ;
  • Kim, Ji Won (Department of Life Sciences, Pohang University of Science and Technology (POSTECH)) ;
  • Ju, Seulgi (School of Life Sciences, Gwangju Institute of Science and Technology (GIST)) ;
  • Choi, Seung Hun (School of Life Sciences, Gwangju Institute of Science and Technology (GIST)) ;
  • Kim, Subin (School of Life Sciences, Gwangju Institute of Science and Technology (GIST)) ;
  • Kim, Na Jin (School of Life Sciences, Gwangju Institute of Science and Technology (GIST)) ;
  • Hong, Semi (School of Life Sciences, Gwangju Institute of Science and Technology (GIST)) ;
  • Kang, Jin Young (Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST)) ;
  • Jin, Mi Sun (School of Life Sciences, Gwangju Institute of Science and Technology (GIST))
  • 투고 : 2022.03.11
  • 심사 : 2022.04.07
  • 발행 : 2022.08.31
PDF 다운로드
⟨ 이전 논문 다음 논문 ⟩

초록

Human ABCB6 is an ATP-binding cassette transporter that regulates heme biosynthesis by translocating various porphyrins from the cytoplasm into the mitochondria. Here we report the cryo-electron microscopy (cryo-EM) structures of human ABCB6 with its substrates, coproporphyrin III (CPIII) and hemin, at 3.5 and 3.7 Å resolution, respectively. Metal-free porphyrin CPIII binds to ABCB6 within the central cavity, where its propionic acids form hydrogen bonds with the highly conserved Y550. The resulting structure has an overall fold similar to the inward-facing apo structure, but the two nucleotide-binding domains (NBDs) are slightly closer to each other. In contrast, when ABCB6 binds a metal-centered porphyrin hemin in complex with two glutathione molecules (1 hemin: 2 glutathione), the two NBDs end up much closer together, aligning them to bind and hydrolyze ATP more efficiently. In our structures, a glycine-rich and highly flexible "bulge" loop on TM helix 7 undergoes significant conformational changes associated with substrate binding. Our findings suggest that ABCB6 utilizes at least two distinct mechanisms to fine-tune substrate specificity and transport efficiency.

키워드

과제정보

We are grateful to Dr. Julian Gross for critical reading of the manuscript. We also thank the staff at the following facilities for assistance in grid screening and data collection using cryoEM: Mrs. Su Jeong Kim in POSTECH, Dr. Hyeongseop Jeong in the Korea Basic Science Institute (KBSI), Dr. Jin-Seok Choi in KAIST Research Analysis Center (KARA). Drs. Ludovic Renault and Wen Yang at The Netherlands Centre for Electron Nanoscopy (NeCEN). This research was supported by grants from the National Research Foundation (NRF) funded by the Ministry of Science, ICT, and Future Planning of Korea (MSIP) (NRF-2017M3A9F6029753, NRF-2019M3E5D6066058, NRF-2019R1A6A1A10073887, and NRF-2021M3A9I4022846), by a grant from the GIST Research Institute (GRI) IIBR funded by the GIST in 2020, and by a postdoctoral fellowship (S.K. [Songwon Kim]) from the NRF funded by MSIP (NRF-2020R1I1A1A01072077).

참고문헌

  1. Adams, P.D., Afonine, P.V., Bunkoczi, G., Chen, V.B., Davis, I.W., Echols, N., Headd, J.J., Hung, L.W., Kapral, G.J., Grosse-Kunstleve, R.W., et al. (2010). PHENIX: a comprehensive Python-based system for macromolecular structure solution. Acta Crystallogr. D Biol. Crystallogr. 66, 213-221. https://doi.org/10.1107/S0907444909052925
  2. Andolfo, I., Alper, S.L., Delaunay, J., Auriemma, C., Russo, R., Asci, R., Esposito, M.R., Sharma, A.K., Shmukler, B.E., Brugnara, C., et al. (2013). Missense mutations in the ABCB6 transporter cause dominant familial pseudohyperkalemia. Am. J. Hematol. 88, 66-72. https://doi.org/10.1002/ajh.23357
  3. Angiulli, G., Dhupar, H.S., Suzuki, H., Wason, I.S., Duong Van Hoa, F., and Walz, T. (2020). New approach for membrane protein reconstitution into peptidiscs and basis for their adaptability to different proteins. Elife 9, e53530.
  4. Bakos, E., Evers, R., Szakacs, G., Tusnady, G.E., Welker, E., Szabo, K., de Haas, M., van Deemter, L., Borst, P., Varadi, A., et al. (1998). Functional multidrug resistance protein (MRP1) lacking the N-terminal transmembrane domain. J. Biol. Chem. 273, 32167-32175. https://doi.org/10.1074/jbc.273.48.32167
  5. Carlson, M.L., Young, J.W., Zhao, Z., Fabre, L., Jun, D., Li, J., Li, J., Dhupar, H.S., Wason, I., Mills, A.T., et al. (2018). The Peptidisc, a simple method for stabilizing membrane proteins in detergent-free solution. Elife 7, e34085.
  6. Chavan, H., Khan, M.M., Tegos, G., and Krishnamurthy, P. (2013). Efficient purification and reconstitution of ATP binding cassette transporter B6 (ABCB6) for functional and structural studies. J. Biol. Chem. 288, 22658-22669. https://doi.org/10.1074/jbc.M113.485284
  7. Chavan, H., Oruganti, M., and Krishnamurthy, P. (2011). The ATP-binding cassette transporter ABCB6 is induced by arsenic and protects against arsenic cytotoxicity. Toxicol. Sci. 120, 519-528. https://doi.org/10.1093/toxsci/kfr008
  8. Chen, V.B., Arendall, W.B., 3rd, Headd, J.J., Keedy, D.A., Immormino, R.M., Kapral, G.J., Murray, L.W., Richardson, J.S., and Richardson, D.C. (2010). MolProbity: all-atom structure validation for macromolecular crystallography. Acta Crystallogr. D Biol. Crystallogr. 66, 12-21. https://doi.org/10.1107/S0907444909042073
  9. Chiabrando, D., Vinchi, F., Fiorito, V., Mercurio, S., and Tolosano, E. (2014). Heme in pathophysiology: a matter of scavenging, metabolism and trafficking across cell membranes. Front. Pharmacol. 5, 61.
  10. Chifflet, S., Torriglia, A., Chiesa, R., and Tolosa, S. (1988). A method for the determination of inorganic phosphate in the presence of labile organic phosphate and high concentrations of protein: application to lens ATPases. Anal. Biochem. 168, 1-4. https://doi.org/10.1016/0003-2697(88)90002-4
  11. Dean, M., Rzhetsky, A., and Allikmets, R. (2001). The human ATP-binding cassette (ABC) transporter superfamily. Genome Res. 11, 1156-1166. https://doi.org/10.1101/gr.184901
  12. Demirel, O., Bangert, I., Tampe, R., and Abele, R. (2010). Tuning the cellular trafficking of the lysosomal peptide transporter TAPL by its N-terminal domain. Traffic 11, 383-393. https://doi.org/10.1111/j.1600-0854.2009.01021.x
  13. Emsley, P. and Cowtan, K. (2004). Coot: model-building tools for molecular graphics. Acta Crystallogr. D Biol. Crystallogr. 60(Pt 12 Pt 1), 2126-2132. https://doi.org/10.1107/S0907444904019158
  14. Frank, G.A., Shukla, S., Rao, P., Borgnia, M.J., Bartesaghi, A., Merk, A., Mobin, A., Esser, L., Earl, L.A., Gottesman, M.M., et al. (2016). Cryo-EM analysis of the conformational landscape of human P-glycoprotein (ABCB1) during its catalytic cycle. Mol. Pharmacol. 90, 35-41. https://doi.org/10.1124/mol.116.104190
  15. Friesner, R.A., Banks, J.L., Murphy, R.B., Halgren, T.A., Klicic, J.J., Mainz, D.T., Repasky, M.P., Knoll, E.H., Shelley, M., Perry, J.K., et al. (2004). Glide: a new approach for rapid, accurate docking and scoring. 1. Method and assessment of docking accuracy. J. Med. Chem. 47, 1739-1749. https://doi.org/10.1021/jm0306430
  16. Friesner, R.A., Murphy, R.B., Repasky, M.P., Frye, L.L., Greenwood, J.R., Halgren, T.A., Sanschagrin, P.C., and Mainz, D.T. (2006). Extra precision glide: docking and scoring incorporating a model of hydrophobic enclosure for protein-ligand complexes. J. Med. Chem. 49, 6177-6196. https://doi.org/10.1021/jm051256o
  17. Fukuda, Y., Aguilar-Bryan, L., Vaxillaire, M., Dechaume, A., Wang, Y., Dean, M., Moitra, K., Bryan, J., and Schuetz, J.D. (2011). Conserved intramolecular disulfide bond is critical to trafficking and fate of ATP-binding cassette (ABC) transporters ABCB6 and sulfonylurea receptor 1 (SUR1)/ABCC8. J. Biol. Chem. 286, 8481-8492. https://doi.org/10.1074/jbc.M110.174516
  18. Goddard, T.D., Huang, C.C., and Ferrin, T.E. (2007). Visualizing density maps with UCSF Chimera. J. Struct. Biol. 157, 281-287. https://doi.org/10.1016/j.jsb.2006.06.010
  19. Haffke, M., Menzel, A., Carius, Y., Jahn, D., and Heinz, D.W. (2010). Structures of the nucleotide-binding domain of the human ABCB6 transporter and its complexes with nucleotides. Acta Crystallogr. D Biol. Crystallogr. 66, 979-987. https://doi.org/10.1107/S0907444910028593
  20. Hansen, S., Stuber, J.C., Ernst, P., Koch, A., Bojar, D., Batyuk, A., and Pluckthun, A. (2017). Design and applications of a clamp for Green Fluorescent Protein with picomolar affinity. Sci. Rep. 7, 16292.
  21. Heimerl, S., Bosserhoff, A.K., Langmann, T., Ecker, J., and Schmitz, G. (2007). Mapping ATP-binding cassette transporter gene expression profiles in melanocytes and melanoma cells. Melanoma Res. 17, 265-273. https://doi.org/10.1097/CMR.0b013e3282a7e0b9
  22. Helias, V., Saison, C., Ballif, B.A., Peyrard, T., Takahashi, J., Takahashi, H., Tanaka, M., Deybach, J.C., Puy, H., Le Gall, M., et al. (2012). ABCB6 is dispensable for erythropoiesis and specifies the new blood group system Langereis. Nat. Genet. 44, 170-173. https://doi.org/10.1038/ng.1069
  23. Henderson, R., Sali, A., Baker, M.L., Carragher, B., Devkota, B., Downing, K.H., Egelman, E.H., Feng, Z., Frank, J., Grigorieff, N., et al. (2012). Outcome of the first electron microscopy validation task force meeting. Structure 20, 205-214. https://doi.org/10.1016/j.str.2011.12.014
  24. Januchowski, R., Zawierucha, P., Andrzejewska, M., Rucinski, M., and Zabel, M. (2013). Microarray-based detection and expression analysis of ABC and SLC transporters in drug-resistant ovarian cancer cell lines. Biomed. Pharmacother. 67, 240-245. https://doi.org/10.1016/j.biopha.2012.11.011
  25. Kiss, K., Brozik, A., Kucsma, N., Toth, A., Gera, M., Berry, L., Vallentin, A., Vial, H., Vidal, M., and Szakacs, G. (2012). Shifting the paradigm: the putative mitochondrial protein ABCB6 resides in the lysosomes of cells and in the plasma membrane of erythrocytes. PLoS One 7, e37378.
  26. Kiss, K., Kucsma, N., Brozik, A., Tusnady, G.E., Bergam, P., van Niel, G., and Szakacs, G. (2015). Role of the N-terminal transmembrane domain in the endo-lysosomal targeting and function of the human ABCB6 protein. Biochem. J. 467, 127-139. https://doi.org/10.1042/BJ20141085
  27. Koch, J., Guntrum, R., Heintke, S., Kyritsis, C., and Tampe, R. (2004). Functional dissection of the transmembrane domains of the transporter associated with antigen processing (TAP). J. Biol. Chem. 279, 10142-10147. https://doi.org/10.1074/jbc.M312816200
  28. Krishnamurthy, P.C., Du, G., Fukuda, Y., Sun, D., Sampath, J., Mercer, K.E., Wang, J., Sosa-Pineda, B., Murti, K.G., and Schuetz, J.D. (2006). Identification of a mammalian mitochondrial porphyrin transporter. Nature 443, 586-589. https://doi.org/10.1038/nature05125
  29. Krossa, S., Scheidig, A.J., Grotzinger, J., and Lorenzen, I. (2018). Redundancy of protein disulfide isomerases in the catalysis of the inactivating disulfide switch in A Disintegrin and Metalloprotease 17. Sci. Rep. 8, 1103.
  30. Kumar, R., Matsumura, H., Lovell, S., Yao, H., Rodriguez, J.C., Battaile, K.P., Moenne-Loccoz, P., and Rivera, M. (2014). Replacing the axial ligand tyrosine 75 or its hydrogen bond partner histidine 83 minimally affects hemin acquisition by the hemophore HasAp from Pseudomonas aeruginosa. Biochemistry 53, 2112-2125. https://doi.org/10.1021/bi500030p
  31. Lee, J.Y., Yang, J.G., Zhitnitsky, D., Lewinson, O., and Rees, D.C. (2014). Structural basis for heavy metal detoxification by an Atm1-type ABC exporter. Science 343, 1133-1136. https://doi.org/10.1126/science.1246489
  32. Lewinson, O. and Bibi, E. (2001). Evidence for simultaneous binding of dissimilar substrates by the Escherichia coli multidrug transporter MdfA. Biochemistry 40, 12612-12618. https://doi.org/10.1021/bi011040y
  33. Loo, T.W., Bartlett, M.C., and Clarke, D.M. (2003). Simultaneous binding of two different drugs in the binding pocket of the human multidrug resistance P-glycoprotein. J. Biol. Chem. 278, 39706-39710. https://doi.org/10.1074/jbc.M308559200
  34. Lynch, J., Fukuda, Y., Krishnamurthy, P., Du, G., and Schuetz, J.D. (2009). Cell survival under stress is enhanced by a mitochondrial ATP-binding cassette transporter that regulates hemoproteins. Cancer Res. 69, 5560-5567. https://doi.org/10.1158/0008-5472.CAN-09-0078
  35. Manolaridis, I., Jackson, S.M., Taylor, N.M.I., Kowal, J., Stahlberg, H., and Locher, K.P. (2018). Cryo-EM structures of a human ABCG2 mutant trapped in ATP-bound and substrate-bound states. Nature 563, 426-430. https://doi.org/10.1038/s41586-018-0680-3
  36. Mattle, D., Zeltina, A., Woo, J.S., Goetz, B.A., and Locher, K.P. (2010). Two stacked heme molecules in the binding pocket of the periplasmic heme-binding protein HmuT from Yersinia pestis. J. Mol. Biol. 404, 220-231. https://doi.org/10.1016/j.jmb.2010.09.005
  37. Mehmood, S., Domene, C., Forest, E., and Jault, J.M. (2012). Dynamics of a bacterial multidrug ABC transporter in the inward- and outward-facing conformations. Proc. Natl. Acad. Sci. U. S. A. 109, 10832-10836. https://doi.org/10.1073/pnas.1204067109
  38. Mense, S.M. and Zhang, L. (2006). Heme: a versatile signaling molecule controlling the activities of diverse regulators ranging from transcription factors to MAP kinases. Cell Res. 16, 681-692. https://doi.org/10.1038/sj.cr.7310086
  39. Minami, K., Kamijo, Y., Nishizawa, Y., Tabata, S., Horikuchi, F., Yamamoto, M., Kawahara, K., Shinsato, Y., Tachiwada, T., Chen, Z.S., et al. (2014). Expression of ABCB6 is related to resistance to 5-FU, SN-38 and vincristine. Anticancer Res. 34, 4767-4773.
  40. Mitsuhashi, N., Miki, T., Senbongi, H., Yokoi, N., Yano, H., Miyazaki, M., Nakajima, N., Iwanaga, T., Yokoyama, Y., Shibata, T., et al. (2000). MTABC3, a novel mitochondrial ATP-binding cassette protein involved in iron homeostasis. J. Biol. Chem. 275, 17536-17540. https://doi.org/10.1074/jbc.275.23.17536
  41. Park, S., Shimizu, C., Shimoyama, T., Takeda, M., Ando, M., Kohno, T., Katsumata, N., Kang, Y.K., Nishio, K., and Fujiwara, Y. (2006). Gene expression profiling of ATP-binding cassette (ABC) transporters as a predictor of the pathologic response to neoadjuvant chemotherapy in breast cancer patients. Breast Cancer Res. Treat. 99, 9-17. https://doi.org/10.1007/s10549-006-9175-2
  42. Paterson, J.K., Shukla, S., Black, C.M., Tachiwada, T., Garfield, S., Wincovitch, S., Ernst, D.N., Agadir, A., Li, X., Ambudkar, S.V., et al. (2007). Human ABCB6 localizes to both the outer mitochondrial membrane and the plasma membrane. Biochemistry 46, 9443-9452. https://doi.org/10.1021/bi700015m
  43. Pettersen, E.F., Goddard, T.D., Huang, C.C., Couch, G.S., Greenblatt, D.M., Meng, E.C., and Ferrin, T.E. (2004). UCSF Chimera--a visualization system for exploratory research and analysis. J. Comput. Chem. 25, 1605-1612. https://doi.org/10.1002/jcc.20084
  44. Polireddy, K., Khan, M.M., Chavan, H., Young, S., Ma, X., Waller, A., Garcia, M., Perez, D., Chavez, S., Strouse, J.J., et al. (2012). A novel flow cytometric HTS assay reveals functional modulators of ATP binding cassette transporter ABCB6. PLoS One 7, e40005.
  45. Punjani, A., Rubinstein, J.L., Fleet, D.J., and Brubaker, M.A. (2017). cryoSPARC: algorithms for rapid unsupervised cryo-EM structure determination. Nat. Methods 14, 290-296. https://doi.org/10.1038/nmeth.4169
  46. Putman, M., Koole, L.A., van Veen, H.W., and Konings, W.N. (1999). The secondary multidrug transporter LmrP contains multiple drug interaction sites. Biochemistry 38, 13900-13905. https://doi.org/10.1021/bi991262k
  47. Rakvacs, Z., Kucsma, N., Gera, M., Igriczi, B., Kiss, K., Barna, J., Kovacs, D., Vellai, T., Bencs, L., Reisecker, J.M., et al. (2019). The human ABCB6 protein is the functional homologue of HMT-1 proteins mediating cadmium detoxification. Cell. Mol. Life Sci. 76, 4131-4144. https://doi.org/10.1007/s00018-019-03105-5
  48. Rohou, A. and Grigorieff, N. (2015). CTFFIND4: fast and accurate defocus estimation from electron micrographs. J. Struct. Biol. 192, 216-221. https://doi.org/10.1016/j.jsb.2015.08.008
  49. Scharschmidt, B.F., Keeffe, E.B., Blankenship, N.M., and Ockner, R.K. (1979). Validation of a recording spectrophotometric method for measurement of membrane-associated Mg- and NaK-ATPase activity. J. Lab. Clin. Med. 93, 790-799.
  50. Scheres, S.H. (2012). RELION: implementation of a Bayesian approach to cryo-EM structure determination. J. Struct. Biol. 180, 519-530. https://doi.org/10.1016/j.jsb.2012.09.006
  51. Soehnlein, O. (2014). The ABC of thrombopoiesis. Arterioscler. Thromb. Vasc. Biol. 34, 700-701. https://doi.org/10.1161/ATVBAHA.114.303365
  52. Song, G., Zhang, S., Tian, M., Zhang, L., Guo, R., Zhuo, W., and Yang, M. (2021). Molecular insights into the human ABCB6 transporter. Cell Discov. 7, 55.
  53. Thomas, C., Aller, S.G., Beis, K., Carpenter, E.P., Chang, G., Chen, L., Dassa, E., Dean, M., Duong Van Hoa, F., Ekiert, D., et al. (2020). Structural and functional diversity calls for a new classification of ABC transporters. FEBS Lett. 594, 3767-3775. https://doi.org/10.1002/1873-3468.13935
  54. Tsuchida, M., Emi, Y., Kida, Y., and Sakaguchi, M. (2008). Human ABC transporter isoform B6 (ABCB6) localizes primarily in the Golgi apparatus. Biochem. Biophys. Res. Commun. 369, 369-375. https://doi.org/10.1016/j.bbrc.2008.02.027
  55. Ulrich, D.L., Lynch, J., Wang, Y., Fukuda, Y., Nachagari, D., Du, G., Sun, D., Fan, Y., Tsurkan, L., Potter, P.M., et al. (2012). ATP-dependent mitochondrial porphyrin importer ABCB6 protects against phenylhydrazine toxicity. J. Biol. Chem. 287, 12679-12690. https://doi.org/10.1074/jbc.M111.336180
  56. Varatharajan, S., Abraham, A., Karathedath, S., Ganesan, S., Lakshmi, K.M., Arthur, N., Srivastava, V.M., George, B., Srivastava, A., Mathews, V., et al. (2017). ATP-binding casette transporter expression in acute myeloid leukemia: association with in vitro cytotoxicity and prognostic markers. Pharmacogenomics 18, 235-244. https://doi.org/10.2217/pgs-2016-0150
  57. Vasiliou, V., Vasiliou, K., and Nebert, D.W. (2009). Human ATP-binding cassette (ABC) transporter family. Hum. Genomics 3, 281-290. https://doi.org/10.1186/1479-7364-3-3-281
  58. Wang, C., Cao, C., Wang, N., Wang, X., Wang, X., and Zhang, X.C. (2020). Cryo-electron microscopy structure of human ABCB6 transporter. Protein Sci. 29, 2363-2374. https://doi.org/10.1002/pro.3960
  59. Wang, L., He, F., Bu, J., Zhen, Y., Liu, X., Du, W., Dong, J., Cooney, J.D., Dubey, S.K., Shi, Y., et al. (2012). ABCB6 mutations cause ocular coloboma. Am. J. Hum. Genet. 90, 40-48. https://doi.org/10.1016/j.ajhg.2011.11.026
  60. Ward, A.B., Szewczyk, P., Grimard, V., Lee, C.W., Martinez, L., Doshi, R., Caya, A., Villaluz, M., Pardon, E., Cregger, C., et al. (2013). Structures of P-glycoprotein reveal its conformational flexibility and an epitope on the nucleotide-binding domain. Proc. Natl. Acad. Sci. U. S. A. 110, 13386-13391. https://doi.org/10.1073/pnas.1309275110
  61. Yasui, K., Mihara, S., Zhao, C., Okamoto, H., Saito-Ohara, F., Tomida, A., Funato, T., Yokomizo, A., Naito, S., Imoto, I., et al. (2004). Alteration in copy numbers of genes as a mechanism for cacquired drug resistance. Cancer Res. 64, 1403-1410. https://doi.org/10.1158/0008-5472.CAN-3263-2
  62. Zeytuni, N., Dickey, S.W., Hu, J., Chou, H.T., Worrall, L.J., Alexander, J.A.N., Carlson, M.L., Nosella, M., Duong, F., Yu, Z., et al. (2020). Structural insight into the Staphylococcus aureus ATP-driven exporter of virulent peptide toxins. Sci. Adv. 6, eabb8219.
  63. Zhang, C., Li, D., Zhang, J., Chen, X., Huang, M., Archacki, S., Tian, Y., Ren, W., Mei, A., Zhang, Q., et al. (2013). Mutations in ABCB6 cause dyschromatosis universalis hereditaria. J. Invest. Dermatol. 133, 2221-2228. https://doi.org/10.1038/jid.2013.145
  64. Zheng, S.Q., Palovcak, E., Armache, J.P., Verba, K.A., Cheng, Y., and Agard, D.A. (2017). MotionCor2: anisotropic correction of beam-induced motion for improved cryo-electron microscopy. Nat. Methods 14, 331-332. https://doi.org/10.1038/nmeth.4193
  65. Zou, P., Bortolus, M., and McHaourab, H.S. (2009). Conformational cycle of the ABC transporter MsbA in liposomes: detailed analysis using double electron-electron resonance spectroscopy. J. Mol. Biol. 393, 586-597. https://doi.org/10.1016/j.jmb.2009.08.050