Abstract
ATP-binding cassette (ABC) transporters, composed of importers and exporters, form one of the biggest protein superfamilies that transport a variety of substrates across the membrane, powered by ATP hydrolysis. Most ABC transporters are composed of two transmembrane domains and two cytoplasmic nucleotide-binding domains. Also, importers from prokaryotes usually have extra solute-binding proteins in the periplasm that are responsible for the binding of substrates1,2. Structures of importers have been reported that suggested a two-state model for the transport mechanism3,4,5,6,7,8,9,10,11. Energy-coupling factor (ECF) transporters belong to a new class of ATP-binding cassette importers. Each ECF transporter comprises an energy-coupling module consisting of a transmembrane T protein (EcfT), two nucleotide-binding proteins (EcfA and EcfA′), and another transmembrane substrate-specific binding S protein12,13,14 (EcfS). Despite the similarities with ABC transporters, ECF transporters have different organizational and functional properties. The lack of solute-binding proteins in ECF transporters differentiates them clearly from the canonical ABC importers15. Previously reported structures of the EcfS proteins RibU and ThiT clearly demonstrated the binding site of substrate riboflavin and thiamine, respectively16,17. However, the organization of the four different components and the transport mechanism of ECF transporters remain unknown. Here we present the structure of an intact folate ECF transporter from Lactobacillus brevis at a resolution of 3 Å. This structure was captured in an inward-facing, nucleotide-free conformation with no bound substrate. The folate-binding protein FolT is nearly parallel to the membrane and is bound almost entirely by EcfT, which adopts an L shape and connects to EcfA and EcfA′ through two coupling helices. Two conserved XRX motifs from the coupling helices of EcfT have a vital role in energy coupling by docking into EcfA–EcfA′. We propose a transport model that involves a substantial conformational change of FolT.
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Acknowledgements
We thank X. Chen and G. Zhao for reading the manuscript and the staff members of Shanghai Synchrotron Radiation Facility for technical assistance in data collection. This work was supported by grants from the Ministry of Science and Technology of China (2013CB127000), the Shanghai Institutes for Biological Sciences, the Chinese Academy of Sciences (2012OHTP, 2009CSP001, 2011KIP101, KSCX2-EW-J-12) and the Shanghai ‘Pujiang Talent’ programme (11PJ1411300).
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P.Z. initiated the project and designed the experiments; K.X., M.Z., Q.Z., F.Y., H.G. and P.Z. performed the bulk of the experiments; C.W. and F.H. contributed to some experiments and discussions; K.X., M.Z., Q.Z., F.Y. and J.D. contributed to data analysis and manuscript preparation; P.Z. wrote the manuscript.
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Xu, K., Zhang, M., Zhao, Q. et al. Crystal structure of a folate energy-coupling factor transporter from Lactobacillus brevis . Nature 497, 268–271 (2013). https://doi.org/10.1038/nature12046
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DOI: https://doi.org/10.1038/nature12046
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