ATP-binding cassette (ABC) transporter proteins carry diverse substrates across cell membranes1. Whereas clinically relevant ABC exporters are implicated in various diseases or cause multidrug resistance of cancer cells2,3, bacterial ABC importers are essential for the uptake of nutrients4, including rare elements such as molybdenum. A detailed understanding of their mechanisms requires direct visualization at high resolution and in distinct conformations. Our recent structure of the multidrug ABC exporter Sav1866 has revealed an outward-facing conformation of the transmembrane domains coupled to a closed conformation of the nucleotide-binding domains, reflecting the ATP-bound state5. Here we present the 3.1 Å crystal structure of a putative molybdate transporter (ModB2C2) from Archaeoglobus fulgidus in complex with its binding protein (ModA). Twelve transmembrane helices of the ModB subunits provide an inward-facing conformation, with a closed gate near the external membrane boundary. The ATP-hydrolysing ModC subunits reveal a nucleotide-free, open conformation, whereas the attached binding protein aligns the substrate-binding cleft with the entrance to the presumed translocation pathway. Structural comparison of ModB2C2A with Sav1866 suggests a common alternating access and release mechanism, with binding of ATP promoting an outward-facing conformation and dissociation of the hydrolysis products promoting an inward-facing conformation.
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We thank C. Schulze-Briese, E. Pohl and T. Tomizaki for assistance with synchrotron data collection, B. Blattmann for assistance with initial crystallization screening and J. P. Rosenbusch for discussions. This work was supported by the Roche Research Fund, the National Center for Competence in Research (NCCR) Structural Biology Zurich, and the Swiss National Science Foundation.
Coordinates and structure factors for ModA with bound MoO4, ModA with bound WO4, and for the ModB2C2A complex have been deposited in the Protein Data Bank with accession codes 2ONR, 2ONS, and 2ONK, respectively.
Coordinates and structure factors for ModA with bound MoO4, ModA with bound WO4, and for the ModB2C2A complex have been deposited in the Protein Data Bank with accession codes 2ONR, 2ONS, and 2ONK, respectively. Reprints and permissions information is available at www.nature.com/reprints. The authors declare no competing financial interests.
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Hollenstein, K., Frei, D. & Locher, K. Structure of an ABC transporter in complex with its binding protein. Nature 446, 213–216 (2007). https://doi.org/10.1038/nature05626
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