title: Mapping daunorubicin-binding Sites in the ATP-binding cassette transporter MsbA using site-specific quenching by spin labels.
abstract: ATP-binding cassette (ABC) transporters transduce the free energy of ATP hydrolysis to power the mechanical work of substrate translocation across cell membranes. MsbA is an ABC transporter implicated in trafficking lipid A across the inner membrane of Escherichia coli. It has sequence similarity and overlapping substrate specificity with multidrug ABC transporters that export cytotoxic molecules in humans and prokaryotes. Despite rapid advances in structure determination of ABC efflux transporters, little is known regarding the location of substrate-binding sites in the transmembrane segment and the translocation pathway across the membrane. In this study, we have mapped residues proximal to the daunorubicin (DNR)-binding site in MsbA using site-specific, ATP-dependent quenching of DNR intrinsic fluorescence by spin labels. In the nucleotide-free MsbA intermediate, DNR-binding residues cluster at the cytoplasmic end of helices 3 and 6 at a site accessible from the membrane/water interface and extending into an aqueous chamber formed at the interface between the two transmembrane domains. Binding of a nonhydrolyzable ATP analog inverts the transporter to an outward-facing conformation and relieves DNR quenching by spin labels suggesting DNR exclusion from proximity to the spin labels. The simplest model consistent with our data has DNR entering near an elbow helix parallel to the water/membrane interface, partitioning into the open chamber, and then translocating toward the periplasm upon ATP binding.
authors.affiliation: Department of Molecular Physiology and Biophysics, Vanderbilt University, Nashville, Tennessee 37232.
authors.lastname: Smriti
authors.affiliation: Department of Molecular Physiology and Biophysics, Vanderbilt University, Nashville, Tennessee 37232.
authors.forename: Ping
authors.initials: P
authors.lastname: Zou
authors.affiliation: Department of Molecular Physiology and Biophysics, Vanderbilt University, Nashville, Tennessee 37232. Electronic address: Hassane.mchaourab@vanderbilt.edu.
authors.forename: Hassane S
authors.initials: HS
authors.lastname: Mchaourab
mesh_terms: D018528:ATP-Binding Cassette Transporters / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D001426:Bacterial Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D001665:Binding Sites / Q000502:physiology; D001692:Biological Transport / Q000502:physiology; D003630:Daunorubicin / Q000737:chemistry* / Q000378:metabolism; D004926:Escherichia coli / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D008050:Lipid A / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D010449:Peptide Mapping / Q000379:methods; D013113:Spin Labels
chemical_list: D018528:ATP-Binding Cassette Transporters; D001426:Bacterial Proteins; D008050:Lipid A; C079726:MsbA protein, Bacteria; D013113:Spin Labels; D000255:Adenosine Triphosphate; D003630:Daunorubicin