title: Apo-intermediate in the transport cycle of lactose permease (LacY).
abstract: The lactose permease (LacY) catalyzes coupled stoichiometric symport of a galactoside and an H(+). Crystal structures reveal 12, mostly irregular, transmembrane α-helices surrounding a cavity with sugar- and H(+)- binding sites at the apex, which is accessible from the cytoplasm and sealed on the periplasmic side (an inward-facing conformer). An outward-facing model has also been proposed based on biochemical and spectroscopic measurements, as well as the X-ray structure of a related symporter. Converging lines of evidence demonstrate that LacY functions by an alternating access mechanism. Here, we generate a model for an apo-intermediate of LacY based on crystallographic coordinates of LacY and the oligopeptide/H(+) symporter. The model exhibits a conformation with an occluded cavity inaccessible from either side of the membrane. Furthermore, kinetic considerations and double electron-electron resonance measurements suggest that another occluded conformer with bound sugar exists during turnover. An energy profile for symport is also presented.
authors.affiliation: Department of Physiology, and Molecular Biology Institute, University of California, Los Angeles, CA 90095, USA.
authors.forename: M Gregor
authors.initials: MG
authors.lastname: Madej
authors.forename: Sonya N
authors.initials: SN
authors.lastname: Soro
authors.forename: H Ronald
authors.initials: HR
authors.lastname: Kaback
mesh_terms: D018360:Crystallography, X-Ray; D026901:Membrane Transport Proteins / Q000737:chemistry / Q000378:metabolism*; D008958:Models, Molecular; D011487:Protein Conformation; D021381:Protein Transport
chemical_list: D026901:Membrane Transport Proteins; C021499:lactose permease