title: Structure-based mechanism for Na(+)/melibiose symport by MelB.
abstract: The bacterial melibiose permease (MelB) belongs to the glycoside-pentoside-hexuronide:cation symporter family, a part of the major facilitator superfamily (MFS). Structural information regarding glycoside-pentoside-hexuronide:cation symporter family transporters and other Na(+)-coupled permeases within MFS has been lacking, although a wealth of biochemical and biophysical data are available. Here we present the three-dimensional crystal structures of Salmonella typhimurium MelBSt in two conformations, representing an outward partially occluded and an outward inactive state of MelBSt. MelB adopts a typical MFS fold and contains a previously unidentified cation-binding motif. Three conserved acidic residues form a pyramidal-shaped cation-binding site for Na(+), Li(+) or H(+), which is in close proximity to the sugar-binding site. Both cosubstrate-binding sites are mainly contributed by the residues from the amino-terminal domain. These two structures and the functional data presented here provide mechanistic insights into Na(+)/melibiose symport. We also postulate a structural foundation for the conformational cycling necessary for transport catalysed by MFS permeases in general.
authors.affiliation: Department of Cell Physiology and Molecular Biophysics, Center for Membrane Protein Research, Texas Tech University Health Sciences Center, Lubbock, Texas 79430, USA.
authors.forename: Abdul S
authors.initials: AS
authors.lastname: Ethayathulla
authors.affiliation: Department of Cell Physiology and Molecular Biophysics, Center for Membrane Protein Research, Texas Tech University Health Sciences Center, Lubbock, Texas 79430, USA
authors.forename: Mohammad S
authors.initials: MS
authors.lastname: Yousef
authors.affiliation: Department of Cell Physiology and Molecular Biophysics, Center for Membrane Protein Research, Texas Tech University Health Sciences Center, Lubbock, Texas 79430, USA.
authors.forename: Anowarul
authors.initials: A
authors.lastname: Amin
authors.affiliation: Department of Physiology, University of California, Los Angeles, California 90095, USA.
authors.forename: Gérard
authors.initials: G
authors.lastname: Leblanc
authors.affiliation: Department of Physiology, University of California, Los Angeles, California 90095, USA.
authors.forename: H Ronald
authors.initials: HR
authors.lastname: Kaback
authors.affiliation: Department of Cell Physiology and Molecular Biophysics, Center for Membrane Protein Research, Texas Tech University Health Sciences Center, Lubbock, Texas 79430, USA.
authors.forename: Lan
authors.initials: L
authors.lastname: Guan
mesh_terms: D001426:Bacterial Proteins / Q000378:metabolism*; D001665:Binding Sites; D002412:Cations / Q000378:metabolism; D018360:Crystallography, X-Ray; D008094:Lithium / Q000378:metabolism*; D008553:Melibiose / Q000378:metabolism*; D008958:Models, Molecular; D017434:Protein Structure, Tertiary; D011522:Protons; D012486:Salmonella typhimurium / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism*; D027981:Symporters / Q000378:metabolism*
chemical_list: D001426:Bacterial Proteins; D002412:Cations; D011522:Protons; D027981:Symporters; C021548:melibiose permease; D008553:Melibiose; D008094:Lithium; D012964:Sodium