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Nat Commun. 2014;5:3009. doi: 10.1038/ncomms4009.

Structure-based mechanism for Na(+)/melibiose symport by MelB.

Author information

  • 1Department of Cell Physiology and Molecular Biophysics, Center for Membrane Protein Research, Texas Tech University Health Sciences Center, Lubbock, Texas 79430, USA.
  • 2Department of Cell Physiology and Molecular Biophysics, Center for Membrane Protein Research, Texas Tech University Health Sciences Center, Lubbock, Texas 79430, USA
  • 3Department of Physiology, University of California, Los Angeles, California 90095, USA.

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.

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