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

Abdul S. Ethayathulla, Mohammad S. Yousef, Anowarul Amin, Gérard Leblanc, H. Ronald Kaback and Lan Guan ()
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Abdul S. Ethayathulla: Center for Membrane Protein Research, Texas Tech University Health Sciences Center
Mohammad S. Yousef: Center for Membrane Protein Research, Texas Tech University Health Sciences Center
Anowarul Amin: Center for Membrane Protein Research, Texas Tech University Health Sciences Center
Gérard Leblanc: University of California
H. Ronald Kaback: University of California
Lan Guan: Center for Membrane Protein Research, Texas Tech University Health Sciences Center

Nature Communications, 2014, vol. 5, issue 1, 1-11

Abstract: 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.

Date: 2014
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DOI: 10.1038/ncomms4009

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