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

• DOI: 10.1038/ncomms4009
• Publication Date: 2014-01-06T12:30:57Z
• Authors: Abdul S. Ethayath...; Mohammad S. Yousef; Anowarul Amin; Gérard Leblanc; H. Ronald Kaback; Lan Guan
• Keywords: Models, Molecular; Salmonella typhimurium; 0301 basic medicine; Protein Structure; Crystallography; Binding Sites; Symporters; Sodium; Molecular; Lithium; Crystallography, X-Ray; Article; Protein Structure, Tertiary; 03 medical and health sciences; Infectious Diseases; Bacterial Proteins; Models; Cations; X-Ray; Protons; Tertiary; Melibiose

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.