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

Area of Science:

• Structural biology
• Biochemistry
• Molecular biology

Background:

• The glycoside-pentoside-hexuronide:cation symporter family, part of the major facilitator superfamily (MFS), lacks detailed structural information despite extensive biochemical data.
• Bacterial melibiose permease (MelB) is a key transporter in this family, crucial for understanding its function.

Purpose of the Study:

• To elucidate the three-dimensional structures of Salmonella typhimurium melibiose permease (MelBSt) in different functional states.
• To provide mechanistic insights into the Na(+)/melibiose symport process.
• To establish a structural basis for the conformational changes in MFS permeases.

Main Methods:

• X-ray crystallography was used to determine the structures of MelBSt.
• Biochemical and biophysical data were integrated with structural findings.

Main Results:

• The crystal structures of MelBSt in outward partially occluded and outward inactive states were determined.
• A novel cation-binding motif involving three conserved acidic residues was identified, forming a site for Na(+), Li(+), or H(+).
• The cation-binding site is located near the sugar-binding site, with both primarily influenced by the N-terminal domain.

Conclusions:

• The presented structures offer mechanistic insights into Na(+)/melibiose symport.
• A structural basis for the conformational cycling essential for MFS permease transport is proposed.
• This study enhances understanding of the major facilitator superfamily (MFS) transporter mechanism.