The melibiose carrier from Escherichia coli (MelB) couples the accumulation of the disaccharide melibiose to the downhill entry of H , Na+, orLi+. In this work, substrate-induced FTIR difference spectroscopy was used in combination with fluorescence spectroscopy to quantitatively compare the conformational properties of MelB mutants, implicated previously in sodium binding, with those of a fully functional Cys-less MelB permease. The results first suggest that Asp55 and Asp59 are essential ligands for Na binding. Secondly, though Asp124 is not essential for Na+ binding, this acidic residue may play a critical role, possibly by its interaction with the bound cation, in the full Na+ -induced conformational changes required for efficient coupling between the ion- and sugar-binding sites; this residue may also be a sugar ligand. Thirdly, Asp19 does not participate in Na+ binding but it is a melibiose ligand. The location of these residues in two independent threading models of MelB is consistent with their proposed role.
|Journal||Proceedings of the National Academy of Sciences of the United States of America|
|Publication status||Published - 21 Dec 2010|
- Infrared spectroscopy
- Ligand binding
- Membrane proteins
- Sugar/cation symporter