Structural insights into the activation mechanism of melibiose permease by sodium binding.
The melibiose carrier from Escherichia coli (MelB) couples the accumulation of the disaccharide melibiose to the downhill entry of H(+), Na(+), or Li(+). 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.
Unitat de Biofísica, Departament de Bioquímica i de Biologia Molecular, Facultat de Medicina, and Centre d'Estudis en Biofísica, Universitat Autònoma de Barcelona, 08193 Bellaterra, Barcelona, Spain.
Meritxell
M
Granell
Xavier
X
León
Gérard
G
Leblanc
Esteve
E
Padrós
Víctor A
VA
Lórenz-Fonfría
D004926:Escherichia coli / Q000737:chemistry* / Q000378:metabolism; D008553:Melibiose / Q000737:chemistry / Q000378:metabolism; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D012964:Sodium / Q000737:chemistry* / Q000378:metabolism; D017550:Spectroscopy, Fourier Transform Infrared / Q000379:methods; D013329:Structure-Activity Relationship; D027981:Symporters / Q000737:chemistry* / Q000378:metabolism
D027981:Symporters; C021548:melibiose permease; D008553:Melibiose; D012964:Sodium