Projection structure at 8 A resolution of the melibiose permease, an Na-sugar co-transporter from Escherichia coli

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Abstract
The ion‐coupled sugar membrane symporter or co‐transporter melibiose permease (MelB), responsible for α‐galactoside accumulation in Escherichia coli, is a representative member of the glycoside–pentoside–hexuronide family of the vast class of electrochemical potential‐driven porters. Pure solubilized preparations of a MelB recombinant protein were subjected to two‐dimensional crystallization trials and several crystal forms were observed. Two of these appeared as large wide tubes suitable for analysis by electron crystallography. Flattened tubes on carbon support film, embedded in amorphous ice prior to electron cryomicroscopy, showed two‐sided plane group symmetries P121 or P2221, with unit cell dimensions a = 89.9 Å, b = 51.6 Å, γ = 91.9° and a = 188.9 Å, b = 48.8 Å, γ = 90°, respectively. The projection map from the P2221 crystals at 8 Å resolution displayed an asymmetric protein unit consisting of two domains lining a central and curve‐shaped cleft. Together, the MelB monomer could host the 12 predicted transmembrane α‐helices. Overall, the MelB helix packing arrangement compared more favorably with that of the Na+/H+ antiporter NhaA than that of the oxalate antiporter.