Structure and Substrate Recognition of the Escherichia coli Transport Protein NupG from the Nucleoside: H+ Symporter (NHS) Family

The nucleoside transporter NupG is one of the two principal transport proteins in the inner membrane of Escherichia coli that enable the organism to scavenge nucleosides from its external environment. NupG functions in a symport manner driven by the proton motive force and is a member of the Nucleoside:H⁺ Symporter (NHS) subfamily of the Major Facilitator Superfamily (MFS) of transporters. NupG has broad substrate specificity, transporting all naturally occurring purine and pyrimidine nucleosides. In studies over many years the nupG gene has been cloned and amplified, and the NupG protein has been purified, subjected to biochemical, biophysical and computational analysis, and its X-ray structure determined in the apo state at 3.0 Å resolution. The NupG structure had a typical MFS fold with twelve transmembrane spanning α-helices and distinct N- and C-terminal domains linked by a flexible loop. Preliminary site-directed mutagenesis and molecular docking studies on NupG identified nine putative nucleoside binding pocket residues (R136, T140, F143, Q225, N228, Q261, E264, Y318, F322) and a mutant (D323A) with 20-fold enhanced uridine binding activity. Further biochemical and structural investigations are necessary to better understand the substrate recognition and molecular mechanism of E. coli NHS family proteins (NupG, XapB, YegT).