Specificity and Topology of the Escherichia coli Xanthosine Permease, a Representative of the NHS Subfamily of the Major Facilitator Superfamily

The specificity of XapB permease was compared with that of the known nucleoside transporters NupG and NupC. XapB-mediated xanthosine uptake is abolished by 2,4-dinitrophenol and exhibits saturation kinetics with an apparent K(m) of 136 μM. A 12-transmembrane-segment model was confirmed by translational fusions to alkaline phosphatase and the α fragment of β-galactosidase.