Structure And Function Of A Sodium And Proton Coupled Nucleobase/Ascorbate Transporter

Members of the nucleobase/ascorbic acid transporter (NAT) gene family are found in all kingdoms of life. In mammals, concentrative vitamin C uptake by members of the NAT family protein is driven by the Na+ gradient, while in bacteria the H+ gradient drives the uptake of nucleobases. Little is known about the mechanism of substrate binding and translocation. Here we report the structure and function of a NAT protein, PaaTCp, from the bacterium Colwellia psychrerythraea. PaaTCp transports purine bases but not pyrimidine bases, and the transport is driven by a H+ gradient. Remarkably, PaaTCp also transports vitamin C in Na+-dependent manner. The structure of PaaTCp was solved by X-ray crystallography to 2.85 A resolution. PaaTCp forms a homodimer, and each protomer is composed of two domains, an interface domain and a transport domain. The interface domain is formed by six helices arranged in a panel-like configuration, while the transport domain is more compact with eight helices. A purine base is present in the structure and defines the location of the substrate binding site. Mutations to a conserved aspartate residue near the substrate binding site suggests that it is likely involved in mediating the H+- and Na+-dependency. Comparing the PaaTCp structure to other structures of NAT family homologs led us to propose a transport model in which conformational changes of the transport domain enable substrate translocation across the membrane.