Crystal structure of a concentrative nucleoside transporter from Vibrio cholerae at 2.4 Å

Nucleosides are required for DNA and RNA synthesis, and the nucleoside adenosine plays a role in a variety of signaling processes (1,2). Transporting nucleosides across cell membranes provides the major source of nucleosides in many cell types and is also responsible for the termination of adenosine signaling. Due to their hydrophilic nature, nucleosides require a specialized class of integral membrane proteins, known as nucleoside transporters (NTs), for specific transport across cell membranes. In addition to nucleosides, NTs are important determinants for the transport of nucleoside-derived drugs across cell membranes (3–5). A wide range of nucleoside-derived drugs has been shown to depend, at least in part, on NTs for transport across cell membranes including anticancer drugs (e.g., Ara-C and gemcitabine) and antiviral drugs (e.g., AZT and ribavirin) (4,6–13). Concentrative nucleoside transporters (CNTs), members of the solute carrier transporter superfamily SLC28, use an ion gradient to actively transport nucleosides as well as nucleoside-derived drugs against their chemical gradients. The structural basis for selective ion-coupled nucleoside transport by CNTs is unknown. Here we present the crystal structure of a concentrative nucleoside transporter from Vibrio cholerae in complex with uridine at 2.4 Å. Our functional data show that the transporter utilizes a sodium gradient for nucleoside transport like its human orthologs. The structure reveals the overall architecture of this class of transporter, unravels the molecular determinants for nucleoside and sodium binding, and provides a framework for understanding the mechanism of nucleoside and nucleoside drug transport across cell membranes.