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

Abstract

The X-ray crystal structure of a bacterial concentrative nucleoside transporter reveals the overall architecture of this class of transporter and provides a framework for understanding how nucleosides and nucleoside-derived drugs traverse cell membranes. Concentrative nucleoside transporters are integral membrane proteins that are responsible for the selective uptake of nucleosides and nucleoside-derived anticancer and antiviral drugs into cells. This paper presents the first X-ray crystal structure of a member of a concentrative nucleoside transporter family, from the cholera pathogen Vibrio cholerae, in complex with uridine. The structure reveals the overall architecture of this class of transporter and the molecular determinants for nucleoside- and sodium-binding. Nucleosides are required for DNA and RNA synthesis, and the nucleoside adenosine has a function in a variety of signalling processes1,2. Transport of nucleosides across cell membranes provides the major source of nucleosides in many cell types and is also responsible for the termination of adenosine signalling. As a result of 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 membranes3,4,5. A wide range of nucleoside-derived drugs, including anticancer drugs (such as Ara-C and gemcitabine) and antiviral drugs (such as zidovudine and ribavirin), have been shown to depend, at least in part, on NTs for transport across cell membranes4,6,7,8,9,10,11,12,13. Concentrative nucleoside transporters, members of the solute carrier transporter superfamily SLC28, use an ion gradient in the active transport of both nucleosides and nucleoside-derived drugs against their chemical gradients. The structural basis for selective ion-coupled nucleoside transport by concentrative nucleoside transporters 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, like its human orthologues, the transporter uses a sodium-ion gradient for nucleoside transport. 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.