Title: Visualizing multistep elevator-like transitions of a nucleoside transporter

Nucleosides are required for DNA and RNA synthesis, and the nucleoside adenosine has a function in a variety of signalling processes. 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 membranes. A wide range of nucleoside-derived drugs, including anticancer drugsmore » (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 membranes. 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 {angstrom}. 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.« less

Kinetic analysis of the binding of {sup 3}Hnitrobenzylthioinosine ({sup 3}H NBMPR) to Ehrlich ascites tumor cell plasma membranes was conducted in the presence and absence of a variety of nucleoside transport inhibitors and substrates. The association of {sup 3}H NBMPR with Ehrlich cell membranes occurred in two distinct phases, possibly reflecting functional conformation changes in the {sup 3}HNBMPR binding site/nucleoside transporter complex. Inhibitors of the equilibrium binding of {sup 3}HNBMPR, tested at submaximal inhibitory concentrations, generally decreased the rate of association of {sup 3}HNBMPR, but the magnitude of this effect varied significantly with the agent tested. Adenosine and diazepam hadmore » relatively minor effects on the association rate, whereas dipyridamole and mioflazine slowed the rate dramatically. Inhibitors of nucleoside transport also decreased the rate of dissociation of {sup 3}HNBMPR, with an order of potency significantly different from their relative potencies as inhibitors of the equilibrium binding of {sup 3}HNBMPR. Dilazep, dipyridamole, and mioflazine were effective inhibitors of both {sup 3}HNBMPR dissociation and equilibrium binding. The lidoflazine analogue R75231, on the other hand, had no effect on the rate of dissociation of {sup 3}HNBMPR at concentrations below 300 microM, even though it was one of the most potent inhibitors of {sup 3}HNBMPR binding tested (Ki less than 100 nM). In contrast, a series of natural substrates for the nucleoside transport system enhanced the rate of dissociation of {sup 3}HNBMPR with an order of effectiveness that paralleled their relative affinities for the permeant site of the transporter. The most effective enhancers of {sup 3}HNBMPR dissociation, however, were the benzodiazepines diazepam, chlordiazepoxide, and triazolam.« less

No abstract prepared.