2.A.57.1.17
Equilibrative nuceloside transporter 2, ENT2, of 418 aas and 11 TMSs. OsENT2 transports adenosine and uridine with high affinity (adenosine, K_m = 3.0 μM; uridine, K_m = 0.7 μm) (Hirose et al. 2005). Purine or pyrimidine nucleosides and 2'-deoxynucleosides strongly inhibit adenosine transport via OsENT2, suggesting that it possesses broad substrate specificity. OsENT2-mediated adenosine transport is resistant to the typical inhibitors of mammalian ENTs, nitrobenzylmercaptopurine ribonucleoside, dilazep, and dipyridamole. Transport activity is maximal at pH 5.0. In competition experiments with various cytokinins, adenosine transport is inhibited by isopentenyladenine riboside (iPR). Direct measurements with radiolabeled cytokinins demonstrated that OsENT2 mediated uptake of iPR (K(m) = 32 microm) and trans-zeatin riboside (K(m) = 660 microm), suggesting that OsENT2 participates in iPR transport in planta. In mature plants, OsENT2 is predominantly expressed in roots. The OsENT2 promoter drives expression in the scutellum during germination and in vascular tissues in germinated plants, suggesting participation in the retrieval of endosperm-derived nucleosides by the germinating embryo and in the long-distance transport of nucleosides in growing plants, respectively (Hirose et al. 2005). Three other ENTs have been partially characterized, Ent4, Ent6 and Ent7; all three proteins exhibit broad substrate specificity and transport the purine nucleosides adenosine and guanosine, as well as the pyrimidine nucleosides cytidine and uridine. The apparent K_m values were in the range 3-94 μM, and transport was inhibited most strongly by deoxynucleosides, and to a lesser extent by nucleobases.  Ent6 is in the plasma membrane (Wormit et al. 2004). Ent2 transports both nucleosides and bases, and is less important than Ent1 (Altaweraqi et al. 2020).