Alternative titles; symbols
HGNC Approved Gene Symbol: SLC29A2
Cytogenetic location: 11q13.2 Genomic coordinates (GRCh38) : 11:66,362,521-66,372,446 (from NCBI)
The uptake of nucleosides by transporters, such as SLC29A2, is essential for nucleotide synthesis by salvage pathways in cells that lack de novo biosynthetic pathways. Nucleoside transport also plays a key role in the regulation of many physiologic processes through its effect on adenosine concentration at the cell surface (Griffiths et al., 1997).
Transcription of delayed-early response (DER) genes is induced by immediate-early response (IER) genes following serum or growth factor stimulation of cells. Williams and Lanahan (1995) reported the cloning of a DER gene that they termed HNP36. They isolated a full-length cDNA from a human heart cDNA library and found that it predicted a 326-amino acid protein. This predicted protein is 50% similar to the C-terminal 331 amino acids of the yeast protein FUN26. Using immunocytochemistry, Williams and Lanahan (1995) found that HNP36 localized specifically to the nucleolus of fibroblasts and was more prominent in serum-stimulated cells. Northern blot analysis showed that HNP36 is a DER gene; the mRNA was barely detectable in quiescent fibroblasts, but was prominent approximately 5 hours after stimulation with serum or growth factors.
By Northern blot analysis, Williams et al. (1997) showed that HNP36 was expressed as a single 2.4-kb transcript in all adult human tissues examined. RT-PCR revealed several bands smaller than the predicted size; these bonds had deletions of specific exons and may correspond to alternatively spliced transcripts.
By PCR of a human placenta cDNA library, Griffiths et al. (1997) cloned full-length SLC29A2, which they called ENT2. The nucleotide sequences of ENT2 and HNP36 are identical except for a 68-nucleotide insertion in ENT2 following nucleotide 337 of HNP36. The deduced 456-amino acid ENT2 protein has a calculated molecular mass of 50.2 kD and shares 46% amino acid identity with ENT1 (SLC29A1; 602193). It has 11 transmembrane (TM) domains connected to one another by hydrophilic loops. Most of the loops are small, except for 2 large loops connecting TM domains 1 and 2 and TM domains 6 and 7. The loop connecting TM domains 1 and 2 has 2 potential N-glycosylation sites. EST database analysis indicated that ENT2 mRNA is expressed in adult ovary and ovarian tumors and in fetal brain and heart.
Crawford et al. (1998) determined that the ENT2 transcript contains 2 potential ORFs. HNP36 is translated from the second start codon and lacks TM domains 1, 2, and 3 compared with full-length ENT2. Northern blot analysis detected high expression of a major 2.6-kb transcript in skeletal muscle, with lower expression in most other tissues examined. A transcript of about 4 kb was also detected in thymus, prostate, heart, brain, lung, skeletal muscle, and pancreas.
Griffiths et al. (1997) found that both ENT1 and ENT2 showed saturable sodium-independent equilibrative transport of adenosine and uridine following expression in Xenopus oocytes. ENT1 was more efficient than ENT2 in transport of both substrates, and ENT1, but not ENT2, was sensitive to inhibition by nitrobenzylmercaptopurine riboside (NBMPR).
Crawford et al. (1998) showed that only full-length ENT2, and not HNP36, conferred uridine transport in transfected COS-1 cells.
Ward et al. (2000) found that human ENT2 showed lower affinity than ENT1 for transport of thymidine, adenosine, cytidine, and guanosine following expression in porcine kidney cells. However, ENT2 showed 4-fold higher affinity than ENT1 for inosine. The nucleobase hypoxanthine inhibited uridine uptake by ENT2, but had minimal effect on ENT1. Ward et al. (2000) concluded that ENT2 may be the major transporter of adenosine and its metabolites (inosine and hypoxanthine) in tissues where ENT2 is predominantly expressed, such as skeletal muscle.
Yao et al. (2001) expressed rat and human ENT1 and ENT2 in Xenopus oocytes and characterized their ability to transport three 3-prime-deoxy-nucleoside analogs used in human immunodeficiency virus (HIV) therapy: 2-prime,3-prime-did eoxycytidine (ddC), 3-prime-azido-3-prime-deoxythymidine (AZT), and 2-prime,3-prime-dideoxyinosine (ddI). Rat and human ENT2 transported ddC, AZT, and ddI, whereas rat and human ENT1 transported ddC and ddI only. Relative to uridine, ENT2 mediated substantially larger fluxes of ddC and ddI compared with ENT1. Fusion of the N-terminal half of rat Ent2 with rat Ent1 allowed Ent1 to transport AZT and enhanced its uptake of ddC and ddI, indicating that the N-terminal region of ENT proteins is the major site of 3-prime-deoxy-nucleoside interaction.
Williams et al. (1997) found that the HNP36 gene consists of 12 exons.
Williams et al. (1997) used fluorescence in situ hybridization to localize the HNP36 gene to human chromosome 11q13, within 80 kb of the D11S913 locus.
Crawford, C. R., Patel, D. H., Naeve, C., Belt, J. A. Cloning of the human equilibrative, nitrobenzylmercaptopurine riboside (NBMPR)-insensitive nucleoside transporter ei by functional expression in a transport-deficient cell line. J. Biol. Chem. 273: 5288-5293, 1998. [PubMed: 9478986] [Full Text: https://doi.org/10.1074/jbc.273.9.5288]
Griffiths, M., Yao, S. Y. M., Abidi, F., Phillips, S. E. V., Cass, C. E., Young, J. D., Baldwin, S. A. Molecular cloning and characterization of a nitrobenzylthioinosine-insensitive (ei) equilibrative nucleoside transporter from human placenta. Biochem. J. 328: 739-743, 1997. [PubMed: 9396714] [Full Text: https://doi.org/10.1042/bj3280739]
Ward, J. L., Sherali, A., Mo., Z.-P., Tse, C.-M. Kinetic and pharmacological properties of cloned human equilibrative nucleoside transporters, ENT1 and ENT2, stably expressed in nucleoside transporter-deficient PK15 cells: ENT2 exhibits a low affinity for guanosine and cytidine but a high affinity for inosine. J. Biol. Chem. 275: 8375-8381, 2000. [PubMed: 10722669] [Full Text: https://doi.org/10.1074/jbc.275.12.8375]
Williams, J. B., Lanahan, A. A. A mammalian delayed-early response gene encodes HNP36, a novel, conserved nucleolar protein. Biochem. Biophys. Res. Commun. 213: 325-333, 1995. [PubMed: 7639753] [Full Text: https://doi.org/10.1006/bbrc.1995.2133]
Williams, J. B., Rexer, B., Sirripurapu, S., John, S., Goldstein, R., Phillips, J. A., III, Haley, L. L., Sait, S. N. J., Shows, T. B., Smith, C. M., Gerhard, D. S. The human HNP36 gene is localized to chromosome 11q13 and produces alternative transcripts that are not mutated in multiple endocrine neoplasia, type 1 (MEN I) syndrome. Genomics 42: 325-330, 1997. [PubMed: 9192854] [Full Text: https://doi.org/10.1006/geno.1997.4751]
Yao, S. Y. M., Ng, A. M. L., Sundaram, M., Cass, C. E., Baldwin, S. A., Young, J. D. Transport of antiviral 3-prime-deoxy-nucleoside drugs by recombinant human and rat equilibrative, nitrobenzylthioinosine (NBMPR)-insensitive (ENT2) nucleoside transporter proteins produced in Xenopus oocytes. Molec. Membr. Biol. 18: 161-167, 2001. [PubMed: 11463208] [Full Text: https://doi.org/10.1080/09687680110048318]