2.A.40 The Nucleobase/Ascorbate Transporter (NAT) or Nucleobase:Cation Symporter-2 (NCS2) Family

The NCS2 family, also called the nucleobase/ascorbate transporter (NAT) family (Koukaki et al. 2005; Karatza et al., 2006), consists of over 1000 sequenced proteins derived from Gram-negative and Gram-positive bacteria, archaea, fungi, plants and animals. Of the five known families of transporters that act on nucleobases, it is the only one that is widespread (;(Gournas et al. 2008; Diallinas and Gournas 2013;  (Frillingos 2012).  Many functionally characterized members are specific for nucleobases including both purines and pyrimidines, but others are purine-specific. However, two closely related rat/mouse/human members of the family, SVCT1 and SVCT2, localized to different tissues of the body, cotransport L-ascorbate and Na+ with a high degree of specificity and high affinity for the vitamin (Diallinas and Gournas 2011). Clustering of NAT/NCS2 family members on the phylogenetic tree is complex with bacterial proteins and eukaryotic proteins each falling into at least three distinct clusters. The plant and animal proteins cluster loosely together, but the fungal proteins branch from one of the three bacterial clusters (Gournas et al. 2008). E. coli possesses four distantly related paralogous members of the NCS2 family.  Evidence that this family is a member of the APC superfamily has been presented (Wong et al. 2012).

Proteins of the NCS2 family are 414-650 amino acyl residues in length and probably possess 14 TMSs. Lu et al. (2011) have concluded from x-ray crystallography that UraA (2.A.40.1.1) has 14 TMSs with two 7 TMS inverted repeats. A pair of antiparallel β-strands is located between TMS 3 and TMS 10 and has an important role in structural organization and substrate recognition. The structure is spatially arranged into a core domain and a gate domain. Uracil, located at the interface between the two domains, is coordinated mainly by residues from the core domain. Structural analyses and relationships to other structurally members of the APC superfamily suggest that alternating access of the substrate may be achieved through conformational changes of the gate domain (Wong et al. 2012). 

The first 3-d structure of a eukaryotic NCS2 family member to be crystalized was that of UapA (Alguel et al. 2016).  This structure is similar to UraA, but additionally revealed that NATs dimerize and that the dimer is probably the functional unit. Dimerization appeared to be critical for specificity. Subsequent publications on UraA showed that this porter is also dimeric (Yu et al. 2017). Further analyses confirmed primary sequence comparitive data showing that the NCB2 family is a member of the APC superfamily (Vastermark et al. 2014). This conclusion has been further verified (Chang and Geertsma 2017).  The 7+7 TMS inverted repeat topology of UapA/UraA is also found in several transporters of the APC suprefamily with little primary amino acid sequence similarity with NATs, such as AzgA-like purine transporters (TC# 2.A.40.7.1), plant boron transporters Bor1-3 (e.g., TC# 2.A.31.3), the human Band3 anion exchanger (TC#2.A.31.1.1), and members of SulP transporter family (TC# 2.A.53). All these may be homodimeric transporters which seem to function via the so-called “elevator mechanism” of transport.

The generalized transport reactions catalyzed by proteins of the NAT/NCS2 are:

Nucleobase (out) +  H+ (out) → Nucleobase (in) + H+ (in)

Ascorbate (out) +  Na+ (out) → Ascorbate (in) +  Na+ (in).

This family belongs to the APC Superfamily.



Alguel, Y., S. Amillis, J. Leung, G. Lambrinidis, S. Capaldi, N.J. Scull, G. Craven, S. Iwata, A. Armstrong, E. Mikros, G. Diallinas, A.D. Cameron, and B. Byrne. (2016). Structure of eukaryotic purine/H+ symporter UapA suggests a role for homodimerization in transport activity. Nat Commun 7: 11336.

Amillis, S., V. Kosti, A. Pantazopoulou, E. Mikros, and G. Diallinas. (2011). Mutational analysis and modeling reveal functionally critical residues in transmembrane segments 1 and 3 of the UapA transporter. J. Mol. Biol. 411: 567-580.

Andersen, P.S., D. Frees, R. Fast and B. Mygind (1995). Uracil uptake in Escherichia coli K-12: Isolation of uraA mutants and cloning of the gene. J. Bacteriol. 177: 2008—2013.

Argyrou, E,. V. Sophianopoulou, N. Schultes, and G. Diallinas. (2001). Functional characterization of a maize purine transporter by expression in Aspergillus nidulans. Plant Cell. 13: 953-964.

Brailoiu, E., R. Hooper, X. Cai, G.C. Brailoiu, M.V. Keebler, N.J. Dun, J.S. Marchant, and S. Patel. (2010). An ancestral deuterostome family of two-pore channels mediates nicotinic acid adenine dinucleotide phosphate-dependent calcium release from acidic organelles. J. Biol. Chem. 285: 2897-2901.

Brynestad, S., L.A. Iwanejko, G.S. Stewart and P.E. Granum (1994). A complex array of Hpr consensus DNA recognition sequences proximal to the enterotoxin gene in Clostridium perfringens type A. Microbiol. 140: 97—104.

Bürzle, M., Y. Suzuki, D. Ackermann, H. Miyazaki, N. Maeda, B. Clémençon, R. Burrier, and M.A. Hediger. (2013). The sodium-dependent ascorbic acid transporter family SLC23. Mol Aspects Med 34: 436-454.

Chang, Y.N. and E.R. Geertsma. (2017). The novel class of seven transmembrane segment inverted repeat carriers. Biol Chem 398: 165-174.

Daruwala, R., J. Song, W.S. Koh, S.C. Rumsey, M. Levine (1999). Cloning and functional characterization of the human sodium-dependent vitamin C transporters hSVCT1 and hSVCT2. FEBS Lett. 460: 480-484.

de Koning, H. and G. Diallinas (2000). Nucleobase transporters. Molec. Memb. Biol. 75:75-94.

Diallinas, G. and C. Gournas. (2013). Structure-function relationships in the nucleobase-ascorbate transporter (NAT) family: lessons from model microbial genetic systems. Channels (Austin) 2: 363-372.

Diallinas, G., J. Valdez, V. Sophianopoulou, A. Rosa and C. Scazzocchio (1998). Chimeric purine transporters of Aspergillus nidulans define a domain critical for function and specificity conserved in bacterial, plant and metazoan homologues. EMBO J. 17: 3827-3837.

Diallinas, G., L. Gorfinkiel, H.N. Arst, Jr., G. Cecchetto and C. Scazzocchio (1995). Genetic and molecular characterization of a gene encoding a wide specificity purine permease of Aspergillus nidulans reveals a novel family of transporters conserved in prokaryotes and eukaryotes. J. Biol. Chem. 270: 8610—8622.

Frillingos, S. (2012). Insights to the evolution of Nucleobase-Ascorbate Transporters (NAT/NCS2 family) from the Cys-scanning analysis of xanthine permease XanQ. Int J Biochem Mol Biol 3: 250-272.

Georgopoulou, E., G. Mermelekas, E. Karena, and S. Frillingos. (2010). Purine substrate recognition by the nucleobase-ascorbate transporter signature motif in the YgfO xanthine permease: ASN-325 binds and ALA-323 senses substrate. J. Biol. Chem. 285: 19422-19433.

Ghim, S.Y. and J. Neuhard (1994). The pyrimidine biosynthesis operon of the thermophile Bacillus caldolyticus includes genes for uracil phosphoribosyltransferase and uracil permease. J. Bacteriol. 176: 3698—3707.

Godoy, A., V. Ormazabal, G. Moraga-Cid, F.A. Zuniga, P. Sotomayor, V. Barra, O. Vasquez, V. Montecinos, L. Mardones, C. Guzman, M. Villagran, L.G. Aguayo, S.A. Onate, A.M. Reyes, J.G. Carcamo, C.I. Rivas, and J.C. Vera. (2007). Mechanistic insights and functional determinants of the transport cycle of the ascorbic acid transporter SVCT2. Activation by sodium and absolute dependence on bivalent cations. J. Biol. Chem. 282: 615-624.

Gorfinkiel, L., G. Diallinas and C. Scazzocchio (1993). Sequence and regulation of the uapA gene encoding a uric acid-xanthine permease in the fungus Aspergillus nidulans. J. Biol. Chem. 268: 23376—23381.

Gournas, C., I. Papageorgiou, and G. Diallinas. (2008). The nucleobase-ascorbate transporter (NAT) family: genomics, evolution, structure-function relationships and physiological role. Mol Biosyst 4: 404-416.

Hou, J., A. Renigunta, J. Yang, and S. Waldegger. (2010). Claudin-4 forms paracellular chloride channel in the kidney and requires claudin-8 for tight junction localization. Proc. Natl. Acad. Sci. USA 107: 18010-18015.

Kalli, A.C., M.S. Sansom, and R.A. Reithmeier. (2015). Molecular dynamics simulations of the bacterial UraA H+-uracil symporter in lipid bilayers reveal a closed state and a selective interaction with cardiolipin. PLoS Comput Biol 11: e1004123.

Karatza, P. and S. Frillingos. (2006). Cloning and functional characterization of two bacterial members of the NAT/NCS2 family in Escherichia coli. Mol. Membr. Biol. 22: 251-261.

Karatza, P., P. Panos, E. Georgopoulou, and S. Frillingos. (2006). Cysteine-scanning analysis of the nucleobase-ascorbate transporter signature motif in YgfO permease of Escherichia coli: Gln-324 and Asn-325 are essential, and Ile-329-Val-339 form an α-helix. J. Biol. Chem. 281: 39881-39890.

Karena, E. and S. Frillingos. (2011). The role of transmembrane segment TM3 in the xanthine permease XanQ of Escherichia coli. J. Biol. Chem. 286: 39595-39605.

Karena, E., E. Tatsaki, G. Lambrinidis, E. Mikros, and S. Frillingos. (2015). Analysis of conserved NCS2 motifs in the Escherichia coli xanthine permease XanQ. Mol. Microbiol. 98: 502-517.

Kim, K.S., J.G. Pelton, W.B. Inwood, U. Andersen, S. Kustu, and D.E. Wemmer. (2010). The Rut pathway for pyrimidine degradation: novel chemistry and toxicity problems. J. Bacteriol. 192: 4089-4102.

Kosti, V., G. Lambrinidis, V. Myrianthopoulos, G. Diallinas, and E. Mikros. (2012). Identification of the Substrate Recognition and Transport Pathway in a Eukaryotic Member of the Nucleobase-Ascorbate Transporter (NAT) Family. PLoS One 7: e41939.

Kosti, V., I. Papageorgiou, and G. Diallinas. (2010). Dynamic elements at both cytoplasmically and extracellularly facing sides of the UapA transporter selectively control the accessibility of substrates to their translocation pathway. J. Mol. Biol. 397: 1132-1143.

Koukaki, M., A. Vlanti, S. Goudela, A. Pantazopoulou, H. Gioule, S. Tournaviti, and G. Diallinas. (2005). The nucleobase-ascorbate transporter (NAT) signature motif in UapA defines the function of the purine translocation pathway. J. Mol. Biol. 350: 499-513.

Kozmin, S.G., E.I. Stepchenkova, S.C. Chow, and R.M. Schaaper. (2013). A critical role for the putative NCS2 nucleobase permease YjcD in the sensitivity of Escherichia coli to cytotoxic and mutagenic purine analogs. MBio 4: e661-66113.

Krypotou E., Lambrinidis G., Evangelidis T., Mikros E. and Diallinas G. (2014). Modelling, substrate docking and mutational analysis identify residues essential for function and specificity of the major fungal purine transporter AzgA. Mol Microbiol. 93(1):129-45.

Krypotou, E. and G. Diallinas. (2014). Transport assays in filamentous fungi: kinetic characterization of the UapC purine transporter of Aspergillus nidulans. Fungal Genet Biol 63: 1-8.

Loh, K.D., P. Gyaneshwar, E. Markenscoff Papadimitriou, R. Fong, K.S. Kim, R. Parales, Z. Zhou, W. Inwood, and S. Kustu. (2006). A previously undescribed pathway for pyrimidine catabolism. Proc. Natl. Acad. Sci. USA 103: 5114-5119.

Lu, F., S. Li, Y. Jiang, J. Jiang, H. Fan, G. Lu, D. Deng, S. Dang, X. Zhang, J. Wang, and N. Yan. (2011). Structure and mechanism of the uracil transporter UraA. Nature 472: 243-246.

Mackenzie, B., A.C. Illing, and M.A. Hediger. (2008). Transport model of the human Na+-coupled L-ascorbic acid (vitamin C) transporter SVCT1. Am. J. Physiol. Cell Physiol. 294: C451-459.

Martinussen, J., J. Schallert, B. Andersen, and K. Hammer. (2001). The pyrimidine operon pyrRPB-carA from Lactococcus lactis. J. Bacteriol. 183: 2785-2794.

Martzoukou O., Karachaliou M., Yalelis V., Leung J., Byrne B., Amillis S. and Diallinas G. (2015). Oligomerization of the UapA Purine Transporter Is Critical for ER-Exit, Plasma Membrane Localization and Turnover. J Mol Biol. 427(16):2679-96.

Moraes, T.F. and R.A. Reithmeier. (2012). Membrane transport metabolons. Biochim. Biophys. Acta. 1818: 2687-2706.

Papageorgiou, I., C. Gournas, A. Vlanti, S. Amillis, A. Pantazopoulou, and G. Diallinas. (2008). Specific interdomain synergy in the UapA transporter determines its unique specificity for uric acid among NAT carriers. J. Mol. Biol. 382: 1121-1135.

Papakostas, K. and S. Frillingos. (2012). Substrate selectivity of YgfU, a uric acid transporter from Escherichia coli. J. Biol. Chem. 287: 15684-15695.

Papakostas, K., E. Georgopoulou, and S. Frillingos. (2008). Cysteine-scanning analysis of putative helix XII in the YgfO xanthine permease: ILE-432 and ASN-430 are important. J. Biol. Chem. 283: 13666-13678.

Papakostas, K., M. Botou, and S. Frillingos. (2013). Functional identification of the hypoxanthine/guanine transporters YjcD and YgfQ and the adenine transporters PurP and YicO of Escherichia coli K-12. J. Biol. Chem. 288: 36827-36840.

Quinn, C.L., B.T. Stephenson and R.L. Switzer (1991). Functional organization and nucleotide sequence of the Bacillus subtilis pyrimidine biosynthetic operon. J. Biol. Chem. 266: 9113—9127.

Saier, M.H., Jr., B.H. Eng, S. Fard, J. Garg, D.A. Haggerty, W.J. Hutchinson, D.L. Jack, E.C. Lai, H.J. Liu, D.P. Nusinew, A.M. Omar, S.S. Pao, I.T. Paulsen, J.A. Quan, M. Sliwinski, T.-T. Tseng, S. Wachi and G.B. Young (1999). Phylogenetic characterization of novel transport protein families revealed by genome analyses. Biochim. Biophys. Acta 1422: 1-56.

Scheers NM. and Sandberg AS. (2011). Iron regulates the uptake of ascorbic acid and the expression of sodium-dependent vitamin C transporter 1 (SVCT1) in human intestinal Caco-2 cells. Br J Nutr. 105(12):1734-40.

Schultz, A.C., P. Nygaard, and H.H. Saxild. (2001). Functional analysis of 14 genes that constitute the purine catabolic pathway in Bacillus subtilis and evidence for a novel regulon controlled by the PucR transcription activator. J. Bacteriol. 183: 3293-3302.

Tsukaguchi, H., T. Tokui, B. Mackenzie, U.V. Berger, X.Z. Chen, Y. Wang, R.F. Brubaker, and M.A. Hediger. (1999). A family of mammalian Na+-dependent L-ascorbic acid transporters. Nature 399: 70-75.

Turner, R.J., Y. Lu and R.L. Switzer (1994). Regulation of the Bacillus subtilis pyrimidine biosynthetic (pyr) gene cluster by an autogenous transcriptional attenuation mechanism. J. Bacteriol. 176: 3708—3722.

Vastermark, A., S. Wollwage, M.E. Houle, R. Rio, and M.H. Saier, Jr. (2014). Expansion of the APC superfamily of secondary carriers. Proteins 82: 2797-2811.

Vlanti, A., S. Amillis, M. Koukaki, and G. Diallinas. (2006). A novel-type substrate-selectivity filter and ER-exit determinants in the UapA purine transporter. J. Mol. Biol. 357: 808-819.

Wong, F.H., J.S. Chen, V. Reddy, J.L. Day, M.A. Shlykov, S.T. Wakabayashi, and M.H. Saier, Jr. (2012). The amino acid-polyamine-organocation superfamily. J. Mol. Microbiol. Biotechnol. 22: 105-113.

Yu, X., G. Yang, C. Yan, J.L. Baylon, J. Jiang, H. Fan, G. Lu, K. Hasegawa, H. Okumura, T. Wang, E. Tajkhorshid, S. Li, and N. Yan. (2017). Dimeric structure of the uracil:proton symporter UraA provides mechanistic insights into the SLC4/23/26 transporters. Cell Res 27: 1020-1033.


TC#NameOrganismal TypeExample

Uracil permease, UraA. The crystal structure of UraA with bound uracil at 2.8 Å resolution is available (PDB: 3QE7) (Lu et al., 2011). UraA has a novel structural fold, with 14 TMSs divided into two inverted repeats. A pair of antiparallel β-strands is located between TMS3 and TMS10 and has an important role in structural organization and substrate recognition. The structure is spatially arranged into a core domain and a gate domain. Uracil, located at the interface between the two domains, is coordinated mainly by residues from the core domain. Structural analysis suggests that alternating access of the substrate may be achieved through conformational changes of the gate domain.  Multiscale molecular dynamics simulations of the UraA symporter in phospholipid bilayers revealed a closed state with 3 high affinity binding sites for cardolipin (Kalli et al. 2015).The crystal structure of UraA bound to uracil in an occluded state at 2.5 A resolution (Yu et al. 2017). UraA shows substantial motions between the core domain and the gate domain as well as intra-domain rearrangements of the gate domain. The occluded UraA forms a dimer wherein the gate domains are sandwiched by two core domains. Dimer formation is necessary for transport activity (Yu et al. 2017).


UraA of E. coli (P0AGM7)


High affinity uracil permease (Martinussen et al. 2001).


PyrP of Lactococcus lactis (gbCAB89870)


Putative pyrimidine permease, RutG (Loh et al., 2006; Kim et al. 2010).


RutG of E. coli (P75892)

2.A.40.1.4Uracil permease BacilliPyrP of Bacillus subtilis

TC#NameOrganismal TypeExample
2.A.40.2.1Purine permease Bacteria YcpX of Clostridium perfringens

Putative xanthine/uracil/vitamin C permease of 529 aas and 13 TMSs.

Putative permease of Methanocorpusculum labreanum


Uncharacterized putative purine permease of 427 aas and 13 or 14 TMSs. 

UP of Pyrococcus furiosus


TC#NameOrganismal TypeExample
2.A.40.3.1Xanthine permease Bacteria PbuX (XanP) of Bacillus subtilis

Uric acid permease of 449 aas and 13 TMSs, PucJ.  May function together with PucK (TC# 2.A.40.3.4) (Schultz et al. 2001).


PucJ of Bacillus subtilis


Uric acid (urate) permease YgfU. YgfU exhibits low affinity (0.5 mM) but high capacity for urate and very poor activity for xanthine.  Essential residues were identified.  Coversion of Thr-100 to ala resulted in efficient xanthine transport (Papakostas and Frillingos 2012).  


YgfU of Escherichia coli


Uric acid uptake porter of 430 aas and 13 TMSs, PucK.  May function together with PucJ (TC# 2.A.40.3.2) (Schultz et al. 2001). 

PucK of Bacillus subtilis


TC#NameOrganismal TypeExample

High affinity uric acid-xanthine permease, UapA. Functionaly critical residues in transmembrane segments 1 and 3 have been identified (Amillis et al., 2011). The substrate recognition and transport pathway have been proposed (Kosti et al., 2012; Kosti et al. 2010).  UapA oligomerization is essential for membrane trafficking and turnover and is a common theme in fungi and mammalian cells (Martzoukou et al. 2015).  Specificity is determined by the interactions of a given substrate with the TMS8-9 loop and by interactions of this loop with TMS1 and TMS12 (Papageorgiou et al. 2008). F528 and Q408 in TMS 12 are important for substrate recognition, and mutation of the former results in high efficiency uptake of several purines and pyrimidines not otherwise transported (Vlanti et al. 2006). A high resolution structure of UapA is available, and it is formed from two domains, a core domain and a gate domain, similar to the previously solved uracil transporter UraA, which belongs to the same family (Alguel et al. 2016). The structure shows UapA in an inward-facing conformation with xanthine bound to residues in the core domain. Unlike UraA, which is a monomer, UapA forms a dimer in the crystals with dimer interactions formed exclusively through the gate domain. Analysis of dominant negative mutants is consistent with dimerization playing a key role in transport. Alguel et al. 2016 postulated that UapA uses an elevator transport mechanism likely to be shared with other structurally homologous transporters including anion exchangers and prestin.


UapA of Emericella (Aspergillus) nidulans

2.A.40.4.2The putative xanthine permease, YicE (Karatza and Frillingos, 2005)BacteriaYicE of E. coli (POAGM9)

The YgfO (XanQ) purine (xanthine) transporter. Residues involved in substrate binding have been identified (Georgopoulou et al., 2010). TMS3 functions in substrate recognition (Karena and Frillingos, 2011).  Many more essential residues have more recently been identified (Karena et al. 2015).


XanQ of E. coli (P67444)


Purine (uric acid and xanthine) permease, UapC.  Present in many Ascomycetes (Krypotou and Diallinas 2014).


UapC of Emericella nidulans


TC#NameOrganismal TypeExample

Putative purine permease, YbbY.  The ybbY gene is in an operon involved with allantoin metabolism, and is flanked by allB, encoding allantoinase, and the glxK gene, encoding glycerate kinase II. Downstream of glxK is YlbA, encoding S-uridoglycine aminohydrolase, the second enzyme involved in allantoin degradation (Moraes and Reithmeier 2012).


YbbY of E. coli


Putative purine permease of 440 aas and 13 TMSs, YwdJ.  It belongs to the HCO3_cotransp/Xan_ur_permease families in CDD.

YwkJ of Bacillus subtilis


Putative xanthine/uracil/vitamin C permease of 431 aas and 12 TMSs.

UP of Sebaldella termitidis


TC#NameOrganismal TypeExample

L-ascorbate:Na+ symporter, SVCT1. (L-ascorbate:Na+= 1:2; Mackenzie et al., 2008). Iron regulates SVCT1 in human intestinal Caco-2 cells (Scheers and Sandberg, 2011).


SVCT1 of Rattus norvegicus


Ca2+/Mg2+-dependent L-ascorbate:Na+ symporter, SVCT2; Na+:ascorbate = 2:1; binding order: Na+, ascorbate, Na+ (Na+ increases the affinity for ascorbate; Ca2+/Mg2+ are required for function) (Godoy et al., 2007; Bürzle et al. 2013).


SLC23A2 of Homo sapiens

2.A.40.6.3High affinity (Km = 30 µM) uric acid-xanthine transporter; leaf permease protein 1, LPE1 (necessary for proper chloroplast development in maize) (Argyrou et al., 2001) PlantsLPE1 of Zea mays (Q41760)

solute carrier family 23 (nucleobase transporters), member 3, SVCT3 or SLC23A3.  Function not certain as of 1/2013 (Bürzle et al. 2013).


SLC23A3 of Homo sapiens


Solute carrier family 23 member 1 (Na+/L-ascorbic acid transporter 1; Sodium-dependent vitamin C transporter 1) (hSVCT1; Yolk sac permease-like molecule 3) (Bürzle et al. 2013).


SLC23A1 of Homo sapiens

2.A.40.6.6Nucleobase-ascorbate transporter 12 (AtNAT12)PlantsNAT12 of Arabidopsis thaliana

TC#NameOrganismal TypeExample

The purine (hypoxanthine/adenine/guanine) transporter, AzgA (Cecchetto et al., 2004).  Topological modeling has revealed a potential substrate binding cavity, and residues important for transport activity have been identified (Krypotou et al. 2014).


AzgA of Aspergillus (Emericella) nidulans (CAE00849)

2.A.40.7.2Hypoxanthine/guanosine uptake transporter, PbuG (Johansen et al., 2003)BacteriaPbuG of Bacillus subtilis (CAB12456)
2.A.40.7.3The purine transporter Azg1 (takes up 8-azadenine and 8-azaguanine but not other toxic nucleobase analogues; similar to Azg2 of A. thaliana (Q84MA8); (Mansfield et al. 2009).


Azg1 of Arabidopsis thaliana (Q9SRK7)


  Adenine permease, YicO.  Also recognizes with low micromolar affinity N(6)-benzoyladenine, 2,6-diaminopurine, and purines (Papakostas et al. 2013).


YicO of Escherichia coli


Purine base permease, GhxP or YjcD.  Transports purines such as guanine, hypoxanthine, and xanthine.  Also transports mutagenic modified purines such as 6-N-hydroxylaminopurine (HAP), 2-amino-HAP (AHAP), 6-mercaptopurine, 6-thioguanine, 1-methylguanine, 8-azaguanine, 6-thioguanine and 2-aminopurine (Kozmin et al. 2013; Papakostas et al. 2013).  


YjcD (GhxP) of E. coli


Adenine permease, PurP.  Also recognize with low micromolar affinity N(6)-benzoyladenine, 2,6-diaminopurine, and purine (Papakostas et al. 2013).


PurP of Escherichia coli


Guanine/hypoxanthine uptake porter of 455 aas, YgfQ.  Also takes up mutagens such as 1-methylguanine, 8-azaguanine, 6-thioguanine, and 6-mercaptopurine (Papakostas et al. 2013).


YgfQ of E. coli