Source: http://tcdb.org/tcfamilybrowse.php?tc=2.A.40
Timestamp: 2019-04-23 13:58:58+00:00

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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.
Ascorbate (out) + Na+ (out) → Ascorbate (in) + Na+ (in).
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High affinity uracil permease (Martinussen et al. 2001).
Putative pyrimidine permease, RutG (Loh et al., 2006; Kim et al. 2010).
Putative xanthine/uracil/vitamin C permease of 529 aas and 13 TMSs.
Uncharacterized putative purine permease of 427 aas and 13 or 14 TMSs.
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).
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.
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).
Purine (uric acid and xanthine) permease, UapC. Present in many Ascomycetes (Krypotou and Diallinas 2014).
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).
Putative purine permease of 440 aas and 13 TMSs, YwdJ. It belongs to the HCO3_cotransp/Xan_ur_permease families in CDD.
Putative xanthine/uracil/vitamin C permease of 431 aas and 12 TMSs.
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).
solute carrier family 23 (nucleobase transporters), member 3, SVCT3 or SLC23A3. Function not certain as of 1/2013 (Bürzle et al. 2013).
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).
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).
2.A.40.7.3 The 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).
Adenine permease, YicO. Also recognizes with low micromolar affinity N(6)-benzoyladenine, 2,6-diaminopurine, and purines (Papakostas et al. 2013).
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).
Adenine permease, PurP. Also recognize with low micromolar affinity N(6)-benzoyladenine, 2,6-diaminopurine, and purine (Papakostas et al. 2013).

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