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 (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 nucelobases, it is the only one that is widespread (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/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 TM3 and TM10 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 rrelationships 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 3-d structure confirmed primary sequence comparitive data showing that the NCB2 family is a member of the APC superfamily (Vastermark et al. 2014), and this conclusion has been further verified (Chang and Geertsma 2017).
The generalized transport reactions catalyzed by proteins of the NAT/NCS2 are:
Nucleobase (out) H+(out) %u2192 Nucleobase (in) H+(in)
Ascorbate (out) Na+(out) %u2192 Ascorbate (in) Na+(in)