2.A.40 The Nucleobase:Cation Symporter-2 (NCS2) Family

The NCS2 family, also called the nucleobase/ascorbate transporter (NAT) family (Karatza et al., 2006), consists of over 100 currently 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).  Most functionally characterized members are specific for nucleobases including both purines and pyrimidines. 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. Clustering of 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. E. coli possesses four distantly related paralogous members of the NCS2 family.

Proteins of the NCS2 family are 414-650 amino acyl residues in length and probably possess 12-14 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 analysis suggests that alternating access of the substrate may be achieved through conformational changes of the gate domain.

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

Nucleobase (out) H⁺(out) %u2192 Nucleobase (in) H⁺(in).

Ascorbate (out) Na⁺(out) %u2192 Ascorbate (in) Na⁺(in).