2.A.39 The Nucleobase:Cation Symporter-1 (NCS1) Family
The NCS1 family consists of over 1000 currently sequenced proteins derived from Gram-negative and Gram-positive bacteria, archaea, yeast, fungi and plants. The bacterial and yeast proteins are widely divergent and do not cluster closely on the NCS1 family phylogenetic tree. B. subtilis possesses two paralogues of the NCS1 family, and S. cerevisiae has several. Two of the yeast proteins (Dal4 and Fur4) cluster tightly together, and three other S. cerevisiae proteins, one of which is the thiamin permease, Thi10, and another of which is the nicotinamide riboside transporter, Nrt1 (Belenky et al., 2008), also cluster tightly together. The latter three proteins are likely to be closely-related thiamin permease isoforms. The yeast cytosine-purine and vitamin B6 transporters cluster loosely together (24% identity; e-50 (Stolz and Vielreicher, 2003). The bacterial proteins are derived from several Gram-negative and Gram-positive species. These proteins exhibit limited sequence similarity with the xanthine permease, PbuX, of Bacillus subtilis which is a member of the NCS2 family. The two families are therefore probably related.
Proteins of the NCS1 family are 419-635 amino acyl residues long and possess twelve putative transmembrane α-helical spanners (TMSs). At least some of them have been shown to function in uptake by substrate:H+ symport. In these respects, and with respect to substrate specificity, these proteins resemble the symporters of the NCS2 family, providing further evidence that the two families represent distant constituents of a single superfamily. The two families probably arose by an early gene duplication event that occurred long before divergence of the three major kingdoms of life. It is possible that they are distant constituents of the MFS (2.A.1).
The nucleobase-cation-symport-1 (NCS1) transporters are essential components of salvage pathways for nucleobases and related metabolites. Weyand et al. 2008 reported the 2.85-angstrom resolution structure of the NCS1 benzyl-hydantoin transporter, Mhp1, from Microbacterium liquefaciens. Mhp1 contains 12 transmembrane helices, 10 of which are arranged in two inverted repeats of five helices. The structures of the outward-facing open and substrate-bound occluded conformations were solved, showing how the outward-facing cavity closes upon binding of substrate. Comparisons with the leucine transporter LeuT(Aa) and the galactose transporter vSGLT reveal that the outward- and inward-facing cavities are symmetrically arranged on opposite sides of the membrane. The reciprocal opening and closing of these cavities is synchronized by the inverted repeat helices 3 and 8, providing the structural basis of the alternating access model for membrane transport (Weyand et al. 2008).
The generalized transport reaction catalyzed by NCS1 family permeases is:
Nucleobase or Vitamin (out) + H+ (out) → Nucleobase or Vitamin (in) + H+ (in)