Molecular evolution of the equilibrative nucleoside transporter family: Identification of novel family members in prokaryotes and eukaryotes

Equilibrative nucleoside transporters (ENTs) are integral membrane proteins which enable the movement of hydrophilic nucleosides and nucleoside analogs down their concentration gradients across cell membranes. ENTs were only recently characterized at the molecular level, and little is known about the tertiary structure or distribution of these proteins in nonmammalian organisms. To identify conserved regions, residues, and motifs of ENTs that may indicate functionally important parts of the protein and to better understand the evolutionary history of this protein family, we conducted an exhaustive analysis to characterize and compare ENTs in taxonomically diverse organisms. We have identified novel ENT family members in humans, mice, fish, tunicates, slime molds, and bacteria. This greatly extends our knowledge on the distribution of the ENTs in eukaryotes, and we have identified, for the first time, family members in bacteria. The prokaryote ENTs are attractive models for future studies on transporter tertiary structure and mechanism of substrate translocation. Using sequence similarities, we have identified regions, residues, and motifs that are conserved across all family members. These areas are presumably correlated with function and therefore are important targets for future analysis. Finally, we propose an evolutionary history for the ENT family which clarifies the origin(s) of multiple isoforms in different taxa.