Nucleosides play other important roles beyond their nucleic acid synthesis building block role. For example, they are involved in energy metabolism; they serve as ligands of purinergic receptors and act as influential signaling molecules. Being hydrophilic, nucleosides cannot simply diffuse across the plasma membrane in order to exert their various functions, but rather need to be physically transported via nucleoside transporters.
Two different transporter families are responsible for transporting nucleosides across the plasma membrane: The Concentrative Nucleoside Transporter proteins (CNT, SLC28 family), which consist of three members, CNT1-3, and act as Na+-dependent symporters. The Equilibrative Nucleoside Transporter proteins ENT1-4 (ENT, SLC29 family), which mediate a Na+-independent facilitated diffusion. Therefore, ENTs act as bidirectional carriers, responsible for the influx and efflux of substrates.