Members of the Equilibrative Nucleoside Transporter (ENT) Family
TC# 2.A.57 are transport proteins that are specific to
nucleosides and
nucleobases, and are part of the
major facilitator superfamily. They generally possess at least 6, typically 10,
transmembrane segments (TMSs) and are 300-600 amino acyl residues in length.
Function
ENTs, including those in
parasitic protozoa
Protozoa (singular: protozoan or protozoon; alternative plural: protozoans) are a group of single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic tissues and debris. Histo ...
, function in nucleoside and nucleobase uptake for salvage pathways of nucleotide synthesis and, in humans, are also responsible for the cellular uptake of nucleoside analogues used in the treatment of
cancers and
viral diseases. By regulating the concentration of adenosine available to cell surface receptors, mammalian ENTs additionally influence physiological processes ranging from cardiovascular activity to
neurotransmission.
Human ENTs
In humans ENT are also known as SLC29, a group of
plasmalemmal transport proteins which transport
nucleoside substrates like
adenosine
Adenosine ( symbol A) is an organic compound that occurs widely in nature in the form of diverse derivatives. The molecule consists of an adenine attached to a ribose via a β-N9-glycosidic bond. Adenosine is one of the four nucleoside building ...
into
cells.
There are four known human ENTs, designated
ENT1,
ENT2,
ENT3, and
ENT4
The plasma membrane monoamine transporter (PMAT) is a low-affinity monoamine transporter protein which in humans is encoded by the ''SLC29A4'' gene. It is known alternatively as the human equilibrative nucleoside transporter-4 (hENT4). Unlike othe ...
.
They are
blocked by
adenosine reuptake inhibitors like
dipyridamole and
dilazep, drugs used clinically for their
vasodilatory properties.
The best-characterized members of the human Ent family,
hENT1 and
hENT2, possess similar broad permeant selectivities for purine and pyrimidine nucleosides, but hENT2 also efficiently transports nucleobases. hENT3 has a similar broad permeant selectivity for nucleosides and nucleobases and appears to function in intracellular membranes, including lysosomes.
Gemcitabine, an anti-cancer drug, is transported by hENT1 and hENT3.
hENT4 is uniquely selective for adenosine, and also transports a variety of organic cations.
Transport Reaction
The generalized transport reaction catalyzed by well characterized ENT family members is:
: Nucleoside (out) → Nucleoside (in)
See also
*
Concentrative nucleoside transporter
Human concentrative nucleoside transporters include SLC28A1, SLC28A2 and SLC28A3 proteins. SLC28A2 is a purine-specific Na+-nucleoside cotransporter localised to the bile canalicular membrane. SLC28A1 is a Na+-dependent nucleoside transporter sele ...
*
Nucleoside transporter
Nucleoside transporters (NTs) are a group of membrane transport proteins which transport nucleoside substrates like adenosine
Adenosine ( symbol A) is an organic compound that occurs widely in nature in the form of diverse derivatives. The ...
Further reading
*
References
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Protein families
Membrane proteins
Transmembrane proteins
Transmembrane transporters
Transport proteins
Integral membrane proteins