Mk. Farmer et al., Cloning and characterization of human NTT5 and v7-3: Two orphan transporters of the Na+/Cl--dependent neurotransmitter transporter gene family, GENOMICS, 70(2), 2000, pp. 241-252
Orphan transporters form a growing subfamily of genes related by sequence s
imilarity to the Na+/Cl(-)dependent neurotransmitter superfamily. Using a c
ombination of database similarity searching and cloning methods, we have id
entified and characterized two novel human orphan transporter genes, v7-3 a
nd NTT5. Similar to other known orphan transporters, v7-3 and NTT5 contain
12 predicted transmembrane domains, intracellular N- and C-terminal domains
, and large extracellular loops between transmembrane (TM) domains 3 and 4
and between TRI domains 7 and 8. Residues within the extracellular loops ar
e also predicted to contain sites for N-linked glycosylation. Human v7-3, t
he species orthologue of rat v7-3, contains an open reading frame (ORF) of
730 amino acids. Human NTT5 is a new member of the orphan transporter famil
y and has an ORF of 736 amino acids. The amino acid sequences of human v7-3
and NTT5 are greater than 50% similar to other known orphan neurotransmitt
er transporters and also show sequence similarity to the human serotonin an
d dopamine transporters. Radiation hybrid mapping located the human v7-3 an
d NTT5 genes on chromosomes 12q21.3-q21.4 and 19q13.1-q13.4, respectively.
Human mRNA distribution analysis by TaqMan reverse transcription-polymerase
chain reaction showed that v7-3 mRNA is predominantly expressed in neurona
l tissues, particularly amygdala, putamen, and corpus callosum, with low-le
vel expression in peripheral tissues. In contrast, NTT5 mRNA was highly exp
ressed in peripheral tissues, particularly in testis, pancreas, and prostat
e. Transient transfection with epitope-tagged transporter constructs demons
trated v7-3 to be expressed at the cell surface, whereas NTT5 was predomina
ntly intracellular, suggestive of a vesicular location. Although the substr
ates transported by these transporters remain unknown, their specific but w
idespread distribution suggests that they may mediate distinct and importan
t functions within the brain and the periphery. (C) 2000 Academic Press.