Cloning and expression of a betaine/GABA transporter from human brain

A cDNA clone encoding a human gamma-aminobutyric acid (GABA) transporter has been isolated from a brain cDNA library, and its functional properties have been examined in mammalian cells. The nucleotide sequence predicts a transporter with 614 amino acids and 12 putative transmembrane domains. The highest degree of amino acid identity is with a betaine/GABA transporter originally cloned from the dog termed BGT-1 (91%) and a related transporter from mouse brain (87%). These identities are similar to those for species homologues of other neurotransmitter transporters and suggest that the new clone represents the human homologue of BGT-1. The transporter displays high affinity for GABA (IC50 of 30 microM) and is also sensitive to phloretin, L-2,4-diaminobutyric acid, and hypotaurine (IC50 values of approximately 150-400 microM). The osmolyte betaine is approximately 25-fold weaker than GABA, displaying an IC50 of approximately 1 mM. The relative potencies of these inhibitors at human BGT-1 differ from those of mouse and dog BGT-1. Northern blot analysis reveals that BGT-1 mRNA is widely distributed throughout the human brain. The cloning of the human homologue of BGT-1 will further our understanding of the roles of GABA and betaine in neural function.