Cloning and characterization of a novel variant of rat GABA(B)R1 with a truncated C-terminus.

The gamma-aminobutyric acid B receptor (GABA(B)R) belong to the G-protein-coupled receptor superfamily and has been identified as a mediator in the transmission of slow inhibitory neurotransmission in the mammalian central nervous system. Two types of GABA(B)R have been cloned, GABA(B)R1 and R2. GABA(B)R2 is co-expressed with GABA(B)R1 in many brain regions and inwardly rectifying potassium channels are activated by GABA(B)R agonists only upon co-expression of GABA(B)R1 with GABA(B)R2. Several splice variants of GABA(B)R1 receptors have been cloned from rat brain library. Using a rat hippocampal cDNA library, we have isolated a novel cDNA clone of GABA(B) receptor containing an insert of 124 bp between exon 3 and exon 4. This insert occurred between the regions encoding the Sushi domain and leucine binding protein (LBP)-like domain. The insert and subsequent frame shift generated a cDNA that codes for a truncated polypeptide of 239 amino acids lacking the C-terminus. Analysis of the deduced amino acid sequence of the new cDNA clone, termed GABA(B)R1g, showed that it was identical to the first 157 amino acids of GABA(B)R1a, but diverged thereafter. The C-terminal region of GABA(B)R1g contained two cysteine residues. GABA(B)R1g was expressed in both brain and peripheral tissues. Northern blot analysis demonstrated that two transcripts (4.5 kb and 4.0 kb) exist in hippocampus. In addition, studies of hippocampus in developing animals indicated that the expression of GABA(B)R1g is maximal at postnatal day four. GABA(B)R1g could be generated by alternative splicing of the GABA(B)R1 gene.