Determination of the minimal functional ligand-binding domain of the GABAB(1b) receptor

2005 
In the mammalian central nervous system, slow inhibitory neurotransmission is largely mediated by metabotropic GABA B receptors (where GABA stands for γ-aminobutyric acid), which belong to the G-protein-coupled receptor gene family. Functional GABA B receptors are assembled from two subunits GABA B(1) (GABA B receptor subtype 1) and GABA B(2) . For the GABA B(1) subunit, which binds the neurotransmitter GABA, two variants GABA B(1a) (GABA B receptor subtype 1 variant a) and GABA B(1b) have been identified. They differ at the very N-terminus of their large glycosylated ECD (extracellular domain). To simplify the structural characterization, we designed truncated GABA B(1) receptors to identify the minimal functional domain which still binds a competitive radioligand and leads to a functional, GABA-responding receptor when co-expressed with GABA B(2) . We show that it is necessary to include all the portion of the ECD encoded by exon 6 to exon 14. Furthermore, we studied mutant GABA B(1b) receptors, in which single or all potential N-glycosylation sites are removed. The absence of oligosaccharides does not impair receptor function, suggesting that the unglycosylated ECD of GABA B(1) can be used for further functional or structural investigations.
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