Alternative Splicing of a 51-Nucleotide Exon that Encodes a Putative Protein Kinase C Phosphorylation Site Generates Two Forms of the Chicken γ-Aminobutyric AcidAReceptor β2 Subunit

: Complementary DNAs that encode two forms of the chicken -γ-aminobutyric acid type A (GABAA) receptor β2 subunit have been isolated. These polypeptides differ by the presence (β2L) or absence (β2S) of 17 amino acids, which contain a possible target for phosphorylation by protein kinase C, in the large intracellular loop between the third and fourth membrane-spanning domains. The extra sequence in the chicken β2L subunit is not found in previously published GABAA receptor β2-subunit sequences. Analysis of genomic DNA has revealed that the two β2-subunit mRNAs arise by alternative splicing of a novel 51-nucleotide exon. Although the two β2-subunit transcripts appear to be present in 1 -day-old chick brain at similar steady-state levels, we have been unable to detect an mRNA for the long form of the β2 subunit in either the bovine or the rat. Because the various GABAA receptor genes are thought to have arisen by duplication of a common ancestor, our data, taken together with that on the γ2 subunit, which occurs in two forms that arise by alternative splicing of a 24-nucleotide exon, suggest that the coding region of the primordial gene or one of its very early descendants contained 10 exons, not nine as previously thought.