Alternative Splicing of the Adenylyl Cyclase Stimulatory G-protein Gαs Is Regulated by SF2/ASF and Heterogeneous Nuclear Ribonucleoprotein A1 (hnRNPA1) and Involves the Use of an Unusual TG 3′-Splice Site

The factors involved in regulating alternative splicing of the human adenylyl cyclase stimulatory G-protein Galpha(s) in different cell types remain undefined. We have designed a Galpha(s) minigene that retains the signals required for Galpha(s) alternative splicing in vivo. Employing transient transfection of human myometrial smooth muscle cells and HeLa cells, as well as in vitro splicing assays, we have provided evidence that the antagonistic splicing factors SF2/ASF and hnRNPA1 act as potent regulators of Galpha(s) isoform expression in these cells. Both SF2/ASF and hnRNPA1 control the selection of competing 5'-splice sites and respectively promote inclusion or skipping of the small cassette-type exon 3 of Galpha(s) transcripts, resulting in the generation of Galpha(s)-long and Galpha(s)-short mRNA isoforms. We have also provided evidence that SF2/ASF and hnRNPA1 play a role in T-splice site selection involving the use of a non-canonical TG 3'-splice site preceding exon 4. Using a score-matrix analysis to identify putative exonic enhancer sequences (ESES), we found multiple high score ESE motifs for SF2/ASF, SC35, and SRp40 in exon 3 of Galpha(s). These results suggest that tissue-specific expression of SF2/ASF and hnRNPA1 governs the expression of alternative isoforms of Galpha(s) in these different cells types.