At least three alternatively spliced mRNAs encoding two α subunits of the G(o) GTP-binding protein can be expressed in a single tissue

Hybridization blot (Northern) analysis of mRNA coding for α subunits of the G(o) signal-transducing protein detects three bands at 5.7, 4.2, and 3.2 kilobases (kb). We showed previously that the largest is a splice variant coding for the type 2 form of the polypeptide (α(o2)) and the two smaller RNAs react with a probe specific for the seventh of the eight exons that code for the type 1 form (α(o1)). In the present work we demonstrate that the 3.2- and 4.2-kb mRNAs also result from alternative splicing, the splice site being located 31 nucleotides downstream from the termination codon of the open reading frame, and that therefore the α(o) mRNA is made up of at least nine exons. All three α(o) mRNAs are expressed in both heart and brain, more in the latter than the former, as well as in the hamster insulin-secreting tumor (HIT) cell from which the cDNAs encoding the splice variants had been cloned. In contrast, in lung and testis we found only the 5.7-kb α(o2) mRNA. The same analysis was unable to detect α(o)-specific sequences in either kidney, pancreas (whole), spleen, or liver, while at the same time detecting strong bands for α(s) mRNA. A comparison of the nucleotide sequences of the 5'- and 3'-untranslated regions of the hamster cDNAs cloned here indicated that previously cloned α(o) cDNAs all belong to the same α(o1A) splice subclass derived from 3.2-kb mRNA. The comparison also revealed that the sequence of the untranslated regions are highly conserved among three species (rat, hamster, and brain). Their 3' tails are 99.1% (HIT versus bovine, 200 known bases) and 99.7% (HIT versus rat, 229 bases) identical, and their 5' leader sequences are 92.7% (HIT versus bovine, 165 known bases) and 90.7% (HIT versus rat, 670 bases) identical. This indicates that untranslated regions of mRNAs need not exhibit high degrees of species variation.