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 sequences 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.