EXPRESSION OF CONE TRANSDUCIN, G(Z)ALPHA, AND OTHER G-PROTEIN ALPHA-SUBUNIT MESSENGER RIBONUCLEIC-ACIDS IN PANCREATIC-ISLETS

The G-proteins are a family of heterotrimeric guanine nucleotide-binding proteins that play important roles in signal transduction and whose expression is regulated in a tissue-specific manner. Here we have surveyed the expression of G-protein alpha-subunits in mouse pancreatic islets. Degenerate oligonucleotide primers corresponding to conserved primary sequences in known G alpha-subunits were used in a reverse transcriptase-polymerase chain reaction, and the amplified complementary DNA (cDNA) fragments were subcloned and sequenced. Over 100 clones were analyzed, from which we determined that islet cells express at least seven G alpha-subunits: G(8) alpha, G(i1)alpha or G(i3)alpha, G(i2)alpha, G(11)alpha, G(14)alpha, G(2) alpha, and G(t2)alpha (cone transducin). In particular, the identification of G(z) alpha and G(t2)alpha was of interest in that previous studies had indicated that the expression of G(z) alpha was restricted mainly to the brain, retina, and adrenal gland, whereas G(t2)alpha was expressed predominantly in retinal cone photoreceptors. By Western blot analysis, we estimated that the amount of G(z) alpha protein present in mouse islets was about 40% of that in retina. To further investigate the expression of G(t2)alpha, mouse G(t2)alpha cDNA was cloned from a retinal library and sequenced. The cDNA was used as a probe for Northern blot analysis, and the results confirmed that mouse islets contained a substantial level of G(t2)alpha messenger RNA (mRNA), albeit less than that found in retina (similar to 5-fold lower). G(t2)alpha mRNA was also shown to be present in a clonal mouse pancreatic alpha-cell line (alpha TC1-6) as well as in adrenal gland, pituitary, and a clonal mouse anterior pituitary cell line (AtT20). In situ hybridization revealed that G(t2)alpha mRNA was expressed essentially throughout the islet, suggesting that it is normally expressed in the abundant islet beta-cells and possibly others. In situ analysis also showed that G(t2)alpha mRNA expressed in the pituitary was limited to the intermediate and anterior lobes. We conclude that islet cells express multiple G-proteins, including several that are normally expressed at high levels in certain neuronal cells.