| Research Abstract |
Heterotrimeric G proteins play a major role in signal transduction from cell-surface receptors to intracellular effectors. The p and y subunits are tightly bound under physiological conditions, and so far, 11 isoforms of gamma have been found. To elucidate the physiological significance of gamma subunit heterogeneity, we searched the differences among gamma isoforms and obtained following results. 1. In fibroblasts, the gamma12 subunit associated with F-actin, while the gamma5 localized at focal adhesions, suggesting functional differences for the two gamma isoforms. 2. The phosphorylation site of gamma12 by protein kinase C was determined to be Ser-2.3. The gamma12 was phosphorylated in the cells by the stimulation with various reagents, such as phorbol ester, serum, lysophosphatidic acid and growth factors. 4. To examine the roles of gamma subunits, various gamma isoforms were overexpressed in NIH 3T3 cells together with the beta1 subunit. Expression of gamma12 induced cell rounding and increase of cell migration, but expression of other gamma subunits did not induce significant changes. 5. Deletion of the N-terminal region or replacement by alanine of Ser-2 of gamma12 eliminated these effects of gamma12, while a mutant in which Ser-2 was replaced with glutamic acid showed effects equivalent to wild-type gamma12. These results indicated that phosphorylation of gamma12 at Ser-2 enhances the motility of cells. 6. Immunohistochemical studies showed that gamma3 was localized in neuronal cells, while gamma12 was localized some glial cells in the brain. 7. We identified the beta4 protein in the preparation of betagamma5 complex purified from bovine lung. The beta4 was shown to interact selectively with gamma5 and gamma12.
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