Differential degradation rates of the G protein alpha o in cultured cardiac and pituitary cells

Signal transduction in biological membranes is modulated by a family of GTP-binding proteins termed G proteins. Differences in the tissue-specific expression of G protein subtypes suggest that the levels of individual G proteins may be an important determinant of the hormonal response in a given cell type. We have used a polyclonal antibody raised against the purified G protein, alpha o to study alpha o in the rat pituitary cell line GH4 and in primary rat cardiocytes in culture by quantitative immunoprecipitation. Biosynthetic labeling and specific immunoprecipitation of alpha o in pulse-chase experiments demonstrated that the t1/2 for alpha o degradation is 28 +/- 7 h (n = 4) in GH4 pituitary cells and is greater than 72 h (n = 4) in cardiocytes. The steady-state level of alpha o protein is similar in both cell types as measured by Western blots. Northern blots of poly(A)-selected mRNA from these two cell types were probed with labeled alpha o cDNA and showed they have similar alpha o mRNA levels. The observation of different degradation rates, but similar steady-state protein levels, suggests that the rate of alpha o synthesis is different in GH4 cells and cardiocytes. Since mRNA levels are approximately equal in both, our studies imply that protein translation controls may be important determinants of G protein alpha subunit concentrations in biological membranes.