Conditionally expressed G alpha(15) couples to endogenous receptors in GH(3) cells

The mammalian G proteins G(15) and G(16) Couple a wide variety of receptors to phospholipase C (PLC) in co-transfected systems, and it has been sugges ted that they can be used as tools in agonist-screening systems. Using the reversed tetracycline-controlled transactivation system we generated rat pi tuitary GH(3) cell clones that expressed G alpha (15) and G alpha (16) cond itionally to study the coupling of endogenous receptors to both G proteins. In cells expressing moderate levels of Ga15, activation of various endogen ous receptors increased inositol phosphate production, whereas conditional expression of G alpha (16) had no significant effect on agonist-dependent P LC activity. Activation of PLC through G alpha (15) in response to carbacho l did not increase cytosolic (Ca2+] ([Ca2+)(i)) but stimulated protein kina se C. While carbachol decreased the secretory activity in non-induced GH3 c ells, it increased secretion in cells expressing G alpha (15). Our data dem onstrate that G alpha (15) has a higher functional promiscuity than G alpha (16) when studied in a system that preserves physiological G protein and r eceptor levels. In addition, G alpha (15)-mediated coupling of a receptor t o PLC can change the cellular response to receptor agonists, indicating tha t downstream cellular functions can be used to detect receptor activation i n screening systems employing a promiscuous G protein.