Conditionally expressed Gα_(15) couples to endogenous receptors in GH_3 cells

Abstract

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 suggested that they can be used as tools in agonist-screening systems. Using the reversed tetracycline-controlled transactivation system we generated rat pituitary GH_3 cell clones that expressed Gα_(15) and Gα_(16) conditionally to study the coupling of endogenous receptors to both G proteins. In cells expressing moderate levels of Gα_(15), activation of various endogenous receptors increased inositol phosphate production, whereas conditional expression of Gα_(16) had no significant effect on agonist-dependent PLC activity. Activation of PLC through Gα_(15) in response to carbachol did not increase cytosolic [Ca2+] ([Ca^(2+)]_i) but stimulated protein kinase C. While carbachol decreased the secretory activity in non-induced GH_3 cells, it increased secretion in cells expressing Gα_(15). Our data demonstrate that Gα_(15) has a higher functional promiscuity than Gα_(16) when studied in a system that preserves physiological G protein and receptor levels. In addition, Gα_(15)-mediated coupling of a receptor to PLC can change the cellular response to receptor agonists, indicating that downstream cellular functions can be used to detect receptor activation in screening systems employing a promiscuous G protein.

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