Distinct roles for G alpha(i)2, G alpha(i)3, and G beta gamma in modulation of forskolin- or G(s)-mediated cAMP accumulation and calcium mobilizationby dopamine D2S receptors

Previous studies have shown that a single G protein-coupled receptor can re gulate different effector systems by signaling through multiple subtypes of heterotrimeric G proteins. In LD2S fibroblast cells, the dopamine D2S rece ptor couples to pertussis toxin (PTX)-sensitive G(i)/G(o) proteins to inhib it forskolin- or prostaglandin E-1-stimulated cAMP production and to stimul ate calcium mobilization, To analyze the role of distinct G alpha(i/o) prot ein subtypes, LD2S cells were stably transfected with a series of PTX-insen sitive G alpha(i/o) protein Cys --> Ser point mutants and assayed for D2S r eceptor signaling after PTX treatment, The level of expression of the trans fected G alpha mutant subunits was similar to the endogenous level of the m ost abundant G alpha(i/o) proteins (G alpha(o), G alpha(i)3). D2S receptor- mediated inhibition of forskolin-stimulated cAMP production was retained on ly in clones expressing mutant G alpha(i)2. In contrast, the D2S receptor u tilized G alpha(i)3 to inhibit PGE(i)-induced (G(s)-coupled) enhancement of cAMP production. Following stable or transient transfection, no single or pair set of mutant G alpha(i/o) subtypes rescued the D2S-mediated calcium r esponse following PTX pretreatment. On the other hand, in LD2S cells stably transfected with GRK-CT, a receptor kinase fragment that specifically anta gonizes G beta gamma subunit activity, D2S receptor-mediated calcium mobili zation was blocked. The observed specificity of G alpha(i)2 and G alpha(i)3 for different states of adenylyl cyclase activation suggests a higher leve l of specificity for interaction of G alpha(i) subunits with forskolin- ver sus G(s)-activated states of adenylyl cyclase than has been previously appr eciated.