Mutant α subunits of G_i2 inhibit cyclic AMP accumulation

Abstract

ONE or more of three G_i proteins, G_i1–3, mediates hormonal inhibition of adenylyl cyclase^1–3. Whether this inhibition is mediated by the α or by the βγ subunits of G_i proteins is unclear^1,2. Mutations inhibiting the intrinsic GTPase activity of another G protein, the stimulatory regulator of adenylyl cyclase (G_s), con-stitutively activate it by replacing either of two conserved amino acids in its α subunit (α_s)^4–7. These mutations create the gsp oncogene which is found in human pituitary and thyroid tumours^5,8. In a second group of human endocrine tumours, somatic mutations in the a subunit of G_i2 replace a residue cognate to one of those affected by gsp mutations⁸. This implies that the mutations convert the α_i2 gene into a dominantly acting oncogene, called gip2 (ref. 8), and that the mutant α_i2 subunits are constitutively active. We have therefore assessed cyclic AMP accumulation in cultured cells which stably or transiently express exogenous wild-type α_i2 complementary DNA or either of two mutant α_i2 cDNAs. The results show that putatively oncogenic mutations in α_i2 constitutively activate the protein's ability to inhibit cAMP accumulation.