MUTANT ALPHA-SUBUNITS OF GI2 INHIBIT CYCLIC-AMP ACCUMULATION

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 alpha or by the beta-gamma-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)), constitutively activate it by replacing either of two conserved amino acids in its alpha-subunit (alpha-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 alpha-subunit of G(i2) replace a residue cognate to one of those affected by gsp mutations 8. This implies that the mutations convert the alpha-i2 gene into a dominantly acting oncogene, called gip2 (ref. 8), and that the mutant alpha-i2 subunits are constitutively active. We have therefore assessed cyclic AMP accumulation in cultured cells which stably or transiently express exogenous wild-type alpha-i2 complementary DNA or either of two mutant alpha-i2 cDNAs. The results show that putatively oncogenic mutations in alpha-i2 constitutively activate the protein's ability to inhibit cAMP accumulation.