Genetic selection in Saccharomyces of mutant mammalian adenylyl cyclases with elevated basal activities

We show that co-expression of rat Gαs together with type I, II, IV, or VI mammalian adenylyl cyclase (AC) can suppress the growth defect of cyr1 strains of Saccharomyces cerevisiae, which lack a functional endogenous AC. Complemention of cyr1 is not observed in the absence of Gαs, indicating that the mammalian ACs retain their normal regulatory behavior in yeast. Selection for Gαs-independent growth of cyr1 strains expressing type IV AC yielded several ACIV mutants with enhanced basal activity, each of which had a single amino acid substitution in the conserved C_1a or C_2a region of the protein. Expression of two of the mutant ACs in HEK293 cells resulted in increased levels of cAMP and elevated adenylyl cyclase activity. Further selection for reverting mutations in one of these constitutively active AC mutants yielded three independent intragenic suppressor mutations. The distribution of the activating and suppressor mutations throughout both C_1a and C_2a is consistent with a model in which the enhanced basal activity results from an increase in the affinity between C_1a and C_2a. These results demonstrate the utility of Saccharomyces as a tool for the identification of informative mutant forms of mammalian ACs.