Constitutively Active Adenylyl Cyclase Mutant Requires Neither G Proteins nor

Receptor-mediated G protein-linked adenylyl cyclase systems are universal signal transducers. We exploited the essential role of this cascade in Dictyostelium devel-opment to screen for random mutations in the catalytic component, ACA. This enzyme is activated by G protein bg-subunits acting in concert with a novel cytosolic reg-ulator, CRAC. By suppression of the CRAC-null pheno-type, we isolated constitutively active versions of the enzyme that require neither exogenous stimuli nor in-ternal regulators. One mutant displayed a 15-fold in-crease in its Vmax. It harbors a single amino acid substi-tution (L394S) affecting a conserved residue located in the first cytoplasmic loop near the N-terminal hydro-phobic domain of ACA. The screening procedure can be adapted for isolation of constitutive mutations in mam