Reconstitution of calmodulin-sensitive adenylate cyclase from bovine brain with phosphatidylcholine liposomes

Protein kinase C alters the responsiveness of adenylyl cyclases to G protein alpha and betagamma subunits

The ability of protein kinase C (PKC) to regulate the responsiveness of adenylyl cyclase to different activators was assessed. Membranes prepared from Sf9 cells infected with recombinant baculoviruses encoding either type II or IV adenylyl cyclase were incubated with recombinant PKCalpha (purified from Sf9 cells), and the effects on adenylyl cyclase activity were measured after reconstitution with Gsalpha, Gbetagamma, or forskolin. PKCalpha treatment of type II adenylyl cyclase leads to increases in basal, forskolin-stimulated, and betagamma-stimulated activities and greater sensitivity to stimulation by Gsalpha. Paradoxically, most of the betagamma potentiation of Gsalpha-stimulated activity is eliminated by pretreatment with PKCalpha. By contrast, treatment of type IV adenylyl cyclase with PKCalpha has little effect on the basal, forskolin-stimulated, or betagamma-stimulated activities but markedly reduces the Gsalpha-stimulated and betagamma-potentiated activity of this isoform. These studies demonstrate that protein kinases can alter both the activity of adenylyl cyclase isoforms and their responsiveness to G protein regulation, thereby altering the ability of adenylyl cyclases to integrate signals derived from multiple hormonal inputs.

Phosphatidylinositol modulates the response of calmodulin-dependent phosphatase to calmodulin

Phosphatidylinositol (PtdIns) and many other phospholipids activated calmodulin (CaM)-dependent phosphatase in the presence or absence of Ca2+, and the stimulation was more pronounced in the presence of Ca2+. In addition, PtdIns modulated the response of phosphatase to CaM: at low and nonstimulatory concentrations (less than 70 microM), PtdIns augmented the activity of phosphatase by a submaximum concentration of CaM, giving a synergistic effect; and at high concentrations (greater than 100 microM), PtdIns suppressed the synergistic effect. Kinetic experiments indicated that PtdIns (both nonstimulatory and stimulatory concentrations) increased the affinity of phosphatase for CaM. In addition to the CaM regulatory site, phosphatase appears to have two PtdIns regulatory sites: a high-affinity site the occupation of which does not stimulate enzyme activity, and a low-affinity site the occupation of which stimulates enzyme activity in the absence of CaM and inhibits it in the presence of CaM. Modulating the response of phosphatase to CaM is not unique to PtdIns, and was observed with other phospholipids, including some that did not stimulate the enzyme. This raises the possibility that certain phospholipids may regulate phosphatase in two ways: (i) direct activation of the enzyme and (ii) modulation of its response to CaM.

Adenosine inhibition of calmodulin-sensitive adenylate cyclase from bovine cerebral cortex

Calmodulin (CaM)-sensitive adenylate cyclase has recently been purified extensively from bovine brain. In this study, the sensitivity of the CaM-sensitive adenylate cyclase to adenosine and adenosine analogs was examined. The highly purified enzyme preparation retained sensitivity to inhibition by adenosine and adenosine analogs with ribose ring modifications, but not to those with purine ring modifications. Adenosine inhibition of this enzyme was not dependent on GTP and was noncompetitive with respect to ATP. Enzyme that had been dissociated from functional guanine nucleotide binding protein interactions by gel filtration in the presence of the zwitterionic detergent 3-[3-(cholamidopropyl)-dimethylammonio]-propanesulfonate and Mn2+ retained sensitivity to adenosine inhibition. The Ki for adenosine inhibition of the CaM-sensitive adenylate cyclase was approximately 2.6 X 10(-4) M. 5'-Guanylylimidodiphosphate and CaM did not affect the Ki of 3'-deoxyadenosine for the enzyme, but the presence of Ca2+ in the millimolar range raised the Ki by a factor of 5. These results show that the CaM-sensitive form of adenylate cyclase from bovine brain is subject to adenosine inhibition, and strongly suggest that this inhibition is due to interaction of ligands with a purine-specific ("P") site located on the catalytic subunit of the enzyme.