THE CROSS-REGULATION OF G(I)-PROTEIN BY CHOLERA-TOXIN INVOLVES A PHOSPHORYLATION BY PROTEIN-KINASE-A

Pretreatment of alveolar macrophages with cholera toxin inhibits the r elease of arachidonic acid induced by the chemotactic peptide N-formyl methionyl-leucyl-phenylalanine. The results presented here show that c holera toxin might exert its inhibitory effect through the phosphoryla tion of G(i) alpha by protein kinase A (PKA). (1) G(i)-proteins from c ells pretreated with cholera toxin showed parallel increases in their sensitivity to ADP-ribosylation by toxins in vitro and in G(i) alpha p hosphorylation. By contrast, the G(i) alpha concentration was unchange d. (2) Cholera toxin pre treatment also decreased the functional activ ity of G(i), as assessed by the inhibition (80 %) of agonist-induced b inding of guanosine-5'-[gamma-thio]triphosphate (GTP[gamma S]). (3) Th ese effects of cholera toxin were blocked by a specific PKA inhibitor, N-(2[methylamino]ethyl)-3-isoquinolinesulphonamide dihydrochloride (H 8) and mimicked by a cyclic AMP (cAMP) analogue and a phosphatase inhi bitor. (4) G(i) alpha was also phosphorylated in vitro by the catalyti c subunit of PKA. In contrast with other cell systems, the stimulation of protein kinase C seems to have no effect on the sensitivity of G(i ) to ADP-ribosylation or on its phosphorylation. Therefore, the phosph orylation of G(i)-proteins by PKA seems to be the actual target of the negative control of arachidonic acid release via the cAMP-mediated pa thway.