Mutational analysis of the potential phosphorylation sites for protein kinase C on the CCKA receptor

1 Many G protein-coupled receptors contain potential phosphorylation sites for protein kinase C (PKC), the exact role of which is poorly understood. In the present study, a mutant cholecystokinin(A) (CCKA) receptor was generated in which the four consensus sites for PKC action were changed in an alanine. Both the wild-type (CCKAWT) and mutant (CCKAMT) receptor were stably expressed in Chinese hamster ovary (CHO) cells. 2 Binding of [H-3]-cholecystokinin-(26-33)-peptide amide (CCK-8) to membranes prepared from CHO-CCKAWT cells and CHO-CCKAMT cells revealed no difference in binding affinity (Kd values of 0.72 nM and 0.86 nM CCK-8, respectively). 3 The dose-response curves for CCK-8-induced cyclic AMP accumulation and inositol 1,4,5-trisphosphate (Ins(1,4,5)P-3) formation were shifted to the left in CHO-CCKAMT cells. This leftward shift was mimicked by the potent inhibitor of protein kinase activity, staurosporine. However, the effect of staurosporine was restricted to CHO-CCKAWT cells. This demonstrates that attenuation of CCK-8-induced activation of adenylyl cyclase and phospholipase C-P involves a staurosporine-sensitive kinase, which acts directly at the potential sites of PKC action on the CCKA receptor in CCK-8-stimulated CHO-CCKAWT cells. 4 The potent PKC activator, 12-O-tetradecanoylphorbol 13-acetate (TPA), evoked a rightward shift of the dose-response curve for CCK-8-induced cyclic AMP accumulation in CHO-CCKAWT cells but not CHO-CCKAMT cells. This is in agreement with the idea that PKC acts directly at the CCKA receptor to attenuate adenylyl cyclase activation. 5 In contrast, TPA evoked a rightward shift of the dose-response curve for CCK-8-induced Ins(1,4,5)P-3 formation in both cell lines. This demonstrates that high-level PKC activation inhibits CCK-8-induced Ins(1,4,5)P-3 formation also at a post-receptor site. 6 TPA inhibition of agonist-induced Ca2+ mobilization was only partly reversed in CHO-CCKAMT cells. TPA also inhibited Ca2+ mobilization in response to the G protein activator, Mas-7. These findings are in agreement with the idea that partial reversal of agonist-induced Ca2+ mobilization is due to the presence of an additional site of PKC inhibition downstream of the receptor and that the mutant receptor itself is not inhibited by the action of PKC. 7 The data presented demonstrate that the predicted sites for PKC action on the CCKA receptor are the only sites involved in TPA-induced uncoupling of the receptor from its G proteins. In addition, the present study unveils a post-receptor site of PKC action, the physiological relevance of which may be that it provides a means for the cell to inhibit phospholipase C-beta activation by receptors that are not phosphorylated by PKC.