KN-93 inhibition of G protein signaling is independent of the ability of Ca2+/calmodulin-dependent protein kinase II to phosphorylate phospholipase Cbeta(3) on (537)-Ser

Stimulation of the phospholipase C beta (PLC) signaling pathway results in intracellular Ca2+ release and subsequent activation of calmodulin (CaM) an d CaM kinase II (CaMK II). KN-93, an inhibitor of CaMK II, reduced the stim ulation of phosphatidylinositide (PI) turnover by G alpha (i)-coupled (form yl-Met-Leu-Phe, fMLP) or G alpha (q)-coupled [M1 muscarinic and oxytocin (O T)] receptors. The inhibitory effect of KN-93 was also observed when PLC be ta (3) was stimulated directly by G alpha (q) or G beta gamma in overexpres sion assays. CaMK II phosphorylated PLC beta (3) but not PLC beta (1) in vi tro. Phosphorylation occurred exclusively on (537)Ser in the X-Y linker reg ion of PLC beta (3). (537)Ser was also phosphorylated in the basal state in cells and phosphorylation was enhanced by ionomycin treatment. However, mu tation of (537)Ser to Glu had no effect on inhibition of G alpha (q) or G b eta gamma -stimulated PLC beta (3) activity by KN-93. KN-93 also inhibited G alpha (q) -stimulated PLC beta (1) activity, even though this enzyme is n ot a substrate for CaMK II. These data indicate that phosphorylation of PLC beta (3) by CaMK II is not directly involved in the inhibitory effect of K N-93 on phosphatidylinositide turnover. (C) 2001 Elsevier Science Ireland L td. All rights reserved.