Inositol Trisphosphate Receptor: Phosphorylation by Protein Kinase C and Calcium Calmodulin-Dependent Protein Kinases in Reconstituted Lipid Vesicles

We have previously demonstrated that the inositol 1,4,5-trisphosphate (IP3) receptor is phosphorylated by cyclic AMP-dependent protein kinase (PKA). In the present study, phosphorylation of IP3receptors has been examined with purified receptor protein reconstituted in liposomes to remove detergent that can inhibit protein kinases. The IP3receptor is stoichiometrically phosphorylated by protein kinase C (PKC) and Ca2+calmodulin-dependent protein kinase II (CaM kinase II) as well as by PKA. Phosphorylation by the three enzymes is additive and involves different peptide sequences. Phosphorylation by PKC, which is stimulated by Ca2+and diacylglycerol, and by CaM kinase II, which requires Ca2+, provides means whereby Ca2+and diacylglycerol, formed during inositol phospholipid turnover, may regulate IP3receptor physiology.