Phosphorylation by protein kinase C decreases catalytic activity of avian phospholipase C-beta

The potential role of protein kinase C (PKC)-promoted phosphorylation has been examined in the G-protein-regulated inositol lipid signalling pathway. Incubation of [32P]Pi-labelled turkey erythrocytes with either the P2Y1 receptor agonist 2-methylthioadenosine triphosphate (2MeSATP) or with PMA resulted in a marked increase in incorporation of 32P into the G-protein-activated phospholipase C PLC-betaT. Purified PLC-betaT also was phosphorylated by PKC in vitro to a stoichiometry (mean+/-S. E.M.) of 1.06+/-0.2 mol of phosphate/mol of PLC-betaT. Phosphorylation by PKC was isoenzyme-specific because, under identical conditions, mammalian PLC-beta2 also was phosphorylated to a stoichiometry near unity, whereas mammalian PLC-beta1 was not phosphorylated by PKC. The effects of PKC-promoted phosphorylation on enzyme activity were assessed by reconstituting purified PLC-betaT with turkey erythrocyte membranes devoid of endogenous PLC activity. Phosphorylation resulted in a decrease in basal activity, AlF4(-)-stimulated activity, and activity stimulated by 2MeSATP plus guanosine 5'-[gamma-thio]triphosphate in the reconstituted membranes. The decreases in enzyme activities were proportional to the extent of PKC-promoted phosphorylation. Catalytic activity assessed by using mixed detergent/phospholipid micelles also was decreased by up to 60% by phosphorylation. The effect of phosphorylation on Gqalpha-stimulated PLC-betaT in reconstitution experiments with purified proteins was not greater than that observed on basal activity alone. Taken together, these results illustrate that PKC phosphorylates PLC-betaT in vivo and to a physiologically relevant stoichiometry in vitro. Phosphorylation is accompanied by a concomitant loss of enzyme activity, reflected as a decrease in overall catalytic activity rather than as a specific modification of G-protein-regulated activity.