Direct labelling of hormone-sensitive phosphoinositides by a plasma-membrane-associated PtdIns synthase in turkey erythrocytes.

We have previously characterized phosphatidylinositol (PtdIns) synthase and PtdIns/myo-inositol-exchange enzyme activities in ghost membranes prepared by hypotonic lysis of turkey erythrocytes [McPhee, Lowe, Vaziri and Downes (1991) Biochem. J. 275, 187-192]. Here we show that PtdIns synthase activity is relatively enriched in plasma-membrane preparations of turkey erythrocytes and that inositol phospholipids labelled by both PtdIns synthase and PtdIns myo-inositol exchange enzymes are susceptible to hydrolysis by the receptor- and G-protein-regulated phospholipase C (PLC), which is present also in ghost preparations. Specific-radioactivity measurements of [3H]PtdIns from ghosts labelled to equilibrium under conditions favouring [3H]inositol incorporation by PtdIns synthase activity indicate that PtdIns synthase can directly access approx. 14% of the total erythrocyte ghost PtdIns. Approx. 16% of the [3H]PtdIns labelled by the PtdIns synthase reaction can be phosphorylated to polyphosphoinositides, which are then hydrolysed by the receptor- and G-protein-stimulated PLC. Since the mass of PtdIns declines to a similar extent as [3H]PtdIns during stimulation in the presence of guanine nucleotides and ATP, it is evident that both the labelled and unlabelled phosphoinositides are susceptible to hydrolysis by the relevant PLC. Phosphoinositides present in nuclei-free plasma membranes were also labelled by [3H]inositol under conditions favouring PtdIns synthase and PtdIns/myo-inositol-exchange enzyme activities respectively. These membranes lack PLC activity [Vaziri and Downes (1992) J. Biol. Chem. 267, 22973-22981], but the labelled lipids were sensitive to purinergic-receptor-stimulated hydrolysis in reconstitution assays using partially purified turkey erythrocyte PLC. The results strongly suggest that at least a portion of the PtdIns synthase in turkey erythrocytes is located in the plasma membrane and has direct access to an agonist-sensitive pool of inositol phospholipids.