Regulation of phosphatidylcholine biosynthesis in mammalian cells. II. Effects of phospholipase C treatment on the activity and subcellular distribution of CTP:phosphocholine cytidylyltransferase in Chinese hamster ovary and LM cell lines.

CTP

phosphocholine cytidylyltransferase was located in both the cytosolic and particulate fractions from Chinese hamster ovary cells. The activity of the cytosolic form of the enzyme was greatly enhanced by incubation with sonicated preparations of several different lipids, although incubations with either phosphatidylcholine or 1,2-sn-diolein did not increase activity. The activation of the cytidylyltransferase in Chinese hamster ovary cells treated with phospholipase C from Clostridium perfringens occurred with a concomitant shift in the subcellular distribution of the enzyme from cytosolic to particulate fractions. This shift was rapid and did not require protein synthesis. Removal of phospholipase C from the cell cultures resulted in a return to basal levels of incorporation of [3H]choline into phosphatidylcholine, a decrease in the activity of cytidylyltransferase, and a loss of the membrane-bound form of the enzyme. Similar experiments with LM cells, which are resistant to exogenous phospholipase C, showed no change in subcellular distribution of cytidylyltransferase, suggesting that the activation of CTP:phosphocholine cytidylyltransferase required a change in membrane phospholipid composition. The results presented are discussed in terms of a mechanism of regulation of phosphatidylcholine production involving monitoring of membrane phospholipid composition.