EXPRESSION OF PHOSPHATIDYLETHANOLAMINE N-METHYLTRANSFERASE-2 CANNOT COMPENSATE FOR AN IMPAIRED CDP-CHOLINE PATHWAY IN MUTANT CHINESE-HAMSTER OVARY CELLS

Phosphatidylcholine is a product of the CDP-choline pathway and the pa thway that methylates phosphatidylethanolamine, We have asked the ques tion: are the two pathways functionally interchangeable? We addressed this question by investigating the expression of phosphatidylethanolam ine N-methyltransferase-2 (PEMT2) of rat liver in mutant Chinese hamst er ovary cells (MT-58) (Esko, J. D., Wermuth, M. M., and Raetz, C. R. H. (1981) J, Biol. Chem, 256, 7388-7393) defective in the CDP-choline pathway for phosphatidylcholine biosynthesis. Cell lines stably expres sing different amounts of PEMT2 activity (up to 700 pmol/min mg protei n) were isolated. A positive correlation between the amount of PEMT2 a ctivity expressed and the incorporation of [H-3]methionine into phosph atidylcholine at both the permissive and restrictive temperatures show ed that PEMT2 was functional in the Chinese hamster ovary MT-58 cells. In contrast to mutant cell lines stably expressing transfected CTP:ph osphocholine cytidylyltransferase, the cell lines stably expressing PE MT2 did not survive at the restrictive temperature, Determination of t he phosphatidylcholine mass in wild type cells, mutant MT-58 cells, an d cells with the highest level of PEMT2 expression showed that PEMT2 w as functional and synthesized the same amount of phosphatidylcholine a s did wild type cells at the restrictive temperature, Indirect immunof luorescence studies showed that localization of the over-expressed cyt idylyltransferase in MT-58 cells was largely nuclear, whereas PEMT2 wa s predominantly located outside the nucleus. Our data show that methyl ation of phosphatidylethanolamine to phosphatidylcholine cannot substi tute for the CDP-choline pathway.