Loss of the product-inhibition for phosphatidylserine biosynthesis in cho cells with a point-mutation in phosphatidylserine synthase

Phosphatidylserine (PtdSer) synthesis in Chinese hamster ovary (CHO) cells occurs through the exchange of L-serine with the base moiety of phosphatidylcholine or phosphaüdylethanolamine. The synthesis is depressed on the addition of PtdSer to the culture medium. A CHO cell mutant named mutant 29, whose PtdSer biosynthesis is highly resistant to thus depression by exogenous PtdSer, has been isolated from CHO-K1 cells. In the present study, the PtdSer-resistant PtdSer biosynthesis in the mutant was traced to a point mutation in the PtdSer synthase I gene, pssA, resulting in the replacement of arginine residue 95 of the synthase by lysine. Introduction of the mutant pssA cDNA, but not the wild-type pssA cDNA, into CHO-K1 cells induced the PtdSer-resistant PtdSer biosynthesis. In a cell-free system, the serine base-exchange activity of the wild-type pssA -transfected cells was inhibited by PtdSer, but that of the mutant pî.s.4-transfected cells was resistant to the inhibition. Like the mutant 29 cells, the mutant pssA -transfected cells grown without exogenous PtdSer exhibited a -2-fold increase in the cellular PtdSer level, compared with that in CHO-K1 cells, although the wild-type pssAtransfected cells did not exhibit such a significant increase. These results indicated that the inhibition of PtdSer synthase I by PtdSer is essential for the maintenance of a normal PtdSer level in CHO-K1 cells and that arginine residue 95 of the synthase is a crucial residue for the inhibition.