CONTROL OF PHOSPHATIDYLSERINE BIOSYNTHESIS THROUGH PHOSPHATIDYLSERINE-MEDIATED INHIBITION OF PHOSPHATIDYLSERINE SYNTHASE-I IN CHINESE-HAMSTER OVARY CELLS

Phosphatidylserine (PtdSer) synthesis in Chinese hamster ovary (CHO) c ells occurs through the exchange of L-serine with the base moiety of p hosphatidylcholine or phosphatidylethanolamine. The synthesis is depre ssed on the addition of PtdSer to the culture medium. A CHO cell mutan t named mutant 29, whose PtdSer biosynthesis is highly resistant to th is depression by exogenous PtdSer, has been isolated from CHO-K1 cells . In the present study, the PtdSer-resistant PtdSer biosynthesis in th e mutant was traced to a point mutation in the PtdSer synthase I gene, pssA, resulting in the replacement of Arg-95 of the synthase by lysin e. Introduction of the mutant pssA cDNA, but not the wild-type pssA cD NA, 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 pssA-transfected cells was resistant to the inhibition. Like th e mutant 29 cells, the mutant pssA-transfected cells grown without exo genous PtdSer exhibited an approximate to 2-fold increase in the cellu lar PtdSer level compared with that in CHO-K1 cells, although the wild -type pssA-transfected cells did not exhibit such a significant increa se. These results indicated that the inhibition of PtdSer synthase I b y PtdSer is essential for the maintenance of a normal PtdSer level in CHO-K1 cells and that Arg-95 of the synthase is a crucial residue for the inhibition.