The phosphatidylserine synthesis in a CHO cell mutant which lacks PSS I,
EC 2.7.8.8, but has normal PSS II activity, is almost completely inhibited by the addition of phosphatidylserine to the culture medium, like that in the wild-type CHO-K1 cells. The phosphatidylserine synthesis in a PSS II-overproducing stable transformant is reduced by 35% upon addition of phosphatidylserine. Residue Arg-97 is critical for the exogenous phosphatidylserine-mediated inhibition