IMMUNOCHEMICAL IDENTIFICATION OF THE PSSA GENE-PRODUCT AS PHOSPHATIDYLSERINE SYNTHASE-I OF CHINESE-HAMSTER OVARY CELLS

We have previously shown that a Chinese hamster ovary (CHO) cell mutan t defective in phosphatidylserine synthase I recovers the enzyme activ ity on transfection with a pssA cDNA clone isolated from the parental CHO-K1. The resultant transfectant, CDT-1, exhibited about 20-fold hig her specific activity of the enzyme in the membrane fraction than CHO- K1 cells. Polyclonal antibodies against two peptides of the predicted pssA product cross-reacted with a membrane protein having an apparent molecular mass of 42 kDa, which was overproduced in CDT-1 cells. By im munoprecipitation with the antibody, phosphatidylserine synthase I act ivity as well as the 42-kDa protein was eliminated from solubilized me mbrane proteins of CDT-1 cells. Both the enzyme activity and the 42-kD a protein of CHO-K1 cells mere enriched in the mitochondria-associated membrane fraction and the microsome fraction, but neither was enriche d in the mitochondria fraction or the cytosol fraction, These results suggest that the pssA gene encodes phosphatidylserine synthase I.