In eukaryotic cells,
phosphatidylserine (PtdSer) is synthesized by two distinct synthases on the endoplasmic reticulum by a base-exchange reaction in which the polar head-group of an existing
phospholipid is replaced with
serine. We report the cloning and expression of a
cDNA for mouse liver
PtdSer synthase-1. The deduced
protein sequence is >90% identical to that of
PtdSer synthase-1 from Chinese hamster ovary cells and a sequence from a human myeloblast cell line.
PtdSer synthase-1 cDNA was stably expressed in M.9.1.1 cells which are mutant Chinese hamster ovary cells defective in
PtdSer synthase-1 activity, are
ethanolamine auxotrophs, and have a reduced content of PtdSer and
phosphatidylethanolamine (PtdEtn). The growth defect of M.9.1.1 cells was eliminated, and a normal
phospholipid composition was restored in the absence of exogenous
ethanolamine, implying that the cloned
cDNA encoded PtdSer synthase. Mouse liver
PtdSer synthase-1 was also expressed in McArdle 7777 rat
hepatoma cells. In addition to a 3-fold higher in vitro
serine-exchange activity, these cells also exhibited enhanced
choline- and
ethanolamine-exchange activities and incorporated more [3H]
serine into PtdSer than did control cells. However, the levels of PtdSer and PtdEtn in cells overexpressing
PtdSer synthase-1 activity were not increased. Excess PtdSer produced by the transfected cells was rapidly decarboxylated to PtdEtn and the degradation of PtdSer, and/or PtdEtn derived from PtdSer, was increased. Moreover, the
CDP-ethanolamine pathway for PtdEtn biosynthesis was inhibited. These data suggest that (i) cellular levels of PtdSer and PtdEtn are tightly controlled, and (ii) the metabolism of PtdSer and PtdEtn is coordinately regulated to maintain
phospholipid homeostasis.