Catalyzes a base-exchange reaction in which the polar head group of phosphatidylethanolamine (PE) or phosphatidylcholine (PC) is replaced by L-serine (PubMed:10432300, PubMed:9516423, PubMed:18343815, PubMed:10938271). Catalyzes mainly the conversion of phosphatidylcholine (PubMed:9516423, PubMed:18343815, PubMed:10432300, PubMed:10938271). Also converts, in vitro and to a lesser extent, phosphatidylethanolamine (PubMed:10432300, PubMed:9516423, PubMed:18343815, PubMed:10938271). a 1,2-diacyl-sn-glycero-3-phosphoethanolamine + L-serine = a 1,2-diacyl-sn-glycero-3-phospho-L-serine + ethanolamine a 1,2-diacyl-sn-glycero-3-phosphocholine + L-serine = a 1,2-diacyl-sn-glycero-3-phospho-L-serine + choline Potently inhibited by choline in the mitochondria-associated membrane (MAM). Very little inhibition by choline in the endoplasmic reticulum (ER) per se. Phospholipid metabolism; phosphatidylserine biosynthesis. Highly enriched in the mitochondria-associated membrane (MAM). Expressed in kidney, testis, lung, skeletal muscle, liver brain, heart and spleen with highest expression in testis, liver, heart and brain. Null mice are viable, fertile and have a normal life span. Toal serine exchange is reduced up to 85%, but apart from in liver, the phosphatatidylserine content was unaltered. Elimination of either Pss1 or Pss2, but not both, is compatible with mouse viability. Mice can tolerate as little as 10% serine-exchange activity and are viable with small amounts of phosphatidylserine and phosphatidylethanolamine content. to phosphatidylethanolamine. Belongs to the phosphatidyl serine synthase family.