Catalyzes a base-exchange reaction in which the polar head group of phosphatidylethanolamine (PE) is replaced by L-serine. a 1,2-diacyl-sn-glycero-3-phosphoethanolamine + L-serine = a 1,2-diacyl-sn-glycero-3-phospho-L-serine + ethanolamine Phospholipid metabolism; phosphatidylserine biosynthesis. Lethal at the first instar larval stage (PubMed:31869331). RNAi-mediated knockdown in salivary glands reduces phosphatidylserine (PS) levels and depletes Akt1 from the plasma membrane contributing to cell growth defects probably by affecting the insulin pathway; causes a shift from phospholipid synthesis to neutral lipid synthesis, which results in ectopic lipid accumulation in third instar larval salivary glands; reduces mitochondrial PS levels thereby impairing mitochondrial protein import and mitochondrial integrity (PubMed:31869331). Belongs to the phosphatidyl serine synthase family.