Reconstituted phosphatidylserine synthase from Escherichia coli is activated by anionic phospholipids and micelle-forming amphiphiles

The activity of phosphatidylserine (PS) synthase (CDP-1,2-diacyl-sn-glycero l:L-serine O-phosphatidyltransferase, EC 2.7.8.8) from Escherichia coli was studied after reconstitution with lipid vesicles of various compositions. PS synthase exhibited practically no activity in the absence of a detergent and with the substrate CDP-diacylglycerol (CDP-DAG) present only in the li pid vesicles. Inclusion of octylglucoside (OG) in the assay mixture increas ed the activity 20- to 1000-fold, the degree of activation depending on the lipid composition of the vesicles. Inclusion of additional CDP-DAG in the assay mixture increased the activity 5- to 25-fold. When the fraction of ph osphatidylglycerol (PG) was increased from 15 to 100 mol% in the vesicles t he activity increased 10-fold using the assay mixture containing OG. The hi ghest activities were exhibited with the anionic lipids synthesized by E. c oli, namely pc, diphosphatidylglycerol (DPG), and phosphatidic acid, while phosphatidylinositol gave a lower activity. Cryotransmission electron micro scopy showed that transformation of the vesicles to micelles brings about a n activation of the enzyme that is proportional to the degree of micellizat ion. Thus, the activity of PS synthase is modulated by the lipid aggregate structure and by the fraction and type of anionic phospholipid in the aggre gates. The increase in the activity caused by PG and DPG is physiologically relevant; it may be part of a regulatory mechanism that keeps the balance between phosphatidylethanolamine, and the sum of PG and DPG, nearly constan t in wildtype E. coil cells. (C) 1999 Elsevier Science B.V. All rights rese rved.