Ribosomal-associated phosphatidylserine synthetase from Escherichia coli: purification by substrate-specific elution from phosphocellulose using cytidine 5'-diphospho-1,2-diacyl-sn-glycerol

Abstract

Cytidine 5'-diphospho-1,2-diacyl-sn-glycerol (CDPdiglyceride):L-serine O-phosphatidyltransferase (EC 2.7.8.8, phosphatidylserine synthetase) is bound tightly to the ribosomes in crude extracts of Escherichia coli. After separation of the enzyme from the ribosomes by the method of Raetz and Kennedy (Raetz, C.R.H., and Kennedy, E.P. (1974), J. Biol. Chem. 249, 5038), we have purified the enzyme to 97% of homogenekty. The major portion of the overall 5500-fold purification was attained by substrate-specific elution from phosphocellulose using CDP-diglyceride in the presence of detergent. The purified enzyme migrated as a single band with an apparent minimum molecular weight of 54 000 when subjected to electrophoresis on polyacrylamide disc gels containing sodium dodecyl sulfate. The purified enzyme catalyzed exchange reactions between cytidine 5'- monophosphate (CMP) and CDP-diglyceride and between serine and phosphatidylserine. The enzyme also catalyzed the hydrolysis of CDP-diglyceride to form CMP and phosphatidic acid. dCDP-diglyceride was equivalent to CDP-diglyceride in all reactions catalyzed by the enzyme. In addition, the purified enzyme catalyzed the formation of phosphatidylglycerol or phosphatidylglycerophosphate at a very slow rate when serine was replaced as substrate by glycerol or sn-glycero-3-phosphate, respectively. These results suggest catalysis occurs via a ping-pong mechanism through the formation of a phosphatidyl-enzyme intermediate.