Identification of a Low Density Membrane Fraction Involved in Storage Triacylglycerol Synthesis

Abstract

It has recently been reported that fatty acid elongase in developing seeds of Lunaria annua and Brassica napus was located in two different fractions of organelles separated by sucrose gradient centriftigation [l].The denser of these fractions was found to be endoplasmic reticulum (E.R.) membrane whereas the nature of the less dense fraction was unclear. Storage triacylglycerol (TAG) biosynthesis in developing seeds of Brassica napus is now being characterised using the same system. The enzymes involved in TAG synthesis were found in two distinct fractions with equilibrium densities of 1.05 and 1.10. The more dense membranes contained the E.R. marker enzymes CDP-choline: DAG cholinephosphotransferase and lyso-phosphatidylcholine acyltransferase (LPC-AT). The less dense membranes did not contain choline phosphotransferase activity and contained only trace levels of LPC-AT. The triacylglycerol/phospholipid (TAG/PL) ratio of the vesicles in the light fraction was 2.9 compared to 0.18 in the E.R.. The fatty acid composition of these two lipid classes were similar in both fractions.