Heterogeneity of the endoplasmic reticulum with respect to lipid synthesis in developing seeds of Brassica napus L

Studies of the sub-cellular location of storage triacylglycerol (TAG) synthesis in developing embryos of oilseed rape (Brassica napus L.) show that there is heterogeneity of the endoplasmic reticulum (ER) with respect to the enzymes of lipid synthesis. The enzymes of TAG synthesis were detected in two membrane fractions (equilibrium densities 1.05 and 1.10 g·ml-1) isolated by sucrose-density-gradient centrifugation of homogenates from developing rape embryos. The synthesis of TAG by the low- density membranes has not been reported previously and was found in this study because the sucrose density gradients began at only 10% (w/w) sucrose. The pattern of activity of the enzymes involved in the synthesis of TAG in the higher-density fraction closely matched the marker enzymes for the ER; lyso-phosphatidylcholine acyltransferase and cytidine diphosphate-choline:diacylglycerol cholinephosphotransferase. The activity of the ER marker enzymes in the low-density membrane fraction, however, was very much lower when compared to those involved in the synthesis of TAG. Analysis of the lipids extracted from the low-density fraction revealed it contained about 50 mol% TAG compared with 15 mol% in the bulk ER, which may account for the low density of the membranes in this fraction. The possibility that the low-density membranes were the result of contamination of ER by oil bodies was ruled out by the use of oleosins as a marker for oil bodies. It is suggested that the low-density membranes are derived from a domain of the ER which is involved in the formation and secretion of TAG.