Aggregated Forms of Malate and Citrate Synthase are Localized in Endoplasmic Reticulum of Endosperm of Germinating Castor Bean

Abstract

The endosperm of 3-day germinated seedlings of R. communis was homogenized in the presence or absence of Mg2+. When the Mg2+-containing homogenate was fractionated on linear, 20-40% sucrose gradients the endoplasmic reticulum (ER) reached equilibrium at a density of 1.146 g/cc3. Absence of Mg2+ in the grinding medium resulted in displacement of the ER in the gradient from a density of 1.146-1.138 g/cc3. At either density, the activities of both malate and citrate synthase overlapped the activity of NADH-cytochrome c reductase (an ER marker) in the gradient. This overlap of activities was observed whether the gradients were centrifuged for 3 or 19 h. An analysis of sedimentation characteristics of the solubilized enzymes revealed that they exist, predominantly, as a 5.2S form (malate synthase) and a 6.8S form (citrate synthase) in the glyoxysomes and cytosol. When the 2 enzymes were released from the ER, they appeared as aggregate forms of 70S and 55S, respectively. Apparently, the synthases are associated with the ER.