Heterogeneous Distribution of Glycosyltransferases in the Endoplasmic Reticulum of Castor Bean Endosperm

Abstract

Endoplasmic reticulum membranes stripped of attached ribosomes were isolated from homogenates of germinating castor bean (R. communis L.) endosperm by sucrose density gradient centrifugation. The isolated endoplasmic reticulum fraction was further separated into 2 major membrane subfraction by centrifugation on a flotation gradient. Both subfractions apparently were derived from the endoplasmic reticulum inasmuch as they share enzymic markers including choline phosphotransferase, NADH-cytochrome c reductase, and glycoprotein fucosyltransferase and phase separation of membrane polypeptides using Triton X-114 revealed a striking similarity in both their hydrophilic and hydrophobic protein components. The endoplasmic reticulum membrane subfractions contain glycoproteins which were readily labeled by incubating intact endosperm tissue with radioactive sugars prior to fractionation. Castor bean endosperm endoplasmic reticulum apparently exhibits a degree of enzymic heterogeneity, however, since the enzymes responsible for the synthesis of dolichol pyrophosphate N-acetylglucosamine and dolichol monophosphate mannose together with their incorporation into the oligosaccharide-lipid precursor of protein N-glycosylation were largely recovered in a single endoplasmic reticulum subfraction.