Localization of GTP-stimulated core glycosylation to fused microsomes

Purified rough microsomes from liver maximally incorporated N-acetyl- [3H]glucosamine into endogenous acceptors from UDP-N-acetyl- [3H]glucosamine substrate, providing the associated ribosomes were removed and 0.5 mM GTP was added. These conditions also led to the coalescence of microsomes into large fused membranes. By measurement of membrane profiles on electron micrographs, a correlation was observed between GTP-stimulated glycosylation and microsomal membrane length (r2 = 0.92). Membrane fusion was not observed in the absence of GTP, with sugar transfer inhibited by greater than 90% for acid-resistant acceptors (protein), and approximately 50% for acid-labile acceptors (lipid-linked intermediates). When radiolabeled acceptors were localized by electron microscope radioautography, high concentrations of silver grains (83 grains/100 microns membrane length) were observed over fused membranes with lower grain densities observed over unfused membranes in the same preparation (20 grains/100 microns). These studies directly link microsomal membrane fusion to GTP-stimulated core glycosylation. The observations extend the suggestion of Godelaine et al. (1979, Eur. J. Biochem. 96:17-26) that physiological levels of GTP promote the translocation of substrate across endoplasmic reticulum membranes which, we propose, occurs via a membrane fusion phenomenon.