Physiological concentrations of GTP stimulate fusion of the endoplasmic reticulum and the nuclear envelope

Incubation of highly purified nuclei with rough microsomes stripped of associated ribosomes and physiological concentrations of guanosine triphosphate (GTP) led to the fusion of outer membranes of nuclei with microsomes to form large irregular membrane extensions. Measurement of membrane profiles in electron micrographs revealed that the outer membranes of nuclei incubated under these conditions increased significantly in length compared with that of outer membranes of unincubated or control incubated nuclei. This morphometric assay for fusion was used to check membrane and tissue specificity. It was found that GTP did not stimulate fusion between other intracellular membranes (e.g. mitochondrial or Golgi) or between such membranes and nuclear envelopes. GTP did, however, stimulate fusion between stripped rough microsomes from rat liver and outer membranes of nuclei from rat brain. These studies have revealed that membranes of the rough endoplasmic reticulum and nuclear envelope possess unique recognition and fusion properties and as such constitute the first demonstration of membrane interaction specificity at the intracellular level.