Intracellular membrane flow: reconstitution of transition vesicle formation and function in a cell-free system

Transfer of membrane between endoplasmic reticulum and Golgi apparatus in situ is considered to occur via 60-nm transition vesicles derived from part-rough, part-smooth transition elements of the endoplasmic reticulum. A procedure is described for the isolation of a fraction enriched in these transition elements from rat liver. The isolated fraction generates small vesicles morphologically resembling transition vesicles when incubated with nucleoside triphosphate at 37 degrees C. In the cell-free system consisting of a donor fraction enriched in transition elements and an acceptor fraction consisting of intact Golgi apparatus immobilized on nitrocellulose strips, transfer in vitro of radiolabeled membranes was demonstrated. Nucleoside triphosphates were required for transfer, and transfer was facilitated by a cytosol fraction of Mr greater than 10,000. In the presence of both nucleoside triphosphate and cytosol, radiolabeled proteins were transferred in a manner dependent upon both time and temperature. Transfer appeared to be both vectorial and specific in that, with Golgi apparatus (or endoplasmic reticulum) as both donor and acceptor, only negligible time and temperature-dependent transfer was observed. The test system described is expected to facilitate further investigation of the transfer process and to provide a convenient assay to guide transition vesicle isolation and characterization.