Two distinct steps in nuclear envelope assembly can be assayed in vitro1-3: the protein-mediated binding4 of nuclear-specific vesicles to chromatin, and the subsequent fusion5 of these vesicles to enclose the chromatin within a double nuclear membrane. Nuclear vesicle fusion, like fusion in the secretory pathway6,7, requires ATP8,9 and cytosol1,3,5 and is inhibited by nonhydrolysable GTP analogues1,2. The sensitivity of nuclear vesicle fusion to GTP-γS requires a GTP-dependent soluble factor, the properties of which are strikingly similar to a GTP-dependent Golgi binding factor (GGBF) that inhibits Golgi vesicle fusion in the presence of GTP-γS and belongs to the ADP-ribosylation factor (ARF) family of small GTPases10,11. In the presence of GTP-γS, ARF proteins and α-, β-, γ-, δ-COP ('coatomer') subunits are associated with Golgi transport vesicles6,12,13, but the exact roles of ARF proteins in secretion are not yet understood. We report here that purified ARF1 and GGBF have GTP-dependent soluble factor activity in the nuclear vesicle fusion assay. Our results show that the function of ARF is not limited to the Golgi apparatus, and indicate that there may be a link between the formation of nuclear vesicles during mitosis and proteins involved in secretion.
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