Vesicles have previously been isolated from cells in which nuclear envelope breakdown has occurred, but it was always possible that the vesicles were formed as part of extract isolation rather than normal physiology. Now, the Utah team has found that the nuclear pore protein Nup153, COP1 coatomer proteins, and ARF GTPase are needed for nuclear envelope breakdown in frog egg extracts. COP1 components, which are better known for driving vesicle formation during intra-Golgi and Golgi-to-ER traffic, were also found to associate with Nup153 and to move from the Golgi to the nuclear periphery at the start of mitosis.
Initiation of nuclear envelope breakdown may be controlled by regulating COP1 access to Nup153, which is primarily exposed in the nucleoplasm. This may occur by the microtubule-mediated ripping of the nuclear envelope or nuclear pore reconfiguration noted by others. The relative contributions of ripping and vesiculation in different cell types remain to be determined. Vesicles are not necessarily seen as an endpoint in nuclear envelope breakdown, but Ullman notes that they may be an intermediate before nuclear membranes fuse with the ER. ▪