The nuclear envelope is composed of an inner and outer membrane that surrounds DNA and associated proteins. The nuclear envelope contains nuclear pores, which are channels that serve as sites of exchange between the nucleus and cytoplasm of the cell. The double membrane nature of the nucleus presents challenges in nuclear envelope and pore assembly that are overcome by membrane fusion events. A recent paper describes inner/outer membrane fusion during nuclear pore assembly and identifies an intermediate step in the process. By using cold temperatures that slow down nuclear pore formation in frog egg extracts, Fichtman and colleagues were able to clarify when and how fusion occurs between the inner and outer membranes. Specifically, the cold temperature treatment blocks inner/outer membrane fusion while allowing only the nuclear side of the pore to assemble. Images above show nuclear pore formation at room temperature (left set of images) and a cold temperature (right set of images). At room temperature, the nuclear membrane (green) and nuclear pores (red) form around chromatin (blue in merged images) after 15 minutes from the start of nuclear assembly. At cold temperatures, nuclear pores form more slowly and don’t appear on the nuclear envelope until 60 minutes.
Fichtman, B., Ramos, C., Rasala, B., Harel, A., & Forbes, D. (2010). Inner/Outer Nuclear Membrane Fusion in Nuclear Pore Assembly: Biochemical Demonstration and Molecular Analysis Molecular Biology of the Cell, 21 (23), 4197-4211 DOI: g/10.1091/mbc.E10-04-0309">10.1091/mbc.E10-04-0309