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First published online 12 February 2008
doi: 10.1242/jcs.019968

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NEP-A and NEP-B both contribute to nuclear pore formation in Xenopus eggs and oocytes

¹ Integrative Cell Biology Laboratories, School of Biological and Biomedical Sciences, The University of Durham, South Road, Durham DH1 3LE, UK
² Laboratory of Nuclear Proteins, University of Wroclaw, Przybyszewskiego 63/77, 51–148 Wroclaw, Poland
³ Department of Morphology and Function of Cell Structure, Institute of Cytology and Genetics, Russian Academy of Sciences, Novosibirk-90, 630090, Russia
⁴ School of Life Sciences, University of Dundee, Dow Street, Dundee DD1 4HN, UK
⁵ CRUK, Department of Structural Cell Biology, Paterson Institute for Cancer Research, Christie Hospital, Wilmslow Road, Manchester M20 9BX, UK
⁶ The Randall Division of Cell and Molecular Biophysics, Kings College, New Hunts House, Guy's Campus, London SE1 1UL, UK
⁷ Centre for Developmental Genetics, Department of Biomedical Sciences, University of Sheffield, Firth Court, Western Bank, Sheffield S10 2TN, UK

^* Author for correspondence (e-mail: c.j.hutchison{at}durham.ac.uk)

Accepted 26 November 2007

In vertebrates, the nuclear envelope (NE) assembles and disassembles during mitosis. As the NE is a complex structure consisting of inner and outer membranes, nuclear pore complexes (NPCs) and the nuclear lamina, NE assembly must be a controlled and systematic process. In Xenopus egg extracts, NE assembly is mediated by two distinct membrane vesicle populations, termed NEP-A and NEP-B. Here, we re-investigate how these two membrane populations contribute to NPC assembly. In growing stage III Xenopus oocytes, NPC assembly intermediates are frequently observed. High concentrations of NPC assembly intermediates always correlate with fusion of vesicles into preformed membranes. In Xenopus egg extracts, two integral membrane proteins essential for NPC assembly, POM121 and NDC1, are exclusively associated with NEP-B membranes. By contrast, a third integral membrane protein associated with the NPCs, gp210, associates only with NEP-A membranes. During NE assembly, fusion between NEP-A and NEP-B led to the formation of fusion junctions at which >65% of assembling NPCs were located. To investigate how each membrane type contributes to NPC assembly, we preferentially limited NEP-A in NE assembly assays. We found that, by limiting the NEP-A contribution to the NE, partially formed NPCs were assembled in which protein components of the nucleoplasmic face were depleted or absent. Our data suggest that fusion between NEP-A and NEP-B membranes is essential for NPC assembly and that, in contrast to previous reports, both membranes contribute to NPC assembly.

Key words: Nuclear Envelope Assembly, Nuclear Pore Complexes, LAP2

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