Vaults – giving us pores?

Nuclear pore complexes (NPCs) traverse the inner and outer nuclear membrane and are the sites of nuclear import and export. Although NPCs are continuously incorporated into the nuclear envelope during interphase, the mechanism has been poorly understood. Now, Marie-Christine Dabauvalle and colleagues (p. 780) show unexpectedly that major vault protein (MVP) – the main component of vaults, which are abundant ribonucleoprotein particles of uncertain function – is important for NPC incorporation. The authors first establish an in vitro assay that enables the separation of nuclear-membrane assembly and NPC incorporation; they do this by fractionating Xenopus egg extract to produce two membrane fractions, one of which forms nuclei that lack NPCs in the presence of chromatin. Insertion of NPCs, they show, proceeds in the presence of the second fraction. The authors next use immunoblotting and mass spectrometry to show that MVP is present in the second fraction, but not the first. In addition, recombinant MVP and purified endogenous vaults both promote NPC insertion when added to pore-free nuclei. Thus, vaults can act as NPC-incorporation factors and might, the authors speculate, help to stabilise newly formed membrane channels early in NPC incorporation. Their data advance our understanding of NPC biogenesis.