A checkpoint coordinates assembly of nuclear pore complexes (NPCs) with formation of the nuclear envelope (NE), according to Wolfram Antonin, Iain Mattaj, and colleagues (EMBL, Heidelberg, Germany).
NPC assembly begins with the binding of the Nup107-160 complex of nucleoporins to chromatin. The EMBL group looked at two other nucleoporins, gp210 and pom121. These nucleoporins span the nuclear membrane and thus might link NPC assembly on the two sides of the NE as it reforms after mitosis.
Although gp210 was not needed for NPC assembly or NE formation in frog egg extracts, pom121 was essential for both processes. After pom121 depletion, vesicles docked onto chromatin but did not fuse with each other to form a complete NE.
But if Nup107-160 was also depleted from the extracts, NE formation proceeded normally, albeit without insertion of NPCs. Thus pom121 is not absolutely required for NE formation, but pom121's absence can block NE membrane fusion events if Nup107-160 is present.
The assembly of Nup107-160 on chromatin happens early and does not require membranes. But prior NE formation does block the access of Nup107-160 to chromatin and thus its assembly. It may be essential to ensure that this initial assembly and its link to a transmembrane component and the outside world are complete before taking the step of sealing off the nucleus as a separate compartment.