FIGURE 2:

A fusion checkpoint for nuclear pore biogenesis. Nuclear pore assembly after nuclear envelope reformation proceeds through an inside-out protrusion mechanism starting from the nuclear side. Early assembly intermediates contain NPC constituents that deform the inner nuclear membrane (INM). This structure needs to expand to bring the INM and outer nuclear membrane (ONM) in close proximity for fusion. Either the integrity of the NPC assembly intermediate is sensed or the transport competence of a late assembly intermediate is required to initiate fusion. This hypothetical checkpoint mechanism would prevent a transient perturbation of NE integrity and can potentially explain why multiple, distinct NPC assembly defects lead to NE blebs resembling late assembly intermediates that got arrested before fusion (see the text).