Multifunctional roles of Brl1-Brr6 in nuclear envelope fusion during nuclear pore complex biogenesis

Brl1 and Brr6 are integral membrane proteins of the yeast nuclear envelope (NE) that transiently associate with nuclear pore complexes (NPCs). The exact roles of Brl1 and Brr6 during NPC assembly are unclear. Here, we demonstrate that Brr6 operates at both early and late stages of NPC assembly. Its early function is supported by amphipathic α-helix mutants, which impact nucleoporin recruitment without nuclear envelope deformation, whereas mutations in conserved cysteine residues result in NE deformation accompanied by defective NE fusion. The N-terminus of Brl1 interacts with the nucleoporin Nic96, promoting Nic96 recruitment to early assembly sites. AlphaFold predictions, the essential role of the conserved PAL motif, and the inhibition of NE fusion upon overexpression of PAL mutants together suggest that the perinuclear domains of Brl1 and Brr6 interact across the perinuclear space. Extending the length of the perinuclear-space regions of Brl1 and Brr6 causes uncontrolled NE fusion, as indicated by nuclear envelope disintegration dependent on the conserved cysteine residues. Together, Brl1 and Brr6 promote NE fusion by bridging the perinuclear space through PAL motif interactions, followed by nuclear envelope fusion and NPC insertion.