Annulate Lamellae biogenesis is essential for nuclear pore function

Nuclear pore complexes (NPCs), large protein assemblies embedded into the nuclear envelope (NE), are crucial for bidirectional transport between the nucleus and cytoplasm, a process often disrupted in human diseases. Besides their presence within the NE, NPCs are also found in stacked cytoplasmic endoplasmic reticulum (ER) membranes called annulate lamellae (AL) ^1,2 . Despite being discovered in the mid-20th century ³ , the function and biogenesis mechanisms of AL have remained largely mysterious. While AL were thought to be restricted to germ, embryonic and malignant cells ^4–12 , we find that AL also exist in the cytoplasm of somatic cells under normal physiological conditions and that they can expand upon specific stimuli. We show that AL merge with the NE, supplying the nucleus with new pores which maintains nuclear pore function and nuclear growth during early interphase. NPC protein RanBP2 (Nup358) and ER-associated Climp63 (CKAP4) trigger AL assembly and their NE-integration. The N-terminal phenylalanine-glycine (FG) repeats of RanBP2 drive the oligomerization of Y-complexes (the NPC outer ring units), and AL-NPCs formation and Climp63 ensures the localization of AL-NPCs to ER sheets and their fusion with the nucleus. These findings uncover a fundamental mechanism of AL biogenesis and highlight the critical role of cytosolic NPCs in the nuclear function and mammalian cellular homeostasis.