Abstract
SummaryNuclear pore complexes (NPCs) span the nuclear envelope (NE) and mediate nucleocytoplasmic transport. In metazoan oocytes and early embryos, NPCs reside not only within the NE, but also at some endoplasmic reticulum (ER) membrane sheets, termed annulate lamellae (AL). Although a role for AL as NPC storage pools has been discussed, it remains controversial whether and how they contribute to the NPC density at the NE. Here, we show that AL insert into the NE as the ER feeds rapid nuclear expansion in Drosophila blastoderm embryos. We demonstrate that NPCs within AL resemble pore scaffolds that mature only upon insertion into the NE. We delineate a topological model in which NE openings are critical for AL uptake that nevertheless occurs without compromising the permeability barrier of the NE. We finally show that this unanticipated mode of pore insertion is developmentally regulated and operates prior to gastrulation.
Highlights
In eukaryotes, the double membranous nuclear envelope (NE) encloses the nucleoplasm and separates it from the cytoplasm
Nuclear Pores Insert from the endoplasmic reticulum (ER) into the NE To investigate whether annulate lamellae (AL)-nuclear pore complexes (NPCs) contribute to the pool of NENPCs, we conducted live-imaging experiments in Drosophila blastoderm embryos before formation of the first somatic cell layer (Figure S2A)
GFP::Nup107 localized to the NE and to prominent foci throughout the cytoplasm (Figure 1A), similar to structures that were previously characterized as AL (Cordes et al, 1996; Daigle et al, 2001; Onischenko et al, 2004, 2005)
Summary
The double membranous nuclear envelope (NE) encloses the nucleoplasm and separates it from the cytoplasm. NPCs consist of multiple copies of $30 different nucleoporins (Nups) that are organized into biochemically distinct sub-complexes (Figures S1A, S1A0, and S1B). Two such modules, the inner ring complex ( called Nup complex) and the Y-complex ( called Nup107 complex) constitute the NPC scaffold that is symmetric across the NE plane. FG-Nups (containing phenylalanine-glycine rich intrinsically disordered protein domains) dock onto the scaffold. They constitute the permeability barrier and interact with translocating cargo complexes. Some of them (e.g., Nup214/88, Nup358 [RanBP2], and Nup153) introduce asymmetry by binding to the cytoplasmic or nuclear face of the NPC, respectively (reviewed in Grossman et al, 2012) (Figure S1B)
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