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Review
. 2010 Mar;2(3):a000539.
doi: 10.1101/cshperspect.a000539.

The Nuclear Envelope

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Free PMC article
Review

The Nuclear Envelope

Martin W Hetzer. Cold Spring Harb Perspect Biol. .
Free PMC article

Abstract

The nuclear envelope (NE) is a highly regulated membrane barrier that separates the nucleus from the cytoplasm in eukaryotic cells. It contains a large number of different proteins that have been implicated in chromatin organization and gene regulation. Although the nuclear membrane enables complex levels of gene expression, it also poses a challenge when it comes to cell division. To allow access of the mitotic spindle to chromatin, the nucleus of metazoans must completely disassemble during mitosis, generating the need to re-establish the nuclear compartment at the end of each cell division. Here, I summarize our current understanding of the dynamic remodeling of the NE during the cell cycle.

Figures

Figure 1.
Figure 1.
Topology of the NE. Inner and outer nuclear membranes (INM and ONM, respectively) are separated by the ER lumen or perinuclear space (PNS). The nuclear lamina interacts with NE proteins and chromatin. INM proteins link the NE to chromatin and the lamina. ONM proteins provide a connection from the nucleus to the cytoskeleton. The lamin B receptor (LBR) interacts both with B-type lamins and chromatin-associated heterochromatin protein 1 (HP1) in conjunction with core histones. Members of the LEM (lamina-associated protein 2 [LAP2], emerin, MAN1)-domain family (pink) bind to lamins and interact with chromatin through barrier-to-autointegration factor (BAF). SUN proteins (SUN 1 and 2) interact with nesprins in the ONM, thereby forming so-called LINC complexes that establish connections to actin and intermediate filaments in the cytoplasm. Nurim is a multi-pass membrane protein with unknown function. Proteomic approaches have identified ∼60 putative transmembrane proteins (NETs), most of which remain uncharacterized.
Figure 2.
Figure 2.
Schematic illustration of NE breakdown. In G2, the cell nucleus has completed DNA replication and NPC duplication. The NE (dark green), which is continuous with the ER network (green), encloses the chromosomes (blue). NPCs (red/blue) mediate nuclear transport. When cells enter mitosis, NPCs disassemble and the NE gets reabsorbed into the ER, which at this stage is composed of tubules. NPC components are dispersed into the cytoplasm and NE proteins are partitioned into the ER (dashed green lines). Centrosomes (orange dots) move to the NE and microtubules (purple) participate in the rupturing of the NE. In metaphase, a subset of NPC components has associated with kinetochores and the spindle is established. At this stage, chromosomes at the metaphase plate are devoid of membranes.
Figure 3.
Figure 3.
Schematic illustration of NE reformation around segregated chromosomes in one of the two daughter cells. In anaphase, ER membranes associate with chromatin (red arrows) and a subset of nucleoporins associates with chromatin. Additional membrane tubules and NE proteins are recruited to the chromatin surface and mediate NE flattening. At this stage, most NE proteins are cleared from the ER network. In late anaphase/early telophase, a closed NE has formed with pores being assembled in a step-wise manner. Once pores become transport competent, the NE expands and cells move into G1.

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