The nuclear envelope: form and reformation - PubMed (original) (raw)

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The nuclear envelope: form and reformation

Amy J Prunuske et al. Curr Opin Cell Biol. 2006 Feb.

Abstract

The membrane system that encloses genomic DNA is referred to as the nuclear envelope. However, with emerging roles in signaling and gene expression, these membranes clearly serve as more than just a physical barrier separating the nucleus and cytoplasm. Recent progress in our understanding of nuclear envelope architecture and composition has also revealed an intriguing connection between constituents of the nuclear envelope and human disease, providing further impetus to decipher this cellular structure and the dramatic remodeling process it undergoes with each cell division.

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Figures

Figure 1

Figure 1

Schematic diagram of the nucleus, highlighting membrane domains of the nuclear envelope (NE) and associated structures. The membrane system of the nuclear envelope consists of the outer nuclear membrane (ONM), the inner nuclear membrane (INM) and the pore membrane (POM). The ONM is contiguous with the endoplasmic reticulum (ER). Portions of the NE extend into the nucleus forming the nucleoplasmic reticulum (NR). The INM contains many distinct proteins (black) that contact the underlying lamina and chromatin. The pore membrane houses integral membrane proteins of nuclear pore complexes (green). Some ONM proteins (yellow) are also present within the ER and others (red) preferentially localize to the ONM and are proposed to bridge INM proteins to such cytoplasmic structures as the centrosome and actin filaments. Finally, another category of protein (blue ovals) is able to diffuse within the perinuclear space and to interact with luminal domains of NE proteins.

Figure 2

Figure 2

Model of reformation of the nuclear envelope (NE) at the end of mitosis. Chromatin-associated RanGEF creates a gradient of RanGTP around DNA, which induces the localized release of nucleoporins (green balls) chaperoned at mitosis by importin β (orange). Importin α (red) also participates in nuclear formation and is, in part, membrane-associated. Some inner nuclear membrane (INM) proteins (grey), present on membrane, target to the chromatin during assembly. Formation of a closed NE requires incorporation of nuclear pore complexes into the fusing membrane. NE growth requires the addition of more membrane and pores as well as import through the nuclear pore complexes. Additional INM proteins (black), synthesized in the ER, target to the INM via the pore membrane (POM).

Figure 3

Figure 3

Mechanistic models of nuclear envelope (NE) disassembly. Key findings in various experimental systems are schematically depicted. Cells are not drawn to scale; it is notable that the oocyte is very large compared to a somatic cell, since size may impose specific constraints on the mechanics of NE breakdown. (a) In starfish oocytes, early alterations in permeability at the nuclear pore (green) have been observed, and correlate with an early phase of nuclear pore complex (NPC) disassembly. During the second phase of disassembly, larger holes in the NE are proposed to emanate from the site of disassembled pores. (b) In embryonic-like nuclei formed in vitro from Xenopus egg extract, nuclear pore proteins recruit the COPI complex (red) to the NE. Local concentration of this coatomer complex may then lead to vesiculation of the NE, as depicted, or to a non-conventional role for COPI. (c) In human tissue culture cells (somatic), microtubules originating from the centrosomes (yellow) connect to the NE via the microtubule motor dynein (black). Dynein-mediated movement is thought to then pull the NE toward the centrosomes, eventually causing a rupture at a distal region of the NE. (d) In Ustilago maydis, a basidiomycete, the NE is dragged from the mother cell to the bud by microtubules and dynein (black). There is an early increase in permeability, suggestive of pore remodeling, and then an obvious opening in the NE near the spindle pole body (yellow). Subsequently, the chromosomes enter the daughter cell where the spindle is formed, and the remnant of the NE collapses into the mother cell.

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