Nuclear envelope pores

The procedures reported here have been used to investigate the development of male pronuclei in cell-free systems of sea urchins and, to a lesser extent, surf clams. Pronuclear formation in vitro is a slower process than that in vivo. This property has been used advantageously to examine the steps of pronuclear formation. Each step can be easily manipulated, but the methods described here may require some adjustments for other organisms. Sea urchin and surf clam male pronuclei formed in vitro are virtually complete, with decondensed chromatin, nuclear envelopes, pores, and lamina. 

The nuclear pore complex, located in the nuclear envelope, contains more than 50 proteins. It allows diffusion of small proteins between cytoplasm and nucleoplasm. Larger molecules (>50kD) are selectively transported by an energy-dependent mechanism. 

FIGURE 1-5 Detail of the nuclear envelope showing a nuclear pore (single arrow) and the outer leaflet connected to the smooth endoplasmic reticulum (ER) (double arrows). Two cisternae of the rough ER with associated ribosomes are also present. X80,000. 

The nucleus (37-44) is the most prominent structure within the cytoplasm. It is bounded by a nuclear envelope containing circular pores that are 30-100 nm in diameter. The outer nuclear envelope may be continuous with the er. 

The nuclear envelope is composed of the nuclear membranes (inner and outer), the nuclear lamina, and the nuclear pore complexes. The inner and outer nuclear membranes are connected at the nuclear pore sites and enclose a flattened sac... 

The nucleus is surrounded by the nuclear envelope, which takes on a lumenal structure connected to the endoplasmic reticulum. The transport of proteins into (and out of) the nucleus occurs through the nuclear pore complex (NPC), a large complex composed of more than 100 different proteins (Talcott and Moore, 1999). Because NPC forms an aqueous pore across the two membranes, small proteins less than 9 nm in diameter can pass through it simply by diffusion. However, most of the transports of both proteins and RNAs are mediated by an active transport mechanism. It is now clear that there is heavy traffic through the NPC in both directions. Proteins are not only imported into the nucleus but also actively exported from it as well. There are many reasons for nuclear export. One reason is to send some shuttle proteins back after their import another is for some viral proteins to export their replicated genomes outside the nucleus. 

Nuclear Envelope The membrane system of the cell nucleus that surrounds the nucleoplasm. It consists of two concentric membranes separated by the perinuclear space. The structures of the envelope where it opens to the cytoplasm are called the nuclear pores (nudear pore). [NIH]... 

Goldberg MW, Allen TD. High resolution scanning electron microscopy of the nuclear envelope demonstration of a new, regular, fibrous lattice attached to the baskets of the nucleoplasmic face of the nuclear pores. J Cell Biol 1992 119(6) 1429-1440. 

Regarding the localization of the majority of proteasomes to the nuclear rim and nuclear envelope, it seems likely, that proteasomal degradation requires nuclear export or translocation processes, which directs nuclear proteins at least to the nuclear pore or to the inner surface of the nucleus. This implies that all components of the ubiquitination machinery have to be active inside the nucleus, v ich seems to be a prerequisite for the specific export process. It is also possible that nucleus-specific kinases or E2/E3 enzymes promote the triggering of nuclear proteins for rapid degradation. 

O-GlcNAc s subcellular localization in rat hepatocytes established that it is highly concentrated at the nuclear envelope, particularly at the nuclear pore complex, but is also abundant and widespread within chromatin (Holt and Hart, 1986). Aside from the biosynthetic intermediates,... 

The nuclear envelope is perforated with huge macromolecular assemblies of 30 different proteins that form nuclear pore complexes with a central channel of 25-30 nm in diameter. This channel allows proteins smaller than 30 kDa to passively traverse the outer and inner nuclear membranes. Larger proteins are actively transported across the nuclear envelope and contain nuclear localization signal (NLS) sequence motifs. These signals consist of one or two clusters of four or five basic residues localized usually within the polypeptide chain. The import of proteins with NLS through the channel is facilitated by the carrier heterodimer of importin-a ( > (Gorlich and Kutay 1999 Pemberton and Paschal... 

Each cell nucleus contains one or more dense nucleoli, regions that are rich in RNA and may contain 10-20% of the total RNA of cells. Nucleoli are sites of synthesis and of temporary storage of ribosomal RNA, which is needed for assembly of ribosomes. The nuclear envelope is a pair of membranes, usually a few tens of nanometers apart, that surround the nucleus. The two membranes of the pair separate off a thin perinuclear space (Fig. 1-7). The membranes contain "pores" -130 ran in diameter with a complex structure (see Fig. 27-8).38/39 There is a central channel -42 ran in diameter, which provides a route for controlled passage of RNA and other large molecules from the nucleus into the cytoplasm and also from the cytoplasm to the nucleus. Smaller -10 nm channels allow passive diffusion of ions and small molecules. 

Figure 27-7 Native nuclear lamina of Xenopus oocytes. Freeze-dried metal-shadowed nuclear envelope extracted with Triton X-100, revealing the nuclear lamina meshwork partially covered with arrays of nuclear pore complexes. Inset, relatively well-preserved area of the meshwork of nearly orthogonal filaments from which pore complexes have been mechanically removed. Bar, 1 pm. From Aebi et al.121...

The nucleus of eukaryotic cells is a very complex structure, containing various components. It is separated from the rest of the cell by two membranes named the nuclear envelope. At regular intervals, the two membranes of the nuclear envelope form pores with a diameter of around 90 nm. These pores regulate flux of macromolecules to and from the cytoplasm. Inside the nucleus is located the nucleolus, which acts to produce ribonucleic acid (RNA), which is the first step for ribosome synthesis. 

The nucleus of the eukaryotic cell is separated from the cytoplasm by the double-membrane nuclear envelope, which provides a continuous boundary between the nucleoplasm and the cytoplasm, except where it is penetrated by nuclear pores, each of which is surrounded by a disklike structure, the nuclear pore complex. These pores serve an export-import function for an exchange of materials between the nucleus and the cytosol. This is necessary in eukaryotic... 

Nuclear Pore A large transporter that spans the nuclear envelope (nuclear membrane). This... 

Erickson, E.S.,Mooren, O.L., Moore-Nichols, D. and Dunn, R.C. (2004)Activation of ryanodine receptors in the nuclear envelope alters the conformation of the nuclear pore complex. Biophys. Chem. 112,1-7. 

Hallbeig, E., Wozniak, R.W. and Blobel, G. (1993) An integral membrane protein of the pore membrane domain of the nuclear envelope contains a nucleoporin-like region. J. Cell Biol. 122, 513-521. 

The increase in nuclear cyclin B/CDKl activity promotes phosphorylation of nuclear substrates that are necessary for mitosis, such as nuclear envelope breakdown, spindle formation, chromatin condensation, and restmcturing of the Golgi and endoplasmic reticulum (85, 86). Numerous cyclin B/CDKl substrates have been dehned, which include nuclear lamins, nucleolar proteins, centrosomal proteins, components of the nuclear pore complex, and microtubule-associated proteins (87-89). Cyclin B/CDKl complexes also phosphorylate MCM4 to block replication of DNA, the TFIIH subunit of RNA polymerase II to inhibit transcription, and the ribosomal S6 protein kinase to prevent translation during mitosis (90-92). 

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