Outer nuclear envelope

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 er is a three-dimensional membrane system (57-62). As visualized in a transmission electron microscope, there are two parallel membranes with an intervening electron transparent space, the lumen. The form and abundance of the er vary. The rer are flattened sacs with numerous attached ribosomes (15-20 nm). In contrast, the smooth er (ser) lacks ribosomes. The er seems to function as a communication system within cells and can be continuous with the outer nuclear envelope. Although the rer is involved in protein synthesis, the ser functions in glycosylation. 

The outer nuclear envelope (ONE) is surrounded by other intermediate fibres, and is essentially just the RER surrounding the nucleus, and continuous with it. The space between the INE and ONE is termed the perinuclear space, and is continuous with the RER cisternae. 

NE, ONE, INE nuclear envelope, irmer/outer nuclear envelope. 

Fig. 3 Isolation of nuclei from suspension tissue culture cells. (A) Low-power SEM image of nucleus isolated from K.S62 tissue culture cell, after isolation and Percoll density-gradient purification. This particular nucleus shows surface indentations due to its position in the cell cycle (approaching prophase), but the overall surface morphology of the nuclear envelope is largely free of cytoplasmic contamination, and numerous pore complexes (arrowed) arc apparent. Scale bar, 1.0 /u,m. (B) Detail of A, showing cytoplasmic surface of the nuclear envelope with clearly visible nuclear pore complexes, and a typical density of ribosomes (remember the outer nuclear envelope is the innermost element of the rough endoplasmic reticulum). Scale bar, 125 nm.

The interaction of retinol-CRBP with die nucleus has been further studied (Liau etal., 1981). Using CRBP labeled with tritium by reductive methylation, it was found that CRBP delivers retinol to the specific binding sites within the nucleus without itself remaining bound. The outer nuclear envelope is apparendy not involved because Triton X-lOO-treated nuclei retained 70% of the retinol binding sites found in intact nuclei. Isolated liver chromatin also had the same... 

A system of membrane enclosed cisternae in the cytoplasm. The ER is continuous with the outer membrane of the nuclear envelope. The part of the ER coated with ribosomes is called rough ER, the other part is called smooth-surfaced ER. The rough ER is the first compartment of the secretory pathway. Here, membrane proteins are integrated into and secretory proteins translocated across the ER membrane. Furthermore,... 

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 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 separated from the cytoplasm by the nuclear envelope, which consists of the outer and inner nuclear membranes. Each of the two nuclear membranes has two layers, and the membranes are separated from each other by the perinuclear space. The outer nuclear membrane is continuous with the rough endoplasmic reticulum and is covered with ribosomes. The inner side of the membrane is covered with a protein layer (the nuclear lamina), in which the nuclear structures are anchored. 

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... 

Krajewski S, Tanaka S, Takayama S, Schibler MJ, Fenton W, Reed JC (1993), Investigation of the subcellular distribution of the Bcl-2 oncoprotein residence in the nuclear envelope, endoplasmic reticulum, and outer mitochondrial membranes, Cancer Res. 53 4701-4714. 

The genome in influenza A and B types is enclosed within an outer lipoprotein envelope (Fig. 17.1). The Ml protein lines the inside of the envelope and is chemically bound to the ribonucleoprotein [14], The Ml protein plays an important role in the mediation of nuclear export of viral ribonucleoproteins and also in virus assembly and budding during the infectious cycle [5, 14, 15], An antigenic protein M2, which functions as a proton-selective ion channel, is present in the viral membrane of influenza A viruses [6, 13], In influenza B, the ion channel activity to aid virus uncoating in the endosome is carried out by the similar integral membrane protein BM2 [11],... 

The endoplasmic reticulum (ER) is composed of flattened sacs and tubes of membranous bilayers that extend throughout the cytoplasm enclosing a large intracellular space. The luminal space (Fig. 1-5) is continuous with the outer membrane of the nuclear envelope (Fig. 1-10). It is involved in the synthesis of proteins and their transport to the cytoplasmic membrane (via vesicles, small spherical particles with an outer bilayer membrane). The rough ER (RER) has flattened stacks of membrane that are studded on the outer (cytoplasmic) face with ribosomes (discussed later in this section) that... 

Bcl2 the prototype for a family of mammalian proteins located in the membranes of the endoplasmic reticulum (ER), nuclear envelope, and in the outer membranes of the mitochondria. They can be either pro-apoptotic (Bax, BAD, Bak and Bok among others) or anti-apoptotic. 

The nucleus is a large membrane-bound cell organelle which houses the chromosomes and which occupies roughly 10% of the volume of all eukaryotic cells. The nucleus is separated from the rest of the cell and the cytoplasm by a double membraue known as the nuclear envelope. The outer layer of the nuclear membrane is studded with small openings called nuclear pores, which allow for the controlled movement of selected molecules in and out of the nucleus. Most of a eukaryotic cell s DNA is found in the chromosomes of the nucleus, while a very small amount of DNA is present in the mitochondria. All plant and animal cells with a nucleus and known as eukaryotic cells, (meaning true nucleus) while bacterial cells which lack a nucleus are known as prokaryotic cells. 

The nucleus (Figure 2.15) consists of nucleoplasm surrounded by a nuclear envelope. The nucleoplasm is a material rich in DNA in which proteins called lamins form a fibrous network that provides structural support. A prominent feature of the nucleoplasm is a network of chromatin fibers composed of DNA and DNA packaging proteins known as the histones. The DNA is believed to be attached to the lamins. The nuclear envelope is composed of two membranes that fuse at structures called nuclear pores. The outer nuclear membrane is continuous with the rough endoplasmic reticulum. The nuclear pores (Figure 2.16) are relatively... 

Q Nuclear envelope, a double membrane, encloses the contents of the nucleus the outer nuclear membrane is continuous with the rough ER. 

In addition, MPF-catalyzed phosphorylation of specific nucleoporins (see Chapter 12) causes nuclear pore complexes to dissociate into subcomplexes during prophase. Similarly, phosphorylation of Integral membrane proteins of the inner nuclear membrane is thought to decrease their affinity for chromatin and contribute to disassembly of the nuclear envelope. The weakening of the associations between the inner nuclear membrane and the nuclear lamina and chromatin may allow sheets of inner nuclear membrane to retract into the endoplasmic reticulum, which is continuous with the outer nuclear membrane. 

Lithgow, T., van, D.R., Bertram, J.F., and Strasser, A. (1994). The protein product of the oncogene bcl-2 is a component of the nuclear envelope, the endoplasmic reticulum, and the outer mitochondrial membrane. Cell Growth Differ. 5, 411-417. 

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