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You are watching: Which of the following statements about the nuclear envelope is false?

Cooper GM. The Cell: A molecular Approach. 2nd edition. Sunderland (MA): Sinauer Associates; 2000.


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The nuclear envelope separates the materials of the nucleus from the cytoplasm and also provides the structural framework of the nucleus. The nuclear membranes, exhilaration as barriers that stop the totally free passage that molecules between the nucleus and also the cytoplasm, keep the nucleus as a unique biochemical compartment. The sole networks through the atom envelope are noted by the nuclear pore complexes, which allow the regulation exchange of molecules between the nucleus and cytoplasm. The selective traffic of proteins and RNAs with the atom pore complexes not only establishes the interior composition that the nucleus, but also plays a an important role in regulating eukaryotic gene expression.


Structure of the nuclear Envelope

The nuclear envelope has actually a complicated structure, consists of two nuclear membranes, an underlying atom lamina, and nuclear pore complexes (Figure 8.1). The nucleus is surrounding by a mechanism of two concentric membranes, called the inner and outer nuclear membranes. The outer nuclear membrane is continuous with the endoplasmic reticulum, so the room between the inner and also outer nuclear membranes is directly linked with the lumen of the absorbent reticulum. In addition, the outer nuclear membrane is functionally comparable to the membrane of the endoplasmic reticulum (see thing 9) and has ribosome bound to its cytoplasmic surface. In contrast, the inner nuclear membrane carries distinctive proteins the are details to the nucleus.


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Figure 8.1

The nuclear envelope. (A) an electron micrograph that a nucleus. The inner and outer atom membranes space joined at atom pore complexes (arrows). (B) one electron micrograph depicting the continually of the outer nuclear membrane through the endoplasmic (more...)


The critical function that the atom membranes is to act together a obstacle that the end the materials of the nucleus from the cytoplasm. Like other cell membranes, the atom membranes are phospholipid bilayers, which room permeable only to tiny nonpolar molecule (see figure 2.49). Various other molecules room unable come diffuse with the phospholipid bilayer. The inner and also outer atom membranes are joined at nuclear pore complexes, the sole channels through which small polar molecules and macromolecules room able to travel v the nuclear envelope (Figure 8.2). As debated in the following section, the atom pore complex is a facility structure the is responsible because that the selective traffic of proteins and RNAs in between the nucleus and also the cytoplasm.


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Figure 8.2

Electron micrograph reflecting nuclear pores. Many nuclear pores (arrows) space visible in this freeze-fracture preparation of the atom envelope. (Photo Researchers, Inc.)


Underlying the inner atom membrane is the atom lamina, a fibrous meshwork that gives structural support to the cell nucleus (Figure 8.3). The nuclear lamina is created of one or an ext related proteins called lamins. Most mammalian cells, for example, save four various lamins, designated A, B1, B2, and C. All the lamins room 60- come 80-kilodalton (kd) fibrous proteins that are concerned the intermediate filament protein of the cytoskeleton (see thing 11). Like other intermediate filament proteins, the lamins associate v each other to kind filaments (Figure 8.4). The an initial stage the this combination is the communication of two lamins to kind a dimer in i m sorry the α-helical areas of two polypeptide chains room wound approximately each various other in a structure referred to as a coiled coil. This lamin dimers then associate v each various other to type the filaments that comprise the atom lamina. The association of lamins through the inner nuclear membrane is helped with by the posttranslational enhancement of lipid—in particular, prenylation the C-terminal cysteine residues (see number 7.30). In addition, the lamins tie to inner atom membrane proteins, i m sorry may assist organize the lamin filaments into a meshwork and also mediate their attachment to the membrane.


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Figure 8.3

Electron micrograph of the nuclear lamina. The lamina is a meshwork that filaments underlying the inner atom membrane. (From U. Aebi, J. Cohn, L. Buhle and L. Gerace, 1986. Nature 323: 560.)


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Figure 8.4

Model of lamin assembly. The lamin polypeptides kind dimers in which the main α-helical areas of 2 polypeptide chains space wound around each other. Additional assembly may involve the head-to-tail combination of dimers to type linear polymers (more...)


In enhancement to offering structural assistance to the nucleus, the nuclear lamina is thought to offer as a website of chromatin attachment. Chromatin within the cell nucleus is organized into big loops of DNA, several of which show up to be bound to the atom envelope. The lamins tie chromatin and may help mediate this interaction.


The atom Pore Complex

The atom pore complexes are the only networks through which tiny polar molecules, ions, and macromolecules (proteins and also RNAs) room able come travel between the nucleus and the cytoplasm. The atom pore complex is one extremely huge structure v a diameter of about 120 nm and also an estimated molecular fixed of about 125 million daltons—about 30 time the size of a ribosome. In vertebrates, the atom pore complex is created of 50 come 100 various proteins. By regulating the website traffic of molecules between the nucleus and also cytoplasm, the atom pore complex plays a fundamental role in the physiology of all eukaryotic bio cells. RNAs that are synthesized in the nucleus have to be effectively exported to the cytoplasm, wherein they role in protein synthesis. Whereas proteins compelled for nuclear functions (e.g., transcription factors) should be transported right into the cell core from your sites of synthesis in the cytoplasm. In addition, numerous proteins spaceship continuously in between the nucleus and the cytoplasm. The regulated traffic of proteins and also RNAs through the atom pore complicated thus identify the composition of the nucleus and plays a vital role in gene expression.

Depending on your size, molecules can travel v the atom pore facility by among two different mechanisms (Figure 8.5). Little molecules and some proteins with molecular mass much less than around 50 kd pass freely throughout the atom envelope in one of two people direction: cytoplasm come nucleus or nucleus come cytoplasm. These molecules diffusive passively through open aqueous channels, estimated to have actually diameters of around 9 nm, in the atom pore complex. Many proteins and RNAs, however, are unable to pass v these open up channels. Instead, this macromolecules pass with the atom pore facility by an active procedure in which proper proteins and RNAs space recognized and selectively transported in only one direction (nucleus come cytoplasm or cytoplasm to nucleus). The website traffic of these molecules occurs with regulated networks in the atom pore complicated that, in response to suitable signals, deserve to open to a diameter of an ext than 25 nm—a size enough to accommodate huge ribonucleoprotein complexes, such as ribosomal subunits. That is through these regulated networks that nuclear proteins space selectively imported native the cytoplasm come the nucleus if RNAs space exported native the nucleus come the cytoplasm.


Figure 8.5

Molecular traffic through nuclear spicy complexes. Little molecules room able to pass quickly through open networks in the nuclear pore complicated by passive diffusion. In contrast, macromolecules are transported through a selective, energy-dependent device that (more...)


Visualization of nuclear pore complexes through electron microscopy reveals a structure with eightfold the opposite organized about a large central channel (Figure 8.6), i beg your pardon is the route through which proteins and also RNAs overcome the nuclear envelope. Detailed structural studies, consisting of computer-based image analysis, have actually led to the breakthrough of three-dimensional models of the nuclear pore facility (Figure 8.7). This studies show that the nuclear pore complicated consists of one assembly of eight spokes arranged around a main channel. The spokes are linked to rings at the nuclear and also cytoplasmic surfaces, and also the spoke-ring assembly is anchored in ~ the atom envelope in ~ sites of fusion between the inner and also outer nuclear membranes. Protein filaments expand from both the cytoplasmic and nuclear rings, developing a distinct basketlike structure on the atom side. The central channel is approximately 40 nm in diameter, i beg your pardon is vast enough come accommodate the biggest particles maybe to cross the nuclear envelope. It consists of a structure dubbed the central transporter, through which the active transport of macromolecules is believed to occur.


Figure 8.6

Electron micrograph of nuclear pore complexes. In this face-on view, isolated atom pore complexes appear to covers eight structural subunits surrounding a central channel. (Courtesy the Dr. Ron Milligan, Scripps research Institute.)


Figure 8.7

Model the the atom pore complex. The complex consists of one assembly the eight spokes fastened to rings on the cytoplasmic and also nuclear political parties of the nuclear envelope. The spoke-ring assembly surrounds a central channel include the main transporter. (more...)


Selective transfer of proteins to and from the Nucleus

The basis because that selective traffic throughout the atom envelope is best understood for proteins that are imported native the cytoplasm to the nucleus. Together proteins space responsible because that all elements of genome structure and function; they include histones, DNA polymerases, RNA polymerases, warrior factors, splicing factors, and also many others. This proteins are targeted come the nucleus by particular amino acid sequences, called nuclear localization signals, that direct their transport through the nuclear pore complex.

The first nuclear localization signal to it is in mapped in information was defined by Alan Smith and colleagues in 1984. These investigators studied simian virus 40 (SV40) T antigen, a virus-encoded protein the initiates famous DNA replication in infected cell (see thing 5). As meant for a replication protein, T antigen usually is localized to the nucleus. The signal responsible because that its atom localization was very first identified by the finding the mutation the a single lysine residue stays clear of nuclear import, resulting instead in the buildup of T antigen in the cytoplasm. Subsequent studies defined the T antigen nuclear localization signal together the seven-amino-acid sequence Pro-Lys-Lys-Lys-Arg-Lys-Val. Not just was this sequence important for the nuclear move of T antigen, yet its enhancement to other, normally cytoplasmic, proteins was likewise sufficient to straight their accumulation in the nucleus.

Nuclear localization signal have because been identified in countless other proteins. Many of this sequences, choose that the T antigen, are quick stretches well-off in straightforward amino acid residues (lysine and arginine). In plenty of cases, however, the amino mountain that kind the atom localization signal room close together but not immediately surrounding to each other. Because that example, the nuclear localization signal of nucleoplasmin (a protein associated in chromatin assembly) consists of two parts: a Lys-Arg pair followed by four lysines located ten amino mountain farther downstream (Figure 8.8). Both the Lys-Arg and Lys-Lys-Lys-Lys sequences are forced for nuclear targeting, but the ten amino acids between these sequences deserve to be mutated there is no affecting atom localization. Because this nuclear localization succession is written of 2 separated elements, the is described as bipartite. Similar bipartite motifs show up to function as the localization signal of many nuclear proteins; therefore they may be an ext common than the easier nuclear localization signal of T antigen. In addition, part proteins, such as ribosomal proteins, contain distinct nuclear localization signals which are unrelated to the straightforward amino acid-rich nuclear localization signal of either nucleoplasmin or T antigen.


Figure 8.8

Nuclear localization signals. The T antigen nuclear localization signal is a solitary stretch of amino acids. In contrast, the nuclear localization signal the nucleoplasmin is bipartite, consist of of a Lys-Arg sequence, followed by a Lys-Lys-Lys-Lys succession (more...)


Protein import v the atom pore facility can be operationally split into 2 steps, differentiated by even if it is they require energy (Figure 8.9). In the very first step, which go not call for energy, proteins that contain nuclear localization signals bind to the atom pore complicated but perform not pass with the pore. In this early step, nuclear localization signal are known by a cytosolic receptor protein, and also the receptor-substrate complex binds to the atom pore. The prototype receptor, dubbed importin, consists of two subunits. One subunit (importin α) binding to the simple amino acid-rich atom localization signals of protein such as T antigen and nucleoplasmin. The second subunit (importin β) binding to the cytoplasmic filaments of the atom pore complex, happen the target protein to the atom pore. Other varieties of atom localization signals, such as those of ribosome proteins, are recognized by distinct receptors i m sorry are related to importin β and duty similarly to importin β during the transport of your target proteins into the nucleus.


Figure 8.9

Protein import through the nuclear pore complex. Proteins are transported with the atom pore complicated in two steps. In the example shown, a protein v a classical simple amino acid-rich atom localization sequence (NLS) is recognized by importin (more...)


The 2nd step in atom import, translocation v the atom pore complex, is one energy-dependent process that calls for GTP hydrolysis. A crucial player in the translocation process is a tiny GTP-binding protein referred to as Ran, i m sorry is regarded the Ras proteins (Figure 8.10). The conformation and activity of Ran is regulation by GTP binding and hydrolysis, favor Ras or numerous of the translate in factors involved in protein synthetic (see number 7.12). Enzymes that stimulate GTP binding come Ran are localized to the nuclear side of the nuclear envelope vice versa, enzymes that stimulate GTP hydrolysis are localized come the cytoplasmic side. Consequently, over there is a gradient of Ran/GTP throughout the nuclear envelope, through a high concentration of Ran/GTP in the nucleus and also a high concentration the Ran/GDP in the cytoplasm. This gradient the Ran/GTP is assumed to identify the directionality of nuclear transport, and GTP hydrolysis through Ran shows up to account for many (if no all) of the power required for nuclear import. Importin β creates a complicated with importin α and its linked target protein on the cytoplasmic next of the atom pore complex, in the visibility of a high concentration the Ran/ GDP. This facility is then transported with the nuclear pore to the nucleus, wherein a high concentration the Ran/GTP is present. In ~ the nuclear side of the pore, Ran/GTP binding to importin β, displacing importin α and also the target protein. Together a result, the target protein is released in ~ the nucleus. The Ran/GTP-importin β complex is then exported come the cytosol, wherein the bound GTP is hydrolyzed come GDP, releasing importin β to participate in one more cycle of nuclear import.


Figure 8.10

Role that the Ran protein in atom import. Transport with the atom pore complicated is pushed by a gradient the Ran/GTP, v a high concentration of Ran/GDP in the cytoplasm and a high concentration that Ran/GTP in the nucleus. Complexes kind between (more...)


Some proteins remain within the nucleus following their import from the cytoplasm, however many rather shuttle ago and forth in between the nucleus and the cytoplasm. Some of these protein act together carriers in the transport of various other molecules, such together RNAs; rather coordinate nuclear and also cytoplasmic attributes (e.g., by regulation the tasks of warrior factors). Proteins room targeted because that export from the cell core by particular amino mountain sequences, referred to as nuclear export signals. Choose nuclear localization signals, nuclear export signal are recognized by receptors within the cell nucleus that straight protein transport v the nuclear pore facility to the cytoplasm. Interestingly, the nuclear export receptors (called exportins) are pertained to importin β. Choose importin β, the exportins bind to Ran, i beg your pardon is compelled for nuclear export and for nuclear income (Figure 8.11). Strikingly, however, Ran/GTP disclosure the formation of steady complexes in between exportins and their target proteins, whereas the dissociates the complexes between importins and also their targets. This impact of Ran/GTP binding top top exportins dictates the motion of proteins containing atom export signal from the nucleus come the cytoplasm. Thus, exportins form stable complexes v their target protein in association v Ran/GTP within the nucleus. Adhering to transport come the cytosolic side of the nuclear envelope, GTP hydrolysis leads to dissociation of the target protein, which is released into the cytoplasm.


Figure 8.11

Nuclear export. Complexes in between target proteins bearing nuclear export signal (NES), exportins, and Ran/ GTP form in the nucleus. Following transport through the atom pore complex, Ran space stimulates the hydrolysis of tied GTP, leading to development (more...)


Regulation of atom Protein Import

An intriguing element of the carry of proteins right into the cell core is the it is an additional level at which the activities of nuclear proteins can be controlled. Transcription factors, because that example, are sensible only once they are current in the nucleus, so regulation that their income to the cell core is a novel way of controlling gene expression. As will certainly be questioned in thing 13, the regulation nuclear import of both transcription factors and protein kinases plays critical role in controlling the habits of cells in an answer to changes in the environment, because it gives a mechanism by which signals received at the cell surface deserve to be sent to the nucleus.

In one device of regulation, transcription components (or various other proteins) associate through cytoplasmic proteins the mask your nuclear localization signals; due to the fact that their signals are no longer recognizable, this proteins remain in the cytoplasm. A great example is noted by the transcription variable NF-κB, i beg your pardon activates transcription of immunoglobulin-κ light chains in B lymphocytes (Figure 8.12). In unstimulated cells, NF-κB is found as one inactive complex with one inhibitory protein (IκB) in the cytoplasm. Binding to IκB appears to mask the NF-κB nuclear localization signal, thus staying clear of NF-κB from being transported into the nucleus. In engendered cells, IκB is phosphorylated and also degraded by ubiquitin-mediated proteolysis, permitting NF-κB to enter the nucleus and also activate transcription of the target genes.


Figure 8.12

Regulation of nuclear income of warrior factors. The transcription aspect NF-κB is preserved as an inactive complex with IκB, i m sorry masks its atom localization succession (NLS), in the cytoplasm. In an answer to proper extracellular (more...)


The nuclear income of other transcription components is regulated straight by their phosphorylation, fairly than through association v inhibitory proteins. For example, the yeast transcription variable SWI5 is imported into the nucleus just at a certain stage of the cell cycle (see figure 8.12). Otherwise, SWI5 is kept in the cytoplasm as a result of phosphorylation at serine residues surrounding to its atom localization signal, preventing nuclear import. Regulated dephosphorylation of this sites activates SWI5 at the proper stage that the cell cycle by permit its translocation to the nucleus.


Transport the RNAs

Whereas many proteins are selectively transported indigenous the cytoplasm right into the nucleus, many RNAs are exported indigenous the nucleus come the cytoplasm. Since proteins space synthesized in the cytoplasm, the violin of mRNAs, rRNAs, and tRNAs is a crucial step in gene expression in eukaryotic bio cells. Favor protein import, the violin of RNAs through nuclear sharp complexes is an active, energy-dependent process that needs the Ran GTP-binding protein.

RNAs space transported across the nuclear envelope as RNA-protein complexes, i beg your pardon in some instances are huge enough come visualize through electron microscopy (Figure 8.13). The substrates for transport room ribonucleoprotein complexes rather than nude RNAs, and also RNAs space targeted for transport from the cell core by atom export signal on the proteins bound come them. This proteins are known by exportins and transported from the nucleus come the cytoplasm as described earlier (see figure 8.11). Pre-mRNAs and also mRNAs are connected with a set of at the very least 20 proteins (forming heterogeneous atom ribonucleoproteins, or hnRNPs) transparent their handling in the nucleus and eventual move to the cytoplasm. At least two of this hnRNP protein contain nuclear export signals and also are believed to role as carrier of mRNAs throughout their export to the cytoplasm. As debated in a later on section that this chapter, ribosome RNAs room assembled v ribosomal proteins in the nucleolus, and intact ribosom subunits are then transported come the cytoplasm. Your export indigenous the nucleus appears to be mediated by nuclear export signals current on ribosomal proteins. Because that tRNAs, the specific proteins the mediate nuclear export stay to be identified.


Figure 8.13

Transport of a ribonucleoprotein complex. Insect salient gland cell produce big ribonucleoprotein complexes (RNPs), i beg your pardon contain 35 come 40 kb the RNA and have a complete mass of approximately 30 million daltons. This collection of electron micrographs reflects (more...)


In comparison to mRNAs, tRNAs, and rRNAs, which duty in the cytoplasm, the snRNAs role within the cell core as contents of the RNA handling machinery. Perhaps surprisingly, these RNAs are at first transported native the nucleus to the cytoplasm, where they associate v proteins to type functional snRNPs and then return to the cell nucleus (Figure 8.14). Proteins that tie to the 5′ hat of snRNAs appear to be involved in the export of the snRNAs to the cytoplasm, vice versa, sequences present on the snRNP proteins room responsible for the carry of snRNPs from the cytoplasm come the nucleus.

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Figure 8.14

Transport the snRNAs in between nucleus and cytoplasm. Little nuclear RNAs are initially exported from the nucleus to the cytoplasm, whereby they associate v proteins to form snRNPs. The assembled snRNPs space then transported earlier into the nucleus.


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