Nuclear pore complex diameter

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. 

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... 

Ions, small metabolites, and globular proteins up to 60 kDa can diffuse through a water-filled channel in the nuclear pore complex these channels behave as if they are -0.9 nm in diameter. However, large proteins and ribonucleoprotein complexes cannot diffuse in and out of the nucleus. Rather, these macromolecules are selectively transported in and out of the nucleus with the assistance of soluble transporter proteins that bind macromolecules and also Interact with certain nucleoporlns. The principles underlying macromolecular transport through nuclear pore complexes were first determined for the Import of individual proteins into the nucleus, which we discuss first before turning to the question of how fully processed mRNAs are transported into the c rt oplasm. 

Low transfection rates are a further problem. Reasons are, among others, insufficient lysosomal release of the therapeutic DNA from its carrier and/or its rapid enzymatic degradation within the lyso-some or in the cytosol. The DNA that was integrated in the Kposomes must be released to become effective (see Fig. 5). Studies on microinjections of Kpid/DNA complexes directly into the nucleus clearly show reduced transfection rates [54]. As the nuclear pores have mean diameters of only 25 to 50 nm and passage for macromolecules is further controlled by the nuclear pore complex, passive diffusion into the karyosol is limited to particle sizes below 45 kDa [51, 55]. Conventional recombinant plasmids normally have a molecular weight ranging from 50 to 100 kDa, and therefore require active transport into the nucleus [55]. 

Figure 10. AFM measurements of nuclear pore complex (NPC) conformation in the < > 0 (A) and closed (B) configuration. Nuclear pores are 12S MDa protein assemblies that control transport across the nuclear envelope. The outer diameter of each NPC is 120 nm. Several groups have found evidence for a calcium dependent displacement of the central granule in the pore region from recessed (A), to extended (B), following calcium release. (Fahrenkrog, Stoffler et al. 2001 Moore-Nichols, Amott et ai. 2002 Perez-Teizic, Pyle et al. 1996 Stoffler, Feja et al. 2003 Wang and Clapham 1999) Repriced with permission from (Vickery et al. 1999). Copyright 2000 Biophysical Society.

Pante, N. and Kann, M., 1998. Nuclear pore complex is able to transport macromolecules with diameters of 39 nm. Mol. Biol. Cell, 13,425-434. 

The exchange of substances between the nucleus and the cytoplasm is mediated by pore complexes with complicated structures, which traverse the nuclear membrane. The nuclear pores consist of numerous proteins that form several connected rings of varying diameter. Low-molecular structures and small... 

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. 

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. 

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