The entry and exit of large molecules from the
cell nucleusIn cell biology, the nucleus , also sometimes referred to as the "control center", is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as...
is tightly controlled by the nuclear pore complexes (NPCs). Although small molecules can enter the nucleus without regulation, macromolecules such as RNA and proteins require association with
karyopherinKaryopherins are a group of proteins involved in transporting molecules through the pores of the nuclear envelope ....
s called
importinImportin is a type of protein that moves other protein molecules into the nucleus by binding to a specific recognition sequence, called the nuclear localization signal . Importin is classified as a karyopherin....
s to enter the nucleus and exportins to exit.
Proteins that must be imported to the nucleus from the cytoplasm carry
nuclear localization signalA nuclear localization signal or sequence is an amino acid sequence which acts like a 'tag' on the exposed surface of a protein. This sequence is used to target the protein to the cell nucleus through the Nuclear Pore Complex and to direct a newly synthesized protein into the nucleus via its...
s (NLS) that are bound by importins.
The entry and exit of large molecules from the
cell nucleusIn cell biology, the nucleus , also sometimes referred to as the "control center", is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as...
is tightly controlled by the nuclear pore complexes (NPCs). Although small molecules can enter the nucleus without regulation, macromolecules such as RNA and proteins require association with
karyopherinKaryopherins are a group of proteins involved in transporting molecules through the pores of the nuclear envelope ....
s called
importinImportin is a type of protein that moves other protein molecules into the nucleus by binding to a specific recognition sequence, called the nuclear localization signal . Importin is classified as a karyopherin....
s to enter the nucleus and exportins to exit.
Proteins that must be imported to the nucleus from the cytoplasm carry
nuclear localization signalA nuclear localization signal or sequence is an amino acid sequence which acts like a 'tag' on the exposed surface of a protein. This sequence is used to target the protein to the cell nucleus through the Nuclear Pore Complex and to direct a newly synthesized protein into the nucleus via its...
s (NLS) that are bound by importins. A NLS is a sequence of amino acids that acts as a tag. They are diverse in their composition and most commonly hydrophilic, although hydrophobic sequences have also been documented. Proteins,
transfer RNATransfer RNA is a small RNA molecule that transfers a specific active amino acid to a growing polypeptide chain at the ribosomal site of protein synthesis during translation. It has a 3' terminal site for amino acid attachment. This covalent linkage is catalyzed by an aminoacyl tRNA synthetase...
, and assembled
ribosomalRibosomes are complexes of RNA and protein that are found in all cells. The ribosome is part of the mechanism that translates the DNA sequence into the protein sequence. Ribosomes from bacteria, archaea and eukaryotes , have significantly different structure and RNA...
subunits are exported from the nucleus due to association with exportins, which bind signaling sequences called
nuclear export signalA nuclear export signal is a short amino acid sequence of 5-6 hydrophobic residues in a protein that targets it for export from the cell nucleus to the cytoplasm through the nuclear pore complex. It has the opposite effect of a nuclear localization signal, which targets a protein located in the...
s (NES). The ability of both importins and exportins to transport their cargo is regulated by the small Ras related
GTPaseGTPases are a large family of hydrolase enzymes that can bind and hydrolyze guanosine triphosphate . The GTP binding and hydrolysis takes place in the highly conserved G domain common to all GTPases.-Functions:...
,
RanRan is a small 25Kda protein that is involved in transport into and out of the cell nucleus during interphase and also involved in mitosis. It is a member of the Ras superfamily....
.
GTPases are enzymes that bind to a molecule called
guanosine triphosphateGuanosine-5'-triphosphate is a purine nucleotide. It can act as a substrate for the synthesis of RNA during transcription process...
(GTP) which they then hydrolyze to create guanosine diphosphate (GDP) and release energy. Ran is in a different conformation depending on whether it is bound to GTP or GDP. In its GTP bound state, Ran is capable of binding karyopherins (importins and exportins). Importins release cargo upon binding to RanGTP, while exportins must bind RanGTP to form a ternary complex with their export cargo. The dominant nucleotide binding state of Ran depends on whether it is located in the nucleus (RanGTP) or the cytoplasm (RanGDP).
Nuclear import
Importin bind their cargo in the cytoplasm, after which they are able to interact with the nuclear pore complex and pass through its channel. Once inside the nucleus, interaction with Ran-GTP causes a
conformational changeA macromolecule is usually flexible and dynamic. It can change its shape in response to changes in its environment or other factors; each possible shape is called a conformation, and a transition between them is called a conformational change...
in the importin that causes it to dissociate from its cargo. The resulting complex of importin and Ran-GTP then translocates to the cytoplasm, where a protein called Ran Binding Protein (RanBP) separates Ran-GTP from importin. Separation allows access to a
GTPase activating proteinGTPase-Activating Proteins, or GAPs, or GTPase-Accelerating Proteins is a family of regulatory proteins whose members can bind to activated G proteins and stimulate their GTPase activity, with the result of terminating the signaling event....
(GAP) that binds Ran-GTP and induces the
hydrolysisHydrolysis is a chemical reaction during which one or more water molecules are split into hydrogen and hydroxide ions, which may go on to participate in further reactions. It is the type of reaction that is used to break down certain polymers, especially those made by step-growth polymerization...
of GTP to GDP. The Ran-GDP produced from this process now binds the nuclear transport factor NTF2 which returns it to the nucleoplasm. Now in the nucleus, the Ran-GDP interacts with a
guanine nucleotide exchange factorNucleotide exchange factors are proteins that stimulate the exchange of nucleoside diphosphates for nucleoside triphosphates bound to other proteins.-Function:...
(GEF) which replaces the GDP with GTP, resulting again in Ran-GTP, and beginning the cycle anew.
Nuclear export
Nuclear export roughly reverses the import process; in the nucleus, the exportin binds the cargo and Ran-GTP and diffuses through the pore to the cytoplasm, where the complex dissociates. Ran-GTP binds GAP and hydrolyzes GTP, and the resulting Ran-GDP complex is restored to the nucleus where it exchanges its bound ligand for GTP. Hence, whereas importins depend on RanGTP to dissociate from their cargo, exportins require RanGTP in order to bind to their cargo.
A specialized mRNA exporter protein moves mature mRNA to the cytoplasm after post-transcriptional modification is complete. This translocation process is not dependent on the Ran protein, although the specific mechanism is not yet well understood. Some particularly commonly transcribed genes are physically located near nuclear pores to facilitate the translocation process.
tRNA export is also dependent on the various modifications it undergoes, thus preventing export of improperly functioning tRNA. This quality control mechanism is important due to tRNA's central role in translation, where it is involved in adding amino acids to a growing peptide chain. The tRNA exporter in vertebrates is called
exportin-t. Exportin-t binds directly to its tRNA cargo in the nucleus, a process promoted by the presence of RanGTP. Mutations that affect tRNA's structure inhibit its ability to bind to exportin-t, and consequentially, to be exported, providing the cell with another quality control step. As described above, once the complex has crossed the envelope it dissociates and releases the tRNA cargo into the cytosol.
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