WordNet

the commercial activity of buying and bringing in goods from a foreign country (同)importation
transfer (electronic data) into a database or document
commodities (goods or services) bought from a foreign country (同)importation
an imported person brought from a foreign country; "the lead role was played by an import from Sweden"; "they are descendants of indentured importees" (同)importee
bring in from abroad

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(…から)…‘を'『輸入する』《+『名』+『from』+『名』》 / 《文》'‘を'意味する,‘の'意味を導入する / 〈U〉(…の)『輸入』《+『of』+『名』》 / 〈C〉《しばしば複数形で》『輸入品』 / 〈U〉《文》重大,重要性 / 〈U〉意味,意義

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2015/09/06 07:09:36」(JST)

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Importin is a type of karyopherin^[1] that transports protein molecules into the nucleus by binding to specific recognition sequences, called nuclear localization sequences (NLS).

Importin has two subunits, importin α and importin β. Members of the importin-β family can bind and transport cargo by themselves, or can form heterodimers with importin-α. As part of a heterodimer, importin-β mediates interactions with the pore complex, while importin-α acts as an adaptor protein to bind the nuclear localisation signal (NLS) on the cargo. The NLS-Importin α-Importin β trimer dissociates after binding to Ran GTP inside the nucleus,^[2] with the two importin proteins being recycled to the cytoplasm for further use.

Discovery

Importin can exist as either a heterodimer of importin-α/β or as a monomer of Importin-β. Importin-α was first isolated in 1994 by a group including Enno Hartmann, based at the Max Delbrück Center for Molecular Medicine.^[1] The process of nuclear protein import had already been characterised in previous reviews,^[3] but the key proteins involved had not been elucidated up until that point. A 60kDa cytosolic protein, essential for protein import into the nucleus, and with a 44% sequence identity to SRP1p, was purified from Xenopus eggs. It was cloned, sequenced and expressed in E.coli and in order to completely reconstitute signal dependent transport, had to be combined with Ran(TC4). Other key stimulatory factors were also found in the study.^[1]

Importin-β, unlike importin-α, has no direct homologues in yeast, but was purified as a 90-95kDa protein and found to form a heterodimer with importin-α in a number of different cases. These included a study lead by Michael Rexach^[4] and further studies by Dirk Görlich.^[5] These groups found that importin-α requires another protein, importin-β to function, and that together they form a receptor for nuclear localization signals (NLS), thus allowing transport into the nucleus. Since these initial discoveries in 1994 and 1995, a host of Importin genes, such as IPO4 and IPO7, have been found that facilitate the import of slightly different cargo proteins, due to their differing structure and locality.

Structure

Importin-α

A large proportion of the importin-α adaptor protein is made up of several armadillo repeats (ARM) arranged in tandem. These repeats can stack together to form a curved shaped structure, which facilitates binding to the NLS of specific cargo proteins. The major NLS binding site is found towards the N-terminus, with a minor site being found at the C-terminus. As well as the ARM structures, Importin-α also contains a 90 amino acid N-terminal region, responsible for binding to Importin-β, known as IBB (Importin-β binding domain). This is also a site of autoinhibition, and is implicated in the release of cargo once importin-α reaches the nucleus.^[6]

Importin-β

Importin-β is the typical structure of a larger superfamily of karyopherins. The basis of their structure is 18-20 tandem repeats of the HEAT motif. Each one of these repeats contains two antiparallel alpha helices linked by a turn, which stack together to form the overall structure of the protein.^[7]

In order to transport cargo into the nucleus, importin-β must associate with the nuclear pore complexes. It does this by forming weak, transient bonds with nucleoporins at their various FG (Phe-Gly) motifs. Crystallographic analysis has shown that these motifs bind to importin-β at shallow hydrophobic pockets found on its surface.^[8]

Nuclear Protein Import Cycle

The primary function of importin is to mediate the translocation of proteins with nuclear localization signals into the nucleus, through nuclear pore complexes (NPC), in a process known as the nuclear protein import cycle.

Cargo Binding

The first step of this cycle is the binding of cargo. Importin can perform this function as a monomeric importin-β protein, but usually requires the presence of importin-α, which acts as an adaptor to cargo proteins (via interactions with the NLS). The NLS is a sequence of basic amino acids that tags the protein as cargo destined for the nucleus. A cargo protein can contain either one or two of these motifs, which will bind to the major and/or minor binding sites on importin-α.^[9]

Overview of the nuclear protein import cycle.

Cargo Transport

Once the cargo protein is bound, importin-β interacts with the NPC, and the complex diffuses into the nucleus from the cytoplasm. The rate of diffusion depends on both the concentration of importin-α present in the cytoplasm and also the binding affinity of importin-α to the cargo. Once inside the nucleus, the complex interacts with the Ras-family GTPase, Ran-GTP. This leads to the dissociation of the complex by altering the conformation of Importin-β. Importin-β is left bound to Ran-GTP, ready to be recycled.^[9]

Cargo Release

Now that the importin-α/cargo complex is free of importin-β, the cargo protein can be released into the nucleus. The N-terminal importin-β-binding (IBB) domain of importin-α contains an auto-regulatory region that mimics the NLS motif. The release of importin-β frees this region and allows it to loop back and compete for binding with the cargo protein at the major NLS-binding site. This competition leads to the release of the protein. In some cases, specific release factors such as Nup2 and Nup50 can be employed to help release the cargo as well.^[9]

Importin Recycling

Finally, in order to return to the cytoplasm, importin-α must associate with a Ran-GTP/CAS (nuclear export factor) complex which facilitates its exit from the nucleus. CAS (cellular apoptosis susceptibility protein) is part of the importin-β superfamily of karyopherins and is defined as a nuclear export factor. Importin-β returns to the cytoplasm, still bound to Ran-GTP. Once in the cytoplasm, Ran-GTP is hydrolysed by RanGAP, forming Ran-GDP, and releasing the two importins for further activity. It is this hydrolysis of GTP that provides the energy for the cycle as a whole. In the nucleus, a GEF will charge Ran with a GTP molecule, which is then hydrolysed by a GAP in the cytoplasm, as stated above. It is this activity of Ran that allows for the unidirectional transport of proteins.^[9]

Importins and Disease

There are several disease states and pathologies that are associated with mutations or changes in expression of importin-α and importin-β.

Importins are vital regulatory proteins during the processes of gametogenesis and embryogenesis. As a result, a disruption in the expression patterns of importin-α has been shown to cause fertility defects in Drosophila melanogaster.^[10]

There have also been studies that link altered importin-α to some cases of cancer. Breast cancer studies have implicated a truncated form of importin-α in which the NLS binding domain is missing.^[11] In addition, importin-α has been shown to transport the tumour suppressor gene, BRCA1 (breast cancer type 1 susceptibility protein), into the nucleus. The overexpression of importin-α has also been linked with poor survival rates seen in certain melanoma patients.^[12]

Importin activity is also associated with some viral pathologies. For instance, in the infection pathway of the Ebola Virus, a key step is the inhibition of the nuclear import of PY-STAT1. This is achieved by the virus sequestering importin-α in the cytoplasm, meaning it can no longer bind its cargo at the NLS.^[13] As a result, importin cannot function and the cargo protein stays in the cytoplasm.

Types of Cargo

Many different cargo proteins can be transported into the nucleus by importin. Often, different proteins will require different combinations of α and β in order to translocate. Some examples of different cargo are listed below.

Cargo	Import Receptor
SV40	Importin-β and importin-α
Nucleoplasmin	Importin-β and importin-α
STAT1	Importin-β and NPI-1 (type of importin-α)
TFIIA	Importin-α not required
U1A	Importin-α not required

Human importin genes

Although importin-α and importin-β are used to describe importin as a whole, they actually represent larger families of proteins that share a similar structure and function. Various different genes have been identified for both α and β, with some of them listed below. Note that often karyopherin and importin are used interchangeably.

Importin: IPO4, IPO5, IPO7, IPO8, IPO9, IPO11, IPO13
Karyopherin-α: KPNA1, KPNA2, KPNA3, KPNA4, KPNA5, KPNA6
Karyopherin-β: KPNB1

References

^ ^a ^b ^c Görlich D, Prehn S, Laskey RA, Hartmann E (1994). "Isolation of a protein that is essential for the first step of nuclear protein import". Cell 79 (5): 767–78. doi:10.1016/0092-8674(94)90067-1. PMID 8001116.
^ Mattaj IW, Englmeier L (1998). "Nucleocytoplasmic transport: the soluble phase". Annu. Rev. Biochem. 67: 265–306. doi:10.1146/annurev.biochem.67.1.265. PMID 9759490.
^ Garcia Bustos J., Heitman J and Hall, M. (1991). "Nuclear Protein Localization". Biochim. Biophys. acta 1071: 83–101.
^ Enenkel C., Blobel G. and Rexach M. (1995). "Identification of a Yeast Karyopherin Heterodimer That Targets Import Substrate to Mammalian Nuclear Pore Complexes". J. Biol. Chem. 270: 16499–502.
^ Görlich D., Kostka S., Kraft R., Dingwall C., Laskey RA. et al. (1995). "Two different subunits of importin cooperate to recognize nuclear localization signals and bind them to the nuclear envelope". Curr. Biol. 5: 383–92.
^ Conti E., Uy, M., Leighton L., Blobel G. and Kuriyan J, (1998). "Crystallographic analysis of the recognition of a nuclear localization signal by the nuclear import factor karyopherin alpha.". Cell 94: 193–204.
^ Lee SJ., Matsuura Y., Liu SM. and Stewart M. (2005). "Structural basis for nuclear import complex dissociation by RanGTP.". Nature 435: 693–6. doi:10.1038/nature03578.
^ Bayliss R., Littlewood T. and Stewart M. (2000). "Structural basis for the interaction between FxFG nucleoporin repeats and importin-beta in nuclear trafficking.". Cell 102: 99–108.
^ ^a ^b ^c ^d Weis K. (1984). "Regulating access to the genome: nucleocytoplasmic transport throughout the cell cycle.". Cell 112: 441–51.
^ Terry LJ., Shows EB. and Wente SR. (2007). "Crossing the nuclear envelope: hierarchical regulation of nucleocytoplasmic transport.". Science 318: 1412–1416. doi:10.1126/science.1142204.
^ Kim IS., Kim DH., Han SM., Chin MU., Nam HJ., Cho HP., Choi SY., Song BJ., Kim ER., Bae YS. et al. (2000). "Truncated form of importin alpha identified in breast cancer cell inhibits nuclear import of p53.". J Biol Chem 275: 23139–23145. doi:10.1074/jbc.M909256199.
^ Winnepenninckx V., Lazar V., Michiels S., Dessen P., Stas M., Alonso SR., Avril MF., Ortiz Romero PL., Robert T., Balacescu O. et al. (2006). "Gene expression profiling of primary cutaneous melanoma and clinical outcome.". J Natl Cancer Inst 98: 472–482. doi:10.1093/jnci/djj103.
^ Sekimoto T., Imamoto N., Nakajima K., Hirano T. and Yoneda Y. (1997). "Extracellular signal-dependent nuclear import of Stat1 is mediated by nuclear pore-targeting complex formation with NPI-1, but not Rch1". EMBO J. 16: 7067–7077. doi:10.1093/emboj/16.23.7067.

External links

http://www.rcsb.org/pdb/101/motm.do?momID=85
http://www.uniprot.org/uniprot/Q14974
http://www.ebi.ac.uk/interpro/entry/IPR002652
Importins at the US National Library of Medicine Medical Subject Headings (MeSH)

This article incorporates text from the public domain Pfam and InterPro IPR002652 This article incorporates text from the public domain Pfam and InterPro IPR001494

English Journal

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Gabryel M1, Skrzypczak-Zielinska M2, Kucharski MA1, Slomski R2,3, Dobrowolska A1.
Scandinavian journal of gastroenterology.Scand J Gastroenterol.2016 Jun;51(6):654-65. doi: 10.3109/00365521.2015.1132336. Epub 2016 Jan 18.
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Distinct functions of the dual leucine zipper kinase depending on its subcellular localization.

Wallbach M1, Duque Escobar J2, Babaeikelishomi R3, Stahnke MJ1, Blume R1, Schröder S2, Kruegel J4, Maedler K5, Kluth O6, Kehlenbach RH7, Miosge N4, Oetjen E8.
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The dual leucine zipper kinase DLK induces β-cell apoptosis by inhibiting the transcriptional activity conferred by the β-cell protective transcription factor cAMP response element binding protein CREB. This action might contribute to β-cell loss and ultimately diabetes. Within its kinase domain
PMID 26776303

Karyopherin Alpha 2 Promotes the Inflammatory Response in Rat Pancreatic Acinar Cells Via Facilitating NF-κB Activation.

Cai Y1,2, Shen Y3,4, Gao L5,6, Chen M7,8, Xiao M9, Huang Z10,11, Zhang D12,13.
Digestive diseases and sciences.Dig Dis Sci.2016 Mar;61(3):747-57. doi: 10.1007/s10620-015-3948-6. Epub 2015 Nov 2.
BACKGROUND: Activation of the transcription factor NF-κB and expression of pro-inflammatory mediators have been considered as major events of acute pancreatitis (AP). Karyopherin alpha 2 (KPNA2), a member of the importin α family, reportedly modulates p65 subcellular localization.AIM: This study a
PMID 26526450

Japanese Journal

核膜孔複合体内部構造とインポーティンβ依存的物質輸送の分子基盤の理解 (特集核-細胞質間分子輸送システム : 基本分子メカニズムの理解とその応用)

小西秀明,吉村成弘
生化学 87(1), 56-63, 2015-02
NAID 40020383578

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宮本洋一,盛山哲嗣
生化学 87(1), 16-21, 2015-02
NAID 40020383453

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木村誠
生化学 87(1), 7-15, 2015-02
NAID 40020383438

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リンク元	「transportin」「karyopherin」「インポーチン」
拡張検索	「importin-alpha」「importin-beta」
関連記事	「import」

「transportin」

Karyopherin (importin) beta 1
	PDB rendering based on 1f59.
Available structures
PDB	Ortholog search: PDBe, RCSB
List of PDB id codes
	1F59, 1IBR, 1M5N, 1O6O, 1O6P, 1QGK, 1QGR, 2P8Q, 2Q5D, 2QNA, 3LWW, 3W5K
Identifiers
Symbols	KPNB1 ; IMB1; IPO1; IPOB; Impnb; NTF97
External IDs	OMIM: 602738 MGI: 107532 HomoloGene: 1707 ChEMBL: 1741199 GeneCards: KPNB1 Gene
Gene ontology
Molecular function	• protein binding; • nuclear localization sequence binding; • zinc ion binding; • Ran GTPase binding; • protein transporter activity; • enzyme binding; • protein domain specific binding; • poly(A) RNA binding;
Cellular component	• nuclear envelope; • nuclear pore; • nucleoplasm; • cytoplasm; • cytosol; • membrane; • nuclear membrane; • extracellular exosome; • endoplasmic reticulum tubular network;
Biological process	• protein import into nucleus, translocation; • apoptotic DNA fragmentation; • protein import into nucleus; • NLS-bearing protein import into nucleus; • ribosomal protein import into nucleus; • apoptotic process; • cellular component disassembly involved in execution phase of apoptosis; • programmed cell death; • viral process; • viral life cycle; • cytokine-mediated signaling pathway; • small molecule metabolic process; • intracellular transport of virus;
	Sources: Amigo / QuickGO
RNA expression pattern
	More reference expression data
Orthologs
	v; t; e;

Karyopherin alpha 1 (importin alpha 5)
	PDB rendering based on 2jdq.
Available structures
PDB	Ortholog search: PDBe, RCSB
List of PDB id codes
	2JDQ, 3TJ3, 4B18
Identifiers
Symbols	KPNA1 ; IPOA5; NPI-1; RCH2; SRP1
External IDs	OMIM: 600686 MGI: 103560 HomoloGene: 55642 GeneCards: KPNA1 Gene
Gene ontology
Molecular function	• protein binding; • nuclear localization sequence binding; • protein transporter activity;
Cellular component	• nucleus; • nuclear pore; • nucleoplasm; • cytoplasm; • cytosol; • dendrite;
Biological process	• regulation of DNA recombination; • apoptotic DNA fragmentation; • NLS-bearing protein import into nucleus; • apoptotic process; • cellular component disassembly involved in execution phase of apoptosis; • programmed cell death; • viral process; • viral life cycle; • cytokine-mediated signaling pathway; • positive regulation of protein import into nucleus; • intracellular transport of virus;
	Sources: Amigo / QuickGO
RNA expression pattern
	More reference expression data
Orthologs
	v; t; e;