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72  structures 576  species 4  interactions 1571  sequences 46  architectures

Family: IBB (PF01749)

Summary: Importin beta binding domain

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This is the Wikipedia entry entitled "Importin". More...

Importin Edit Wikipedia article

Karyopherin alpha 1 (importin alpha 5)
Protein KPNA1 PDB 2jdq.png
PDB rendering based on 2jdq.
Available structures
PDB Ortholog search: PDBe, RCSB
Symbols KPNA1 ; IPOA5; NPI-1; RCH2; SRP1
External IDs OMIM600686 MGI103560 HomoloGene55642 GeneCards: KPNA1 Gene
RNA expression pattern
PBB GE KPNA1 213741 s at tn.png
PBB GE KPNA1 202055 at tn.png
PBB GE KPNA1 202058 s at tn.png
More reference expression data
Species Human Mouse
Entrez 3836 16646
Ensembl ENSG00000114030 ENSMUSG00000022905
UniProt P52294 Q60960
RefSeq (mRNA) NM_002264 NM_008465
RefSeq (protein) NP_002255 NP_032491
Location (UCSC) Chr 3:
122.14 – 122.23 Mb
Chr 16:
35.98 – 36.04 Mb
PubMed search [1] [2]
Karyopherin (importin) beta 1
Protein KPNB1 PDB 1f59.png
PDB rendering based on 1f59.
Available structures
PDB Ortholog search: PDBe, RCSB
Symbols KPNB1 ; IMB1; IPO1; IPOB; Impnb; NTF97
External IDs OMIM602738 MGI107532 HomoloGene1707 ChEMBL: 1741199 GeneCards: KPNB1 Gene
RNA expression pattern
PBB GE KPNB1 213803 at tn.png
PBB GE KPNB1 208974 x at tn.png
PBB GE KPNB1 208975 s at tn.png
More reference expression data
Species Human Mouse
Entrez 3837 16211
Ensembl ENSG00000108424 ENSMUSG00000001440
UniProt Q14974 P70168
RefSeq (mRNA) NM_001276453 NM_008379
RefSeq (protein) NP_001263382 NP_032405
Location (UCSC) Chr 17:
45.73 – 45.76 Mb
Chr 11:
97.16 – 97.19 Mb
PubMed search [3] [4]

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.


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.



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 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.

See also


  1. ^ 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. 
  2. ^ 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. 
  3. ^ Garcia Bustos J., Heitman J and Hall, M. (1991). "Nuclear Protein Localization". Biochim. Biophys. acta 1071: 83–101. 
  4. ^ 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. 
  5. ^ 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. 
  6. ^ 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. 
  7. ^ 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. 
  8. ^ 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. 
  9. ^ a b c d Weis K. (1984). "Regulating access to the genome: nucleocytoplasmic transport throughout the cell cycle.". Cell 112: 441–51. 
  10. ^ 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. 
  11. ^ 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. 
  12. ^ 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. 
  13. ^ 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

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

This page is based on a Wikipedia article. The text is available under the Creative Commons Attribution/Share-Alike License.

This tab holds the annotation information that is stored in the Pfam database. As we move to using Wikipedia as our main source of annotation, the contents of this tab will be gradually replaced by the Wikipedia tab.

Importin beta binding domain Provide feedback

This family consists of the importin alpha (karyopherin alpha), importin beta (karyopherin beta) binding domain. The domain mediates formation of the importin alpha beta complex; required for classical NLS import of proteins into the nucleus, through the nuclear pore complex and across the nuclear envelope. Also in the alignment is the NLS of importin alpha which overlaps with the IBB domain [4].

Literature references

  1. Conti E, Uy M, Leighton L, Blobel G, Kuriyan J; , Cell 1998;94:193-204.: Crystallographic analysis of the recognition of a nuclear localization signal by the nuclear import factor karyopherin alpha. PUBMED:9695948 EPMC:9695948

  2. Weis K; , Trends Biochem Sci 1998;23:185-189.: Importins and exportins: how to get in and out of the nucleus [published erratum appears in Trends Biochem Sci 1998 Jul;23(7):235] PUBMED:9612083 EPMC:9612083

  3. Gorlich D; , EMBO J 1998;17:2721-2727.: Transport into and out of the cell nucleus. PUBMED:9582265 EPMC:9582265

  4. Moroianu J, Blobel G, Radu A; , Proc Natl Acad Sci U S A 1996;93:6572-6576.: The binding site of karyopherin alpha for karyopherin beta overlaps with a nuclear localization sequence. PUBMED:8692858 EPMC:8692858

  5. Gorlich D, Henklein P, Laskey RA, Hartmann E; , EMBO J 1996;15:1810-1817.: A 41 amino acid motif in importin-alpha confers binding to importin- beta and hence transit into the nucleus. PUBMED:8617226 EPMC:8617226

External database links

This tab holds annotation information from the InterPro database.

InterPro entry IPR002652

The exchange of macromolecules between the nucleus and cytoplasm takes place through nuclear pore complexes within the nuclear membrane. Active transport of large molecules through these pore complexes require carrier proteins, called karyopherins (importins and exportins), which shuttle between the two compartments.

Members of the importin-alpha (karyopherin-alpha) family can form heterodimers with importin-beta. As part of a heterodimer, importin-beta mediates interactions with the pore complex, while importin-alpha acts as an adaptor protein to bind the nuclear localisation signal (NLS) on the cargo through the classical NLS import of proteins. Proteins can contain one (monopartite) or two (bipartite) NLS motifs. Importin-alpha contains several armadillo (ARM) repeats, which produce a curving structure with two NLS-binding sites, a major one close to the N terminus and a minor one close to the C terminus.

Ran GTPase helps to control the unidirectional transfer of cargo. The cytoplasm contains primarily RanGDP and the nucleus RanGTP through the actions of RanGAP and RanGEF, respectively. In the nucleus, RanGTP binds to importin-beta within the importin/cargo complex, causing a conformational change in importin-beta that releases it from importin-alpha-bound cargo. The N-terminal importin-beta-binding (IBB) domain of importin-alpha contains an auto-regulatory region that mimics the NLS motif [PUBMED:8692858]. The release of importin-beta frees the auto-regulatory region on importin-alpha to loop back and bind to the major NLS-binding site, causing the cargo to be released [PUBMED:17170104].

This entry represents the N-terminal IBB domain of importin-alpha that contains the auto-regulatory region.

More information about these proteins can be found at Protein of the Month: Importins [PUBMED:].

Gene Ontology

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Domain organisation

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Seed source: Pfam-B_544 (release 4.2)
Previous IDs: none
Type: Family
Author: Bashton M, Bateman A
Number in seed: 197
Number in full: 1571
Average length of the domain: 90.20 aa
Average identity of full alignment: 33 %
Average coverage of the sequence by the domain: 17.81 %

HMM information View help on HMM parameters

HMM build commands:
build method: hmmbuild -o /dev/null HMM SEED
search method: hmmsearch -Z 80369284 -E 1000 --cpu 4 HMM pfamseq
Model details:
Parameter Sequence Domain
Gathering cut-off 23.8 23.8
Trusted cut-off 23.9 23.9
Noise cut-off 23.7 23.7
Model length: 90
Family (HMM) version: 16
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Species distribution

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Arm Arm IBN_N Cse1


For those sequences which have a structure in the Protein DataBank, we use the mapping between UniProt, PDB and Pfam coordinate systems from the PDBe group, to allow us to map Pfam domains onto UniProt sequences and three-dimensional protein structures. The table below shows the structures on which the IBB domain has been found. There are 72 instances of this domain found in the PDB. Note that there may be multiple copies of the domain in a single PDB structure, since many structures contain multiple copies of the same protein seqence.

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