Importin is a type of

karyopherin Karyopherins are protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme c ...

that transports

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

molecules from the cell's

cytoplasm The cytoplasm describes all the material within a eukaryotic or prokaryotic cell, enclosed by the cell membrane, including the organelles and excluding the nucleus in eukaryotic cells. The material inside the nucleus of a eukaryotic cell a ...

to the nucleus. It does so by binding to specific

recognition sequence A recognition sequence is a DNA sequence to which a structural motif of a DNA-binding domain exhibits binding specificity. Recognition sequences are palindromes. The transcription factor Sp1 for example, binds the sequences 5'-(G/T)GGGCGG(G/A)( ...

s, called

nuclear localization sequence A nuclear localization signal ''or'' sequence (NLS) is an amino acid sequence that 'tags' a protein for import into the cell nucleus by nuclear transport. Typically, this signal consists of one or more short sequences of positively charged lysines ...

s (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

heterodimer In biochemistry, a protein dimer is a macromolecular complex or multimer formed by two protein monomers, or single proteins, which are usually non-covalently bound. Many macromolecules, such as proteins or nucleic acids, form dimers. The word ...

s with importin-α. As part of a

, importin-β mediates interactions with the pore complex, while importin-α acts as an adaptor protein to bind the

nuclear localization signal A nuclear localization signal ''or'' sequence (NLS) is an amino acid sequence that 'tags' a protein for import into the cell nucleus by nuclear transport. Typically, this signal consists of one or more short sequences of positively charged lysin ...

(NLS) on the cargo. The NLS-Importin α-Importin β trimer dissociates after binding to Ran GTP inside the nucleus, with the two importin proteins being recycled to the

for further use.

Discovery

Importin can exist as either a

of importin-α/β or as a

monomer A monomer ( ; ''mono-'', "one" + '' -mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or two- or three-dimensional network in a process called polymerization. Classification Chemis ...

of Importin-β. Importin-α was first isolated in 1994 by a group includin
Enno Hartmann
based at the Max Delbrück Center for Molecular Medicine. The process of nuclear protein import had already been characterised in previous reviews, but the key proteins involved had not been elucidated up until that point. A 60 kDa

cytosol The cytosol, also known as cytoplasmic matrix or groundplasm, is one of the liquids found inside cells ( intracellular fluid (ICF)). It is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondri ...

ic protein, essential for protein import into the nucleus, and with a 44% sequence identity t
SRP1p
was purified from ''

Xenopus ''Xenopus'' () (Gk., ξενος, ''xenos'' = strange, πους, ''pous'' = foot, commonly known as the clawed frog) is a genus of highly aquatic frogs native to sub-Saharan Africa. Twenty species are currently described with ...

'' 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. Importin-β, unlike importin-α, has no direct homologues in yeast, but was purified as a 90-95 kDa protein and found to form a

with importin-α in a number of different cases. These included a study led by Michael Rexach and further studies b
Dirk Görlich
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 Importin-7 is a protein that in humans is encoded by the ''IPO7'' gene. The importin-alpha/beta complex and the GTPase Ran mediate nuclear import of proteins with a classical nuclear localization signal. The protein encoded by this gene is a me ...

, 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 Tandem, or in tandem, is an arrangement in which two or more animals, machines, or people are lined up one behind another, all facing in the same direction. ''Tandem'' can also be used more generally to refer to any group of persons or objects w ...

. 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 The N-terminus (also known as the amino-terminus, NH2-terminus, N-terminal end or amine-terminus) is the start of a protein or polypeptide, referring to the free amine group (-NH2) located at the end of a polypeptide. Within a peptide, the amin ...

, with a minor site being found at the

C-terminus The C-terminus (also known as the carboxyl-terminus, carboxy-terminus, C-terminal tail, carboxy tail, C-terminal end, or COOH-terminus) is the end of an amino acid chain (protein Proteins are large biomolecules and macromolecules that comp ...

. As well as the ARM structures, Importin-α also contains a 90

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the 22 α-amino acids incorporated into proteins. Only these 22 a ...

N-terminal The N-terminus (also known as the amino-terminus, NH2-terminus, N-terminal end or amine-terminus) is the start of a protein or polypeptide, referring to the free amine group (-NH2) located at the end of a polypeptide. Within a peptide, the amin ...

region, responsible for binding to Importin-β, known as the Importin-β binding (IBB)domain. This is also a site of autoinhibition, and is implicated in the release of cargo once importin-α reaches the nucleus.

Importin-β

Importin-β is the typical structure of a larger superfamily of

s. The basis of their structure is 18-20 tandem repeats of the

HEAT In thermodynamics, heat is energy in transfer between a thermodynamic system and its surroundings by such mechanisms as thermal conduction, electromagnetic radiation, and friction, which are microscopic in nature, involving sub-atomic, ato ...

motif. Each one of these repeats contains two antiparallel

alpha helices An alpha helix (or α-helix) is a sequence of amino acids in a protein that are twisted into a coil (a helix). The alpha helix is the most common structural arrangement in the secondary structure of proteins. It is also the most extreme type of l ...

linked by a turn, which stack together to form the overall structure of the

. 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

nucleoporin Nucleoporins are a family of proteins which are the constituent building blocks of the nuclear pore complex (NPC). The nuclear pore complex is a massive structure embedded in the nuclear envelope at sites where the inner and outer nuclear memb ...

s at their various F G (Phe-Gly) motifs. Crystallographic analysis has shown that these motifs bind to importin-β at shallow

hydrophobic In chemistry, hydrophobicity is the chemical property of a molecule (called a hydrophobe) that is seemingly repelled from a mass of water. In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, thu ...

pockets found on its surface.

Nuclear protein import cycle

The primary function of importin is to mediate the translocation of

s 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

ic importin-β

, 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

s that tags the

as cargo destined for the nucleus. A cargo

can contain either one or two of these motifs, which will bind to the major and/or minor binding sites on importin-α. 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

. The rate of

diffusion Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...

depends on both the concentration of importin-α present in the cytoplasm and also the

binding affinity In biochemistry and pharmacology, a ligand is a substance that forms a complex with a biomolecule to serve a biological purpose. The etymology stems from Latin ''ligare'', which means 'to bind'. In protein-ligand binding, the ligand is usuall ...

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.

Cargo release

Now that the importin-α/cargo complex is free of importin-β, the cargo protein can be released into the nucleus. The

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

. In some cases, specific release factors such a
Nup2
and Nup50 can be employed to help release the cargo as well.

Recycling

Finally, in order to return to the

, 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

s and is defined as a nuclear export factor. Importin-β returns to the

, still bound to Ran- GTP. Once in the

, Ran- GTP is hydrolysed by Ran GAP, 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

, as stated above. It is this activity of Ran that allows for the unidirectional transport of

Disease

There are several disease states and pathologies that are associated with

mutation In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, ...

s or changes in expression of importin-α and importin-β. Importins are vital regulatory

s during the processes of gametogenesis and

embryogenesis An embryo ( ) is the initial stage of development for a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male ...

. As a result, a disruption in the expression patterns of importin-α has been shown to cause fertility defects in ''

Drosophila melanogaster ''Drosophila melanogaster'' is a species of fly (an insect of the Order (biology), order Diptera) in the family Drosophilidae. The species is often referred to as the fruit fly or lesser fruit fly, or less commonly the "vinegar fly", "pomace fly" ...

''. There have also been studies that link altered importin-α to some cases of

cancer Cancer is a group of diseases involving Cell growth#Disorders, abnormal cell growth with the potential to Invasion (cancer), invade or Metastasis, spread to other parts of the body. These contrast with benign tumors, which do not spread. Po ...

Breast cancer Breast cancer is a cancer that develops from breast tissue. Signs of breast cancer may include a Breast lump, lump in the breast, a change in breast shape, dimpling of the skin, Milk-rejection sign, milk rejection, fluid coming from the nipp ...

studies have implicated a truncated form of importin-α in which the NLS binding domain is missing. 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 Melanoma is the most dangerous type of skin cancer; it develops from the melanin-producing cells known as melanocytes. It typically occurs in the skin, but may rarely occur in the mouth, intestines, or eye (uveal melanoma). In very rare case ...

patients. Importin activity is also associated with some viral pathologies. For instance, in the infection pathway of the

Ebola virus ''Orthoebolavirus zairense'' or Zaire ebolavirus, more commonly known as Ebola virus (; EBOV), is one of six known species within the genus ''Ebolavirus''. Four of the six known ebolaviruses, including EBOV, cause a severe and often fatal vira ...

, a key step is the inhibition of the nuclear import of PY-STAT1. This is achieved by the virus sequestering importin-α in the

, meaning it can no longer bind its cargo at the NLS. As a result, importin cannot function and the cargo protein stays in the cytoplasm.

Types of cargo

Many different cargo

s 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.

Human importin genes

Although importin-α and importin-β are used to describe importin as a whole, they actually represent larger

families Family (from ) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictability, structure, and safety as ...

s 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

and importin are used interchangeably. * Importin: IPO4, IPO5,

, IPO8, IPO9, IPO11, IPO13 * Karyopherin-α:

KPNA1 Importin subunit alpha-5 is a protein that in humans is encoded by the ''KPNA1'' gene. Interactions Importin subunit alpha-5 has been shown to interact with KPNB1 Importin subunit beta-1 is a protein that in humans is encoded by the ''KPNB1'' ...

, KPNA2, KPNA3, KPNA4, KPNA5, KPNA6 * Karyopherin-β:

KPNB1 Importin subunit beta-1 is a protein that in humans is encoded by the ''KPNB1'' gene. Function Nucleocytoplasmic transport, a signal- and energy-dependent process, takes place through nuclear pore complexes embedded in the nuclear envelope. T ...

References

External links

* * {{Membrane transport proteins Protein families Transport proteins