Importin is a type of karyopherin
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 with importin-α. As part of a heterodimer, importin-β mediates interactions with the nuclear pore, while importin-α acts as an adaptor protein to bind the nuclear localization signal (NLS) on the cargo. The NLS-Importin α-Importin β protein trimer dissociates after binding to Ran GTP inside the Cell nucleus,
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
target="_blank" rel="nofollow"> 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
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
Escherichia 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 heterodimer with importin-α in a number of different cases. These included a study led by Michael Rexach
Structure
Importin-α
A large proportion of the importin-α adaptor protein is made up of several
armadillo repeat arranged in
tandem repeat. 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
armadillo repeat structures, Importin-α also contains a 90
amino acid n-terminus 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
Cell nucleus.
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]
Importin-β
Importin-β is the typical structure of a larger superfamily of
. The basis of their structure is 18-20 tandem repeats of the HEAT motif. Each one of these repeats contains two antiparallel
alpha helix linked by a turn, which stack together to form the overall structure of the
protein.
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]
In order to transport cargo into the Cell nucleus, importin-β must associate with the nuclear pore. It does this by forming weak, transient chemical bonds with at their various phenylalanineglycine (Phe-Gly) motifs. Crystallographic analysis has shown that these sequence motif bind to importin-β at shallow hydrophobe pockets found on its surface.
Nuclear protein import cycle
The primary function of importin is to mediate the translocation of
with nuclear localization signals into the
Cell nucleus, through
nuclear pore, 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
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
that tags the
protein as cargo destined for the
Cell nucleus. A cargo
protein can contain either one or two of these
sequence motif, which will bind to the major and/or minor binding sites on importin-α.
Cargo transport
Once the cargo protein is bound, importin-β interacts with the
nuclear pore, and the complex diffuses into the
Cell 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
Cell nucleus, the complex interacts with the
Ras superfamily, 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
Cell nucleus. The
n-terminus 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.
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
Cell nucleus. CAS (cellular apoptosis susceptibility protein) is part of the importin-β superfamily of
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
hydrolysis 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
Cell 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
.
Disease
There are several disease states and pathologies that are associated with
or changes in expression of importin-α and importin-β.
Importins are vital regulatory 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.[
]
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.[
]
In addition, importin-α has been shown to transport the tumour suppressor gene, BRCA1 (breast cancer type 1 susceptibility protein), into the Cell nucleus. The overexpression of importin-α has also been linked with poor survival rates seen in certain melanoma patients.[
]
Importin activity is also associated with some pathogen. 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.[
]
As a result, importin cannot function and the cargo protein stays in the cytoplasm.
Types of cargo
Many different cargo
can be transported into the
Cell 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.
|
|
| Importin-β and importin-α |
| Importin-β and importin-α |
| Importin-β and NPI-1 (type of importin-α) |
| Importin-α not required |
| 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
protein family of
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.
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Importin: IPO4, IPO5, IPO7, IPO8, IPO9, IPO11, IPO13
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Karyopherin-α: KPNA1, KPNA2, KPNA3, KPNA4, KPNA5, KPNA6
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Karyopherin-β: KPNB1
See also
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Karyopherin
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Nuclear localization sequence
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Nuclear pore complex
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Nuclear transport
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Ran (gene)
External links
