Importin

Importin 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 (NLS). Importin is classified as a karyopherin.

Importin has two subunits, importin α and importin β. Members of the importin-beta family can bind and transport cargo by themselves, or can form heterodimers with importin-alpha. 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. The NLS-Importin α-Importin β trimer dissociates after binding to Ran GTP inside the nucleus. 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. Importin alpha also contains an N-terminal importin beta binding domain that contains an auto-regulatory region. Importin-beta is a helicoidal molecule constructed from 19 HEAT repeats. Many nuclear pore proteins contain FG sequence repeats that can bind to HEAT repeats within importins, which is important for importin-beta mediated transport.

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. 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.

Read more about Importin:  Human Importin Genes

Other articles related to "importin":

RANBP5
... Importin-5 is a protein that in humans is encoded by the IPO5 gene ... a nuclear localization signal (NLS) requires the NLS import receptor, a heterodimer of importin alpha and beta subunits also known as karyopherins ... Importin alpha binds the NLS-containing cargo in the cytoplasm and importin beta docks the complex at the cytoplasmic side of the nuclear pore complex ...
Phospholipid Scramblase - Nuclear Localisation Sequence
... The structure of the nuclear localisation sequence of scramblase PLSCR1 complexed to importin was determined using X-ray diffraction with a resolution of 2.20 Ångströms ... The importin structure is composed of many alpha helices that integrate the protein into membranes ... The role of importin is to move proteins such as scramblase into the nucleus ...
Human Importin Genes
... Importin IPO4, IPO7, IPO8, IPO9, IPO11, IPO13 Karyopherin-α KPNA1, KPNA2, KPNA3, KPNA4, KPNA5, KPNA6 Karyopherin-β KPNB1 ...
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 ... targets that share a sequence motif related to the Ran-binding site of importin-beta ...
KPNA6
... Importin subunit alpha-7 is a protein that in humans is encoded by the KPNA6 gene ... requires the NLS import receptor, a heterodimer of importin alpha and beta subunits also known as karyopherins ... Importin alpha binds the NLS-containing cargo in the cytoplasm and importin beta docks the complex at the cytoplasmic side of the nuclear pore complex ...