Importin α

Importin alpha, or karyopherin alpha proteins are involved in the import of proteins into the cell nucleus. They are a sub-family of karyopherin proteins.

References

Importin

Importin is a type of karyopherin 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, 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. 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 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.