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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.〔 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 led by (Michael Rexach )〔 〕 and further studies by (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, have been found that facilitate the import of slightly different cargo proteins, due to their differing structure and locality. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Importin」の詳細全文を読む スポンサード リンク
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