Importin

Importin refers to a group of nuclear transport receptors that mediate the import of proteins from the cytoplasm into the nucleus through the nuclear pore complex (NPC). These receptors are part of the broader karyopherin family, which also includes exportins responsible for nuclear export. Importins recognize cargo proteins that carry specific nuclear localization signals (NLS) and shuttle them into the nucleus.

In the classical import pathway, importin-α (a cargo adaptor) binds proteins bearing a classical NLS and then associates with importin-β (also called importin-β1). The resulting complex docks at the NPC and traverses the pore via interactions with FG-nucleoporins. Inside the nucleus, the Ran GTPase system induces cargo release: RanGTP binding to importin-β causes dissociation of the complex and release of the cargo. Importins are then recycled to the cytoplasm, where RanGAP catalyzes GTP hydrolysis to return importins to their cargo-binding-receptive state.

Apart from the classical α/β pathway, other importins (beta family members) recognize non-classical NLSs and direct various cargoes into the nucleus. Examples include transportin-1 (TNPO1), which binds specific non-classical signals such as the M9 domain, and other karyopherins like IPO7 and IPO5. The specificity and regulation of importins contribute to developmental processes and cellular signaling.

Dysregulation or mutation of importins has been linked to diseases including cancer and neurodegenerative disorders, and some viruses co-opt importins to facilitate the nuclear entry of their proteins. The term importin is often used to describe these receptors collectively, while individual members show distinct cargo preferences and regulatory mechanisms.