Importin

Importin

Diagram of the nuclear protein import cycle involving importin.

Importin is a type of karyopherin that is involved in the transport of proteins into the cell nucleus. This process is essential for the regulation of gene expression and the maintenance of cellular function.

Structure

Importins are composed of two subunits: importin _ and importin _. Importin _ acts as an adaptor that binds to the nuclear localization signal (NLS) of the cargo protein, while importin _ mediates the interaction with the nuclear pore complex (NPC).

Function

The primary function of importin is to facilitate the import of proteins that contain an NLS into the nucleus. This process is energy-dependent and requires the hydrolysis of GTP by the small GTPase Ran.

Nuclear Import Cycle

The nuclear import cycle begins with the recognition of the NLS by importin _. The importin _/cargo complex then binds to importin _, forming a trimeric complex. This complex is transported through the NPC into the nucleus.

Once inside the nucleus, the complex encounters Ran-GTP, which binds to importin _, causing a conformational change that releases the cargo protein. Importin _ and importin _ are then recycled back to the cytoplasm, where Ran-GTP is hydrolyzed to Ran-GDP, allowing the cycle to begin anew.

Role in Disease

Dysregulation of importin function can lead to various diseases, including cancer and viral infections. Certain viruses exploit the importin pathway to transport their proteins into the nucleus, facilitating viral replication.

Related Pages

• Nuclear localization signal
• Nuclear pore complex
• Ran (GTPase)
• Karyopherin