Transport of a protein (cargo) from the cytoplasm into the nucleus occurs throug

Question

Transport of a protein (cargo) from the cytoplasm into the nucleus occurs through the translocation of a complex of proteins including the cargo bound to importin proteins ( and ). What mechanism ensures that this complex does not exit the nuclear pore complex (NPC) once it has passed through the nuclear pore and into the nucleus? 1.) No specific mechanism is required because transport of proteins is always unidirectional through the NPC. 2.) The shape of the NPC is such that the basket region on the inner surface of the nuclear membrane allows passage of this protein complex in one direction into the nucleus but not out. 3.) Export from the nucleus requires distinct NPCs and distinct proteins called exportins. 4.) Ran-GTP is enriched in the nucleus and binds the complex, causing the disassociation of cargo and importins and trapping the cargo in the nucleus. 5.) Ran-GTP is enriched in the nucleus and hydrolyzes GTP to GDP to phosphorylate the importins, causing the disassociation of cargo and importins and trapping the cargo in the nucleus.

Explanation / Answer

Answer is 4

Nuclear importins are composed of alpha and beta subunits. The alpha subunit forms helical motifs that are arranged either as large arches or snail shaped coils. These areas are the site for Cargo or Ran-GTP binding. When these importins have entered the nucleus, the Ran-GTP will displace the Cargo and get bound to the importin which is then taken across the nuclear membrane to the cytoplasm(here in the cytoplasm the phosphorylation of importin occurs and Ran-GDP gets released from the bound state and the cycle repeats).

Please refer to the link provided for further study.

https://www.ncbi.nlm.nih.gov/books/NBK26932/

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