16.5:
Nuclear Export
Nuclear export involves recognizing nuclear export signals or NES on the cargo protein.
NES are of three types: The classical 10-residue leucine-rich signal, a 38-residue M9 signal, and a 24-residue KNS signal.
NES are recognized by specialized nuclear transport receptors that shuttle the cargo through the nuclear pore complexes, or the NPC, on the nuclear membrane. Protein transport across the nucleus depends on the receptor binding to the small GTPase, Ran.
Protein exporting receptor— exportin 1 forms a complex with Ran-GTP and undergoes a conformational change. Exportin-Ran-GTP then binds the cargo to form a three-member complex.
The exportin-Ran-GTP-cargo complex shuttles through the NPC by interacting with the FG repeats on its inner lining and disrupts the gel-like selective barrier of the NPC to diffuse into the cytosol.
A GTPase activating protein called Ran-GAP present on the cytoplasmic fibrils of the NPC initiates hydrolysis of Ran-GTP to Ran-GDP. Ran-GDP induces a conformational change in exportin 1 that weakens its binding to the cargo and the complex disassembles.
After the receptors release the protein in the cytosol, exportin 1 and Ran-GDP move back to the nucleus for subsequent rounds of nuclear export.
16.5:
Nuclear Export
The nucleus restricts several proteins within and allows others to pass. The restricted proteins possess a nuclear retention sequence or NRS, anchoring them to the nuclear lamins and preventing their transport to the cytosol. The non-restricted proteins, after their synthesis, are transported to their site of action, such as the cytosol or other organelles, with the help of nuclear export signals or NES.
NES are of three types- the canonical 10-residue long leucine-rich signal and other non-canonical NES such as the M9 and KNS signal found in heteronuclear ribonucleoproteins or hnRNPs. Leucine-rich NES is present at the N-terminal end of a polypeptide and is characterized by stretches of leucine and isoleucine residues interspersed with small polar residues. The M9 signal sequence is 38-residues long and present in the C terminal domain of hnRNP A1. M9 signal sequence transports proteins and protein-bound mRNAs bidirectionally across the nucleus in a temperature-sensitive manner. M9 export signal can allow even nuclear-restricted proteins to move to the cytosol at high temperatures and restrict them inside the nucleus at low temperatures. The third non-canonical NES is the 24 amino acid-long KNS signal found in hnRNP K. Like M9, the KNS signal promotes bidirectional nuclear trafficking of proteins and RNA.
Specific nuclear exporters recognize each of these NES. For example, nuclear exporters such as chromosome region maintenance 1 (CRM1), also called exportin 1, recognize and export cargo with leucine-rich signal sequences. During cargo export, Ran-GTP binds to CRM1 to form the CRM1-RanGTP complex. This enables CRM1 and cargo interaction and the formation of a trimeric export complex. The cargo-exporter-RanGTP complex then interacts with the nucleoporins on the cytoplasmic fibrils of the nuclear pore complex or NPC to move across the nucleus. Nup358-RanBP complex on the NPC accelerates Ran GTP to GDP hydrolysis to disassemble the export complex and release the cargo in the cytosol. CRM1 and RanGDP are recycled back to the nucleus for multiple rounds of cargo export.
Suggested Reading
- Iain W. Mattaj and Ludwig Englmeier, NUCLEOCYTOPLASMIC TRANSPORT: The Soluble Phase. Annu. Rev. Biochem. 1998. 67:265–306.
- Elisa Izaurralde, et al., A Role for the M9 Transport Signal of hnRNP A1 in mRNA Nuclear Export. The Journal of Cell Biology, Volume 137, Number 1, April 7, 1997, 27–35
- Amy M. Brownawell and Ian G. Macara, Exportin-5, a novel karyopherin, mediates nuclear export of double-stranded RNA binding proteins. The Journal of Cell Biology, Volume 156, Number 1, January 7, 2002, 53–64
- Ulrike Kutay and Stephan Gu¨ttinger, Leucine-rich nuclear-export signals: born to be weak. TRENDS in Cell Biology Vol.15 No.3 March 2005
- Saskia Hutten and Ralph H. Kehlenbach, CRM1-mediated nuclear export: to the pore and beyond. TRENDS in Cell Biology Vol.17 No.4
- W.Matthew Michael et al., The K nuclear shuttling domain: a novel signal for nuclear import and nuclear export in the hnRNP K protein. The EMBO Journal Vol.16 No.12 pp.3587–3598, 1997
- W. Matthew Michael et al., A Nuclear Export Signal in hnRNP Al: A Signal-Mediated, Temperature-Dependent Nuclear Protein Export Pathway. Cell, Vol. 83, 415-422, November 3, 1995.
- Sara Nakielny and Gideon Dreyfuss. Nuclear export of proteins and RNA. Current Opinion in Cell Biology 1997, 9:420-429.
- Katharine S. Ullman et al., Nuclear Export Receptors: From Importin to Exportin. Cell, Vol. 90, 967–970, September 19, 1997.