|MoBio||Nuclear Transport||Chapter 5|
After RNA molecules (mRNA, tRNA and rRNA) are produced in the nucleus, they must be exported to the cytoplasm for protein synthesis. On the other hand, many proteins operating in the nucleus must be imported from the cytoplasm. The traffic through the nuclear envelope is mediated by a protein family which can be divided into exportins and importins. Binding of a molecule (a "cargo") to exportins facilitates its export to the cytoplasm. Importins facilitate import into the nucleus.
The function of exportins and importins is regulated by a G protein called "Ran". There are two types of G proteins: heterotrimeric G proteins and monomeric G proteins (or small G proteins). The latter includes Ras, Rho, Rab, Arf, and Ran. Like other G proteins, Ran can switch between GTP-bound and GDP-bound states. Transition from the GTP-bound to the GDP-bound state is catalyzed by a GTPase-activating protein (GAP) which induces hydrolysis of the bound GTP. The reverse transition is catalyzed by guanine nucleotide exchange factor (GEF) which induces exchange between the bound GDP and the cellular GTP.
The GEF of Ran (denoted by RanGEF) is located predominantly in the nucleus while RanGAP is located almost exclusively in the cytoplasm. Therefore, in the nucleus Ran will be mainly in the GTP-bound state due to the action of RanGEF while cytoplasmic Ran will be mainly loaded with GDP. This asymmetric distribution has led to the following model for the function of exportins and importins.
It is thought that binding between an exportin or importin and its cargo depends on their interaction with Ran: RanGTP enhances binding between an exportin and its cargo but stimulates release of importin's cargo; RanGDT has the opposite effect, namely, it stimulates the release of exportin's cargo, but enhances the binding between an importin and its cargo. Therefore, the exportin and its cargo may move together with RanGTP inside the nucleus, but the cargo will be released as soon as the complex moves into the cytoplasm (through nuclear pores), since RanGTP will be converted to RanGDP in the cytoplasm. By contrast, the importin and its cargo may move together with RanGDP in the cytoplasm, but the cargo will be released in the nucleus since RanGDP will be converted to RanGTP in the nucleus.
A protein destined for the nucleus and/or cytoplasm contains a specific sequence which can be recognized directly by importin/exportin or through an adaptor protein. For example, importin β is a well characterized importin. It cannot recognize the specific sequence but can be assisted by importin α which is an adaptor. By contrast, the importin for the heterogeneous nuclear ribonucleoprotein (hnRNP) can recognize directly the specific sequence in hnRNP. This importin is named "transportin".
Nuclear Localization Signal (NLS)
Two major types of signals have been identified for the nuclear import of proteins: SV40 type and bipartite type. The former was first found in the large T antigen of the SV40 virus. It has the following sequence
This type of signal is characterized by a few consecutive basic residues and in many cases also contains a proline residue.
The bipartite type was first identified in Xenopus nucleoplasmin with the following NLS:
Its characteristic pattern is: two basic residues, 10 spacer residues, and another basic region consisting of at least 3 basic residues out of 5 residues.
Nuclear Export Signal (NES)
The signal for nuclear export is a leucine-rich domain which can be recognized by a class of exportins called exportin 1 or Crm1. An example is given below:
This NES is found in the rev protein of HIV-1.
Some proteins which need to shuttle between the nucleus and cytoplasm contain both NLS and NES. Examples include the rev protein of HIV-1 and the nuclear factor of activated T cells (NFAT). The rev protein plays a key role in the regulation of viral expression. NFAT is the target of immunosuppressive drugs widely used in organ transplantation.