Must pass through the nuclear pores if do not diffuse through the membrane. The pores are made/filled with the nuclear pore complex (NPC), which controlls exit/entry. It varies from 10 nm to 25 nm in diameter. It is made of nucleoporins (Nups), which are large proteins with generic protein - protein interaction domains. About half contain phenylalanine-glycine multiply repeted regions. Some components of the NPC may be mobile. It is not known if a given NPC is unidirectional or bidirectional, or if it changes with time, etc.
There are two major type of materials imported/exported from the nucleus: Cargo and Transporters (or transport receptors). Transporters act on the cargo (proteins, RNA, etc.) to effect the transport. Some cargos (Ribosomal subunits and mRNA), use either other pathways or none at all.
Cargos may have one of two types of signals: nuclear localization signals (NLS) for entry or nuclear export signals (NES) for exit. There are many different signals of each type. These signals can be made of protein or RNA, or a combination of the two. A given signal may be recognized by one or more of a family of transporters. Some cargo signals bind directly to a transporter, but some bind indirectly, by use of adaptor protein(s). Cargos that share a given adaptor or transporter (or both) have the same signal (e.g. an N-terminal importin β-binding domain).
Transporters have specific binding regions, which recognize (and bind to) cargo signals. There are many different receptors (about as many as there are signals), which exist in families of similar types, which might share 45% to 85% homology. In yeast, only four transporters are essential. Some signals are one part and some consist of many parts (usually those specific for RNP transport). They are usually large, acidic proteins which share considerable sequence homology (15% to 25%) within a species. They all have an N-terminal Ran GTP binding domain, a C-terminal cargo binding domain and the ability to bind NPC components. Sometimes (such as the case of histone H1) it takes more than one kind of transporter to transport a protein (H1 uses a complex of importin β and importin 7). In this case, one acts as an adaptor for the other, but other transport receptors may also function in pairs. In some cases, there are bidirectional (NES and NLS) signals, such as in mRNA binding proteins, which have no known export receptor.
The system is run by the Ran GTPase. Ran is found inside the nucleus at steady state, and has little activity by itself. It is activated by both a guanine nucleotide exchange factor (RanGEF) that is associated with chromatin and a GTPase activating protein (RanGAP), which is cytoplasmic. Ran binding protein 1 (RanBP1) is found mostly in the cytoplasm and the cytoplasmic filaments of the NPC where it can bind the Nup358/RanBP2 complex. When a transporter binds RanGTPase, the condition of it (whether bound to GTP or GDP) is an indicator of where in the cell the transporter is.
Since the Ran GTP exchange factor (exchanges GTP for GDP) is bound to chromatin in the nucleus, and the RanGAP is in the cytoplasm, a gradient of Ran is set up, with the GTP form mostly in the nucleus and the GDP form mostly in the cytoplasm.
When an import receptor arrives in the nucleus, binding RanGTP triggers dissociation of the complex and thereleasing of the cargo. The import receptor (transporter) is returned to the cytoplasm as a RanGTP complex. Export receptors bind their cargo more tightly when bound to RanGTP, and then are passed through to the cytoplasm.
When export receptors reach the cytoplasm, they encounter RanGAP and RanBP1. This dissociates the complex, releasing the cargo, and RanGDP. In order to return Ran back to the nucleus, NTF2 protein binds RanGDP in the cytoplasm and by interacting with the Nups, returns it back to the nucleus. In both cases, other factors might interact to facilitate the loading and unloading of cargo, as well as transport of the complexes. Proteins may be moved through the NPC without energy usage (in the form of NTP hydrolysis). Ran GTP hydrolysis in the cytoplasm is all the energy that is required.
hnRNP protein family are implicated in mRNA export from the nucleus. Some of these proteins contain a nuclear recognition signal (NRS) which overides the NES and keeps it in the nucleus, while others can shuttle.
Although it is probably not the only control system at this transition, Ran is involved in inhibiting reinitiation of DNA synthesis at the G1 - S phase transition in the cell cycle by blocking the formation of prereplication complexes (pre RCs) (specificaly MCM helicase). It does this by associating MCM helicase with RanGTP and the exportin 1/Crm1. MCM does not necessarily leave the nucleus, but it is removed form the prereplication complex, and so initiation of S phase is inhibited. This effect requires both high RanGTP concentration and cdk 2 activity. Lowering the RanGTP concentration releases MCM to join the preinitiation complex. Inactivation requires cdk2 activity, as well as RanGTP. RanGTP is not enough by itself. Therefore, lowering the cdk2 activity near the G1 - S boundry would lead to free MCM helicase and preinitiation (pre replication) complex formation. Activation of cdk2 at or just before the transition would prevent rereplication of the DNA.
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