NUCLEAR TRANSPORT

A striking property of nuclear pore complexes is their ability to mediate bi-directional nucleocytoplasmic traffic of proteins and RNAs. In the past year, several new nuclear pore proteins have been identified, but their precise functions remain to be established. Cytosolic factors responsible for the recognition and docking of substrates for nuclear transport are also being characterized. It appears that different factors are required for the import of karyophilic proteins versus small nuclear ribonucleoprotein particles. Furthermore, the GTPase Ran/TC4 has been shown to play a key role in translocation across the nuclear pore complex. Specific RNAs require different sets of factors for their export from the nucleus, although a common export route appears to be utilized by different RNA species. In contrast, nuclear retention has been found to have an influence in controlling the rate of protein exit from the nucleus.