NUCLEAR TRANSPORT

A striking property of nuclear pore complexes is their ability to medi ate 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 facto rs responsible for the recognition and docking of substrates for nucle ar transport are also being characterized. It appears that different f actors are required for the import of karyophilic proteins versus smal l 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 the ir export from the nucleus, although a common export route appears to be utilized by different RNA species. In contrast, nuclear retention h as been found to have an influence in controlling the rate of protein exit from the nucleus.