RAN TC4 MUTANTS IDENTIFY A COMMON REQUIREMENT FOR SNRNP AND PROTEIN IMPORT INTO THE NUCLEUS/

Kinetic competition experiments have demonstrated that at least some f actors required for the nuclear import of proteins and U snRNPs are di stinct. Both import processes require energy, and in the case of prote in import, the energy requirement is known to be at least partly met b y GTP hydrolysis by the Ran GTPase. We have compared the effects of no nhydrolyzable GTP analogues and two mutant Ran proteins on the nuclear import of proteins and U snRNPs in vitro. The mutant Ran proteins hav e different defects; Q69L(glutamine 69 changed to leucine) is defectiv e in GTP hydrolysis while T24N (threonine 24 changed to asparagine) is defective in binding GTP. Both protein and snRNP import are sensitive either to the presence of the two mutant Ran proteins, which act as d ominant negative inhibitors of nuclear import, or to incubation with n onhydrolyzable GTP analogues. This demonstrates that there is a requir ement for a GTPase activity for the import of U snRNPs, as well as pro teins, into the nucleus. The dominant negative effects of the two muta nt Ran proteins indicate that the pathways of protein and snRNP import share at least one common component.