Summary

The molecular dynamics of nuclear protein import were examined in a solution binding assay by testing for interactions between a protein containing a nuclear localization signal (NLS), the transport factors kary-opherin a, karyopherin p, and Ran, and FXFG or GLFG repeat regions of nucleoporins. We found that kary-opherins a and f5 cooperate to bind FXFG but not GLFG repeat regions. Binding of the NLS protein to karyoph-erin a was enhanced by karyopherin p. Two novel reac-tions were discovered. First, incubation of a karyoph-erin heterodimer-NLS protein complex with an FXFG repeat region stimulated the dissociation of the NLS protein from the karyopherin heterodimer. Second, in-cubation of the karyopherin heterodimer with RanGTP (or with a Ran mutant that cannot hydrolyze GTP) led to the dissociation of karyopherin a from fi and to an association of Ran with karyopherin p; RanGDP had no effect. We propose that movement of NLS proteins across the nuclear pore complex is a stochastic pro-cess that operates via repeated association-dissocia-tion reactions