We investigated the nuclear transport of a fusion protein consisting of a nuclear localization signal linked to beta-galactosidase, normally a cytoplasmic protein. We microinjected the radiolabeled fusion protein into the cytoplasm of living Xenopus oocytes or supplied it directly to the surface of the oil-isolated oocyte nucleus and measured its transport into the nucleus. Our data confirm that a nuclear localization signal is sufficient to entrain a protein's facilitated transport through the nuclear pore complex and its subsequent nuclear accumulation. Moreover, nuclear envelope micropuncture experiments determine that the fusion protein's accumulation results from its intranuclear binding, demonstrating that no specific region of a transported protein--other than the nuclear localization signal itself--is required for facilitated transport and intranuclear binding. Finally, we present evidence that the intranuclear binding of a transported protein requires not only its nuclear localization signal, but also its prior facilitated transport through the nuclear pore complex.