We have developed an in vitro nuclear protein import reaction from semi-intact yeast cells. The reaction uses cells that have been permeabilized by freeze-thaw after spheroplast formation. Electron microscopic analysis and antibody-binding experiments show that the nuclear envelope remains intact but the plasma membrane is perforated. In the presence of ATP and cytosol derived from yeast or mammalian cells, a protein containing the nuclear localization sequence (NLS) of SV40 large T-antigen is transported into the nucleus. Proteins with mutant NLSs are not imported. In the absence of cytosol, binding of NLS-containing proteins occurs at the nuclear envelope. N-ethylmaleimide treatment of the cytosol as well as antibodies to the nuclear pore protein Nsp1 inhibit import but not binding to the nuclear envelope. Yeast mutants defective in nuclear protein transport were tested in the in vitro import reaction. Semi-intact cells from temperature-sensitive nsp1 mutants failed to import but some binding to the nuclear envelope was observed. On the other hand, no binding and thus no import into nuclei was observed in semi-intact nsp49 cells which are mutated in another nuclear pore protein. Np13 mutants, which are defective for nuclear protein import in vivo, were also deficient in the binding step under the in vitro conditions. Thus, the transport defect in these mutants is at the level of the nucleus and the point at which nuclear transport is blocked can be defined.

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