Nuclear import of yeast Gcn4p requires karyopherins Srp1p and Kap95p.

The yeast transcription factor Gcn4p contains two stretches of amino acid residues, NLS1 and NLS2, which are independently able to relocate the cytoplasmic protein chorismate mutase into the nucleus. Only NLS2 is conserved among fungi. A truncated version of CPCA (the counterpart of Gcn4p in Aspergillus nidulans), which lacks the conserved NLS, accumulates in the cytoplasm instead of the nucleus. Nuclear uptake mediated by the NLS1 of Gcn4p is impaired by defects in genes for several different karyopherins, whereas NLS2-dependent nuclear import specifically requires the alpha-importin Srp1p and the beta-importin Kap95p. Yeast strains that are defective in either of these two karyopherins are unable to respond to amino acid starvation. We have thus identified Gcn4p as a substrate for the Srp1p/Kap95p transport complex. Our data suggest that NLS2 is the essential and specific nuclear transport signal; NLS1 may play only an unspecific or accessory role.