Identification of a yeast karyopherin heterodimer that targets import substrate to mammalian nuclear pore complexes

Abstract

Targeting of import substrate to nuclear pore complexes of permeabilized vertebrate cells was previously shown to require a protein complex composed of two subunits, termed karyopherin. Yeast contain a homologue of karyopherin α named Srp1p, which was initially identified as a genetic suppressor of mutations in a subunit of RNA polymerase I. To determine whether yeast contain a karyopherin complex that includes Srp1p as the karyopherin α homologue, we genetically replaced Srp1p with a Srp1-Protein A chimera. Cytosol from this strain contained a complex, composed of the chimera and a protein of 95 kDa, that was purified using affinity chromatography on IgG Sepharose. Microsequence analysis showed that the 95-kDa protein was identical with a yeast protein encoded by gene L8300.15 on chromosome XII. Sequence comparison revealed that the L8300.15 gene product is the closest structural homologue of vertebrate karyopherin β. The yeast α and β karyopherin subunits were expressed in Escherichia coli and were purified. When combined, they formed a heterodimeric complex and were active in targeting import substrate to nuclear envelopes of mammalian cells. We propose that all karyopherins function as α/β heterodimers.