ISOLATION OF A PROTEIN THAT IS ESSENTIAL FOR THE FIRST STEP OF NUCLEAR-PROTEIN IMPORT

We have purified a cytosolic protein from Xenopus eggs that is essenti al for selective protein import into the cell nucleus. The purified pr otein, named importin, promotes signal-dependent binding of karyophili c proteins to the nuclear envelope. We have cloned, sequenced, and exp ressed a corresponding cDNA. Importin shows 44% sequence identity with SRP1p, a protein associated with the yeast nuclear pore complex. Comp lete, signal-dependent import into HeLa nuclei can be reconstituted by combining importin purified from Xenopus eggs or expressed in E. coli with Ran/TC4. Evidence for additional stimulatory factors is provided .