THE GTP-BINDING PROTEIN RAN TC4 IS REQUIRED FOR PROTEIN IMPORT INTO THE NUCLEUS/

Two cytosolic fractions (A and B) from Xenopus oocytes are sufficient to support protein import into the nuclei of digitonin-permeabilized c ells1. Fraction A recognizes the nuclear localization sequence (NLS) a nd binds the import substrate to the nuclear envelope, whereas fractio n B mediates the subsequent passage of the bound substrate into the nu cleus. Here we report that two interacting components are required for full fraction-B activity, purify one of these components to homogenei ty, and show that it is the highly abundant GTP-binding protein Ran (R as-related nuclear protein)/TC4.