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 cells1. Fraction A recognizes the nuclear localization sequence (NLS) and binds the import substrate to the nuclear envelope, whereas fraction B mediates the subsequent passage of the bound substrate into the nucleus. Here we report that two interacting components are required for full fraction-B activity, purify one of these components to homogeneity, and show that it is the highly abundant GTP-binding protein Ran (Ras-related nuclear protein)/TC4.