The GTP-binding protein Ran/TC4 is required for protein import into the nucleus

Abstract
TWO cytosolic fractions (A and B) fromXenopus 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.