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

Protein import into the nuclei of digitonin permeabilized cells is supported by the A and B cytosolic fractions from Xenopus oocytes. The nuclear localization sequence is recognized by the fraction A, which supports the import to the nuclear envelope. The consequent movement of the binding substrates into the nucleus is mediated by the fraction B. The activity of the fraction B requires two interacting constituents. The purification of one of these constituents to homogeneity reveals that it is made up of the GTP-binding protein Ran/TC4, which is abundant in the human body.