Importin beta can mediate the nuclear import of an arginine-rich nuclear localization signal in the absence of importin alpha.

The import of proteins into the nucleus is dependent on cis-acting targeting sequences, nuclear localization signals (NLSs), and members of the nuclear transport receptor (importin-beta-like) superfamily. The most extensively characterized import pathway, often termed the classical pathway, is utilized by many basic-type (lysine-rich) NLSs and requires an additional component, importin alpha, to serve as a bridge between the NLS and the import receptor importin beta. More recently, it has become clear that a variety of proteins enter the nucleus via alternative import receptors and that their NLSs bind directly to those receptors. By using the digitonin-permeabilized cell system for protein import in vitro, we have defined the import pathway for the Rex protein of human T-cell leukemia virus type 1. Interestingly, the arginine-rich NLS of Rex uses importin beta for import but does so by a mechanism that is importin alpha independent. Based on the ability of the Rex NLS to inhibit the import of the lysine-rich NLS of T antigen and of both NLSs to be inhibited by the domain of importin alpha that binds importin beta (the IBB domain), we infer that the Rex NLS interacts with importin beta directly. In addition, and in keeping with other receptor-mediated nuclear import pathways, Rex import is dependent on the integrity of the Ran GTPase cycle. Based on these results, we suggest that importin beta can mediate the nuclear import of arginine-rich NLSs directly, or lysine-rich NLSs through the action of importin alpha.