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.