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Molecular and Cellular Biology, February 1999, p. 1218-1225, Vol. 19, No. 2
0270-7306/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.

Importin beta  Can Mediate the Nuclear Import of an Arginine-Rich Nuclear Localization Signal in the Absence of Importin alpha

Diana Palmeri1 and Michael H. Malim2,3,4,*

Cell and Molecular Biology Graduate Group,1 Departments of Microbiology2 and Medicine,3 and Howard Hughes Medical Institute,4 University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104-6148

Received 17 August 1998/Returned for modification 14 October 1998/Accepted 27 October 1998

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 .

* Corresponding author. Mailing address: Departments of Microbiology and Medicine, University of Pennsylvania Medical School, Philadelphia, PA 19104-6148. Phone: (215) 573-3493. Fax: (215) 573-2172. E-mail: malim{at}mail.med.upenn.edu.

Molecular and Cellular Biology, February 1999, p. 1218-1225, Vol. 19, No. 2
0270-7306/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.


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