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J. Biol. Chem., Vol. 279, Issue 15, 15356-15367, April 9, 2004

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Nuclear Export of the Oncoprotein v-ErbA Is Mediated by Acquisition of a Viral Nuclear Export Sequence^*

Laura J. DeLong, Ghislain M. C. Bonamy¶||, Erin N. Fink, and Lizabeth A. Allison**

From the Department of Biology, College of William and Mary, Williamsburg, Virginia 23187 and ^¶Universite Paris 7-Denis-Diderot, Paris Cedex 05, France 75251

v-ErbA, an oncogenic derivative of the thyroid hormone receptor (TR) carried by the avian erythroblastosis virus, contains several alterations including fusion of a portion of avian erythroblastosis virus Gag to its N terminus, N- and C-terminal deletions, and 13 amino acid substitutions. Nuclear export of v-ErbA occurs through a CRM1-mediated pathway. In contrast, nuclear export of TR and another isoform, TR, is CRM1-independent. To determine which amino acid changes in v-ErbA confer CRM1-dependent nuclear export, we expressed a panel of green and yellow fluorescent protein-tagged mutant and chimeric proteins in mammalian cells. The sensitivity of subcellular trafficking of these mutants to leptomycin B (LMB), a specific inhibitor of CRM1, was assessed by fluorescence microscopy. Our data showed that a nuclear export sequence resides within a 70-amino acid domain in the C-terminal portion of the p10 region of Gag, and in vitro binding assays demonstrated that Gag interacts directly with CRM1. However, a panel of ligand-binding domain mutants of v-ErbA lacking the Gag sequence exhibited greater nuclear localization in the presence of LMB, suggesting that the various amino acid substitutions/deletions may cause a conformation shift, unmasking an additional CRM1-dependent nuclear export sequence. In contrast, the altered DNA-binding domain of the oncoprotein did not contribute to CRM1-dependent nuclear export. Heterokaryon experiments revealed that v-ErbA did not undergo nucleocytoplasmic shuttling when the CRM1 export pathway was blocked by LMB treatment, suggesting that the ability to follow the export pathway used by TR has been lost by the oncoprotein during its evolution. Our findings thus point to the intriguing possibility that acquisition of altered nuclear export capabilities contributes to the oncogenic properties of v-ErbA.

Received for publication, July 28, 2003 , and in revised form, January 15, 2004.

^* This work was supported in part by National Institutes of Health Grant DK058028-01A1, National Science Foundation Grant MCB0090923, Jeffress Memorial Trust Grant J-477, and the Horsley Cancer Research Fund (to L. A. A.); by a Howard Hughes Medical Institute grant through the Undergraduate Biological Sciences Education Program to the College of William and Mary; and by an Endocrine Society Maurice Raben Summer Research Fellowship (to L. J. D.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

Both authors contributed equally to this work.

^|| This work is part of the Ph.D. dissertation research under the supervision of A. Guichon Mantel and L. A. Allison.

^** To whom correspondence should be addressed: Dept. of Biology, College of William and Mary, P. O. Box 8795, Millington Hall 116, Williamsburg, VA 23187-8795. Tel.: 757-221-2232; Fax: 757-221-6583; E-mail: laalli{at}wm.edu.

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