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Journal of Virology, September 2002, p. 9505-9515, Vol. 76, No. 18
0022-538X/02/$04.00+0     DOI: 10.1128/JVI.76.18.9505-9515.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

Baculovirus Transregulator IE1 Requires a Dimeric Nuclear Localization Element for Nuclear Import and Promoter Activation

Victoria A. Olson, Justin A. Wetter, and Paul D. Friesen*

Institute for Molecular Virology and Department of Biochemistry, Graduate School and College of Agricultural and Life Sciences, University of Wisconsin—Madison, Madison, Wisconsin 53706

Received 27 December 2001/ Accepted 2 June 2002

Immediate-early protein IE1 is a principal regulator of viral transcription and a contributor to origin-specific DNA replication of the baculovirus Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV). Since these viral functions involve interaction of dimeric IE1 with palindromic homologous region (hr) enhancer-origin elements of the AcMNPV genome within the nucleus, it is presumed that proper nuclear transport of IE1 is essential for productive infection. To investigate the mechanisms of IE1 nuclear import, we analyzed the effect of site-directed mutations on IE1 subcellular distribution. As demonstrated by fluorescence microscopy and biochemical fractionation of plasmid-transfected cells, wild-type IE1 localized predominantly to the nucleus. Substitution or deletion of amino acid residues within a positively charged domain (residues 534 to 538) adjacent to IE1's oligomerization motif impaired nuclear import and caused loss of transactivation. Moreover, upon coexpression, these import-defective mutations prevented nuclear entry of wild-type IE1. In contrast, double-mutated IE1 defective for both nuclear import and dimerization failed to block nuclear entry or transactivation by wild-type IE1. Thus, import-defective IE1 dominantly interfered with wild-type IE1 by direct interaction and cytosolic trapping. Collectively, our data indicate that the small basic domain encompassing residues R537 and R538 constitutes a novel nuclear localization element that functions only upon IE1 dimerization. These findings support a model wherein IE1 oligomerizes within the cytosol as a prerequisite for nuclear entry and subsequent high-affinity interaction with the symmetrical binding sites comprising AcMNPV hr enhancer-origin elements.

* Corresponding author. Mailing address: Institute for Molecular Virology, Bock Laboratories, University of Wisconsin—Madison, 1525 Linden Dr., Madison, WI 53706-1596. Phone: (608) 262-7774. Fax: (608) 262-7414. E-mail: pfriesen{at}wisc.edu.

Journal of Virology, September 2002, p. 9505-9515, Vol. 76, No. 18
0022-538X/02/$04.00+0     DOI: 10.1128/JVI.76.18.9505-9515.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.



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