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J. Biol. Chem., Vol. 280, Issue 15, 15257-15266, April 15, 2005

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Zhangfei Is a Potent and Specific Inhibitor of the Host Cell Factor-binding Transcription Factor Luman^*

Vikram Misra, Noreen Rapin, Oksana Akhova, Matthew Bainbridge¶, and Paul Korchinski||

From the Department of Microbiology, Western College of Veterinary Medicine, the ^¶Department of Computer Science, University of Saskatchewan, Saskatoon, Saskatchewan S7N5B4, Canada

Host cell factor (HCF) was initially discovered as a cellular co-factor required for the activation of herpes simplex virus immediate early gene expression by the virion associated transactivator VP16. HCF also participates in a variety of cellular processes, although the mechanism of its action is not known. VP16 binds to HCF through a 4-amino acid motif (EHAY), which closely resembles the HCF binding domain of two cellular basic leucine-zipper proteins, Luman and Zhangfei. Luman is a powerful transcription factor that, in transient expression assays, activates promoters containing cAMP or unfolded protein response elements (UPRE). In contrast, Zhangfei neither binds consensus recognition elements for basic leucine-zipper proteins nor does it activate promoters containing them. Here we show that Zhangfei suppresses the ability of Luman to activate transcription. HCF appeared to be required for efficient suppression. A mutant of Zhangfei, which was unable to bind HCF, was impaired in its ability to suppress Luman. Zhangfei did not suppress ATF6, a transcription factor closely related to Luman but that does not bind HCF, unless the HCF binding motif of Luman was grafted onto it. Zhangfei inhibited the HCF-dependent activation of a UPRE-containing promoter by a Gal4-Luman fusion protein but was unable to inhibit the HCF-independent activation by Gal4-Luman of a promoter that contained Gal4 binding motifs. Binding of HCF by Zhangfei was required for the co-localization of Luman and Zhangfei to nuclear domains, suggesting that HCF might target the proteins to a common location.

Received for publication, January 20, 2005 , and in revised form, February 7, 2005.

^* This work was supported in part by a Natural Sciences and Engineering Research Council (NSERC) of Canada Discovery grant (to V. M.) and a post-doctoral fellowship (awarded to O. A.) by the Saskatchewan Health Research Foundation. 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.

^|| Supported by a summer undergraduate research scholarship by the NSERC.

To whom correspondence should be addressed: Dept. of Microbiology, Western College of Veterinary Medicine, University of Saskatchewan, Saskatoon, Saskatchewan S7N5B4, Canada. Tel.: 306-966-7218; Fax: 306-966-7244; E-mail: vikram.misra{at}usask.ca.

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