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Ini, a Small Nuclear Protein that Enhances the Response of the Connexin43 Gene to Estrogen

Department of Biochemistry and Molecular Biology, University of Miami School of Medicine, Miami, Florida 33101

Address all correspondence and requests for reprints to: Rudolf Werner, Department of Biochemistry and Molecular Biology, University of Miami School of Medicine, P.O. Box 016129, Miami, Florida 33101.

This article describes the structural and functional characterization of Ini (AF495522), a novel highly conserved zinc-finger protein that had been identified by screening an estrogen-induced rat myometrial expression library. Ini localizes to the nucleus of HeLa cells and binds to the proximal connexin43 (cx43) promoter, as demonstrated by EMSA. In addition, transient transfection experiments performed with estrogen receptor (ER) cDNA show that overexpression of Ini enhances, in a dose-dependent fashion, the up-regulation of the cx43 gene by estrogen. On binding to the cx43 promoter, Ini stimulates the transcriptional activating function (AF)-1, but not the AF-2, of the ER. This makes Ini one of the few known coactivators specific for AF-1. Because estrogen up-regulates Ini mRNA in the myometrium, it is likely that Ini’s physiological role in this tissue is to modulate the response of the cx43 gene to estrogen. Transfection studies with an Ini antisense construct seem to indicate that Ini plays an additional role in the cellular response to estrogen affecting both AF-1 and AF-2 activities of the ER. This broader effect may be associated with cell cycle progression that in yeast has been shown to require Ini.

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