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J. Biol. Chem., Vol. 280, Issue 26, 24849-24856, July 1, 2005

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Immediate Early Gene X1 (IEX-1) Is Organized in Subnuclear Structures and Partially Co-localizes with Promyelocytic Leukemia Protein in HeLa Cells^*

Marie-Luise Kruse, Alexander Arlt, Alexander Sieke, Frauke Grohmann, Maike Grossmann, Jörg Minkenberg, Ulrich R. Fölsch, and Heiner Schäfer¶

From the Laboratory of Molecular Gastroenterology and Hepatology, First Department of Medicine, Christian-Albrechts-University of Kiel, Schittenhelmstrasse 12, D-24105 Kiel, Germany

Immediate early gene X1 (IEX-1) represents a stress response gene involved in growth control and modulation of apoptosis. Here, we report a detailed analysis of IEX-1 with respect to its intracellular localization. By means of confocal laser scanning microscopy, a green fluorescent protein-IEX-1 fusion protein transfected into HeLa cells, as well as endogenous IEX-1, could be detected in distinct subnuclear structures. This particular subnuclear localization of IEX-1 was not observed with a green fluorescent protein-IEX-1 fusion protein lacking a putative nuclear localization sequence, along with a decreased effect on apoptosis. Double immunofluorescence staining revealed a partial co-localization of endogenous promyelocytic leukemia protein (PML) and IEX-1 in these subnuclear structures. Nuclear localization of IEX-1 is also enhanced upon treatment of cells with leptomycin B, an inhibitor of the nuclear exporter CRM1. These observations indicate that IEX-1 is specifically shuttled to and from the nucleus. Overexpression experiments using PML isoforms III and IV revealed distinct intranuclear interaction of IEX-1 and PML. Coprecipitation experiments showed physical interaction between IEX-1 and PML. The close structural relation of IEX-1-containing nuclear subdomains and PML nuclear bodies suggests a function of IEX-1 related to the multiple functions of these unique subnuclear regions, particularly during stress response and growth control.

Received for publication, February 10, 2005 , and in revised form, April 15, 2005.

^* This work was supported by a grant from the Deutsche Forschungsgemeinschaft (DFG/SFB 415-A13). 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.

Shared first authorship.

This work is part of an M. D. thesis.

^¶ To whom correspondence should be addressed. Tel.: 49-431-597-1394; Fax: 49-431-597-1427; E-mail: hschaef{at}1med.uni-kiel.de.

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