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J. Biol. Chem., Vol. 278, Issue 23, 21146-21154, June 6, 2003

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Transport Signals and Transcription-dependent Nuclear Localization of the Putative DEAD-box Helicase MDDX28^*,

Rut Valgardsdottir  and Hans Prydz 

From the Biotechnology Centre of Oslo, University of Oslo, Gaustadalleen 21, N0349 Oslo, Norway

The human protein MDDX28 is a putative RNA helicase and a nucleocytoplasmic shuttling protein also localized to the mitochondria. Its localization is novel among RNA helicases. We have studied its intracellular targeting signals and show that the first 20 amino acids of MDDX28 are necessary and sufficient for both mitochondrial import and nuclear export of the protein. Mutation of the five leucines in the sequence to alanines abolished the mitochondrial targeting signal as well as greatly reducing the nuclear export signal, indicating that these signal sequences are highly overlapping. Two short stretches of basic amino acids separated by 44 residues were both necessary and sufficient for full nuclear localization. However, they were not absolutely essential, because the protein was present in 7% of the nuclei when both signals were mutated. This indicates that MDDX28 contains another unidentified weak nuclear localization signal(s). Three basic domains in the N-terminal half of the protein and its RNA binding ability were essential for nucleolar localization as well as transcription-inhibition-dependent localization to nuclear subcompartments. Two of these basic domains were the same as those constituting the nuclear localization signal, suggesting that they are responsible for bringing the protein into the nucleus to the sites of RNA binding. Our results indicate that MDDX28 nucleo-cytoplasmic shuttling is dependent on the availability of nascent RNA.

Received for publication, January 27, 2003 , and in revised form, March 18, 2003.

^* This work was supported by grants from the Research Council of Norway, the Norwegian Cancer Society, and the Jahre Foundation (to H. P.). 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.

The on-line version of this article (available at http://www.jbc.org) contains a video.

A Research Fellow of the Norwegian Cancer Society.

To whom correspondence should be addressed. Tel.: 47-22840532; Fax: 47-22840501; E-mail: hans.prydz{at}biotek.uio.no.

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