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J. Biol. Chem., Vol. 277, Issue 8, 6280-6286, February 22, 2002

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Dephosphorylation and Subcellular Compartment Change of the Mitotic Bloom's Syndrome DNA Helicase in Response to Ionizing Radiation^*

Stéphanie Dutertre^§, Redha Sekhri^¶, Lionel A. Tintignac^¶, Rosine Onclercq-Delic, Bruno Chatton^**, Christian Jaulin, and Mounira Amor-Guéret^§§

From the  CNRS, Unité Mixte de Recherche 1598,  Unité Mixte de Recherche 1599, Institut Gustave Roussy, 39 Rue Camille Desmoulins, Villejuif Cedex 94 805, ^** CNRS, Unité Mixte de Recherche 7100, Ecole Superieure de Biotechnologie de Strasbourg, Boulevard Sébastien Brandt, Strasbourg Illkirch F-67400, France

Bloom's syndrome is a rare human autosomal recessive disorder that combines a marked genetic instability and an increased risk of developing all types of cancers and which results from mutations in both copies of the BLM gene encoding a RecQ 3'-5' DNA helicase. We recently showed that BLM is phosphorylated and excluded from the nuclear matrix during mitosis. We now show that the phosphorylated mitotic BLM protein is associated with a 3'-5' DNA helicase activity and interacts with topoisomerase III. We demonstrate that in mitosis-arrested cells, ionizing radiation and roscovitine treatment both result in the reversion of BLM phosphorylation, suggesting that BLM could be dephosphorylated through the inhibition of cdc2 kinase. This was supported further by our data showing that cdc2 kinase activity is inhibited in -irradiated mitotic cells. Finally we show that after ionizing radiation, BLM is not involved in the establishment of the mitotic DNA damage checkpoint but is subjected to a subcellular compartment change. These findings lead us to propose that BLM may be phosphorylated during mitosis, probably through the cdc2 pathway, to form a pool of rapidly available active protein. Inhibition of cdc2 kinase after ionizing radiation would lead to BLM dephosphorylation and possibly to BLM recruitment to some specific sites for repair.

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