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J. Biol. Chem., Vol. 283, Issue 13, 8406-8411, March 28, 2008

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The Three Methyl-CpG-binding Domains of AtMBD7 Control Its Subnuclear Localization and Mobility^*

Assaf Zemach, Recipient of the Israeli Ministry of Science Eshkol Fellowship for Ph.D. students¹, Ofer Gaspan, and Gideon Grafi

From the Department of Plant Sciences, The Weizmann Institute of Science, Rehovot 76100, Israel and the Albert Katz Department of Dryland Biotechnologies, Jacob Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Sede Boqer Campus 84990, Israel

Three methyl-CpG-binding domain (MBD) proteins in Arabidopsis, AtMBD5, AtMBD6, and AtMBD7, are functional in binding methylated CpG dinucleotides in vitro and localize to the highly CpG-methylated chromocenters in vivo. These proteins differ, however, in their subnuclear localization pattern; AtMBD5 and AtMBD6, each containing a single MBD motif, show preference for two perinucleolar chromocenters, whereas AtMBD7, a naturally occurring poly-MBD protein containing three MBD motifs, localizes to all chromocenters. Here we studied the significance of multiple MBD motifs for subnuclear localization and mobility in living cells. We found that the number of MBD motifs determines the subnuclear localization of the MBD protein. Furthermore, live kinetic experiments showed that AtMBD7-green fluorescent protein (GFP) has lower mobility than AtMBD5-GFP and AtMBD6-GFP, which is conferred by cooperative activity of its three MBD motifs. Thus, the number of MBD motifs appears to affect not only binding affinity and mobility within the nucleus, but also the subnuclear localization of the protein. Our results suggest that poly-MBD proteins can directly affect chromatin structure by inducing intra- and inter-chromatin compaction via bridging over multiple methylated CpG sites.

Received for publication, July 30, 2007 , and in revised form, January 22, 2008.

^* This work was supported in part by the Israel Science Foundation and by the Jewish Colonization Association. 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.

¹ To whom correspondence should be addressed. Fax: 972-8-9344181; E-mail: assaf.zemach{at}weizmann.ac.il.

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