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J. Biol. Chem., Vol. 281, Issue 16, 10727-10736, April 21, 2006

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Formins Regulate Actin Filament Flexibility through Long Range Allosteric Interactions^*

Beáta Bugyi, Gábor Papp, Gábor Hild¹, Dénes Lôrinczy, Elisa M. Nevalainen^¶, Pekka Lappalainen^¶, Béla Somogyi, and Miklós Nyitrai²

From the Research Group for Fluorescence Spectroscopy, Office for Academy Research Groups Attached to Universities and Other Institutions at the Department of Biophysics, Faculty of Medicine, University of Pécs, Pécs, Szigeti str. 12, H-7624, Hungary and the ^¶Program in Cellular Biotechnology, Institute of Biotechnology, University of Helsinki, 00014 Helsinki, Finland

The members of the formin family nucleate actin polymerization and play essential roles in the regulation of the actin cytoskeleton during a wide range of cellular and developmental processes. In the present work, we describe the effects of mDia1-FH2 on the conformation of actin filaments by using a temperature-dependent fluorescence resonance energy transfer method. Our results revealed that actin filaments were more flexible in the presence than in the absence of formin. The effect strongly depends on the mDia1-FH2 concentration in a way that indicates that more than one mechanism is responsible for the formin effect. In accordance with the more flexible filament structure, the thermal stability of actin decreased and the rate of phosphate dissociation from actin filaments increased in the presence of formin. The interpretation of the results supports a model in which formin binding to barbed ends makes filaments more flexible through long range allosteric interactions, whereas binding of formin to the sides of the filaments stabilizes the protomer-protomer interactions. These results suggest that formins can regulate the conformation of actin filaments and may thus also modulate the affinity of actin-binding proteins to filaments nucleated/capped by formins.

Received for publication, September 19, 2005 , and in revised form, February 16, 2006.

^* This work was supported by the Hungarian Academy of Sciences and by grants from the National Research Foundation (OTKA Grants T43103 [GenBank] (to M. N.) and D048545 (to G. H.)). 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 Supported by the János Bolyai Research Fellowship of the Hungarian Academy of Sciences.

² A European Molecular Biology Organisation/Howard Hughes Medical Institute Scientist. To whom correspondence should be addressed. Tel.: 36-72-536267; Fax: 36-72-536261; E-mail: miklos.nyitrai{at}aok.pte.hu.

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