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Conformational Changes in Actin Filaments Induced by Formin Binding to the Barbed End

Gábor Papp ^*, Beáta Bugyi ^*, Zoltán Ujfalusi ^*, Szilvia Barkó ^*, Gábor Hild , Béla Somogyi ^*  and Miklós Nyitrai ^*

^* Department of Biophysics, Faculty of Medicine, and Research Group for Fluorescence Spectroscopy, Office for Academy Research Groups, University of Pécs, Pécs, Hungary

Correspondence: Address reprint requests to Miklós Nyitrai, Dept. of Biophysics, Faculty of Medicine, University of Pécs, Pécs, Szigeti str. 12, H-7624, Hungary. Tel.: 36-72-536267; Fax: 36-72-536261; E-mail: miklos.nyitrai{at}aok.pte.hu.

Formins bind actin filaments and play an essential role in the regulation of the actin cytoskeleton. In this work we describe details of the formin-induced conformational changes in actin filaments by fluorescence-lifetime and anisotropy-decay experiments. The results show that the binding of the formin homology 2 domain of a mammalian formin (mouse mDia1) to actin filaments resulted in a less rigid protein structure in the microenvironment of the Cys³⁷⁴ of actin, weakening of the interactions between neighboring actin protomers, and greater overall flexibility of the actin filaments. The formin effect is smaller at greater ionic strength. The results show that formin binding to the barbed end of actin filaments is responsible for the increase of flexibility of actin filaments. One formin dimer can affect the dynamic properties of an entire filament. Analyses of the results obtained at various formin/actin concentration ratios indicate that at least 160 actin protomers are affected by the binding of a single formin dimer to the barbed end of a filament.