The Effect of Heavy Meromyosin on the Flexibility of Formin-Bound Actin Filaments

Abstract

Formins are conservative proteins with important roles in the regulation of the microfilamental system in eukaryotic cells. They have several domains including FH1, FH2, GPB and DAD domains. In the interaction between actin and formin the FH2 domain plays a key role. This domain builds antiparallel dimers with the help of the ‘linker region’ between FH1 and FH2 domains. The ‘mammalian Diaphanous-related 1′ constitutes one of the subfamilies of the formins. It was shown that formins could make the actin filaments more flexible (Bugyi et al. 2006), and another actin binding protein, tropomyosin reduces this effect (Ujfalusi et al. 2008). In our work we investigated whether the flexibility of the filaments could be restored by the binding of the common binding partner of actin, myosin. Skeletal muscle and non-muscle 2B isoforms of HMM were used. Temperature dependent Förster-type resonance energy transfer (FRET) and fluorescence anisotropy decay experiments showed that the formin (mDia1-FH2) induced an increase in the flexibility of actin filaments, which was reversed by the binding of heavy meromyosin HMM. Our previous measurements showed that tropomyosin had a similar stabilizing effect on the formin-bound actin filaments. These observations together indicated that actin-binding proteins played a central role in the molecular mechanisms that regulate the dynamic properties of the actin filaments. This mechanism provides the opportunity for the formin-induced actin filaments to become similar to those that are polymerized in the absence of formins.