Direct observation of the conformational states of formin mDia1 at actin filament barbed ends and along the filament

The fine regulation of actin polymerization is essential to control cell motility, architecture and to perform essential cellular functions. Formins are key regulators of actin filament assembly, known to processively elongate filament barbed ends and increase their polymerization rate. Based on indirect observations, different models have been proposed to describe the molecular mechanism governing the processive motion of formin FH2 domains at polymerizing barbed ends. Using electron microscopy, we directly identified two conformations of the mDia1 formin FH2 domains in interaction with the barbed ends of actin filaments. These conformations agree with the open and closed conformations of the “stair stepping” model proposed by Otomo and colleagues¹. We observed the FH2 dimers to be in the open conformation for 79% of the data, interacting with the two terminal actin subunits of the barbed end, while they interact with three actin subunits in the closed conformation. Further, our data reveal that the open state encompasses a continuum of states where the orientation of the leading FH2 domain with respect to the filament long axis varies from 108 to 135 degrees. In addition, we identified FH2 domains encircling the core of actin filaments, providing structural information for mDia1 away from the barbed end. Based on these direct observations, we propose a model of formin in interaction with the growing filament end, as well as with the core of the filament.