Rotational movement of formins evaluated by using single-molecule fluorescence polarization

Formin homology proteins (formins) are responsible for the formation of actin structures such as actin stress fibers, actin cables, and cytokinetic contractile rings. Formins are the major actin filament (F-actin) nucleators in the cell. Because formins remain bound to the barbed end after nucleating an actin filament, it was expected that formins might rotate along the double-helical structure of F-actin during processive actin elongation (helical rotation). Here, we describe a method to detect the rotational movement of F-actin elongating from immobilized formins using single-molecule fluorescence polarization (FL_P). Tetramethylrhodamine (TMR) attached to Cys-374 of actin emits polarized fluorescence at ≈ 45 with respect to the filament axis. When the TMR-labeled F-actin laying at 45 in the visual field rotates, the vertical- and horizontal-polarized fluorescence (FL_V and FL _H, respectively) of TMR alternately become bright. This technique allowed us to demonstrate the helical rotation of mDia1, a mammalian formin. Adenosine triphosphate (ATP) hydrolysis in actin subunits is not required for helical rotation; however, ATP appears to contribute to accelerating actin elongation by mDia1. When helical rotation is limited by trapping both mDia1 and the pointed-end side, the processive filament elongation is blocked. Thus, mDia1 faithfully rotates along the long-pitch helix of F-actin. In this chapter, we introduce the theoretical concept of single-molecule FL_P, the optical setup, the preparation of adenosine diphosphate-bound actin, and the procedure to observe the rotational movement of F-actin elongating from immobilized formins.