Featured Article - March 2011
Short description: Single-molecule fluorescence polarization shows how mDia1 rotates along actin filaments.
Actin is necessary for cell integrity and movement, and it is regulated by Rho family GTPases and actin nucleators. The major nucleators are formins, which extend actin filaments by associating with the fast-growing (barbed) end. As each new actin molecule associates with the filament, formin moves to the new end. It does this with remarkable fidelity, and this process is thought to involve rotation of formins along the double helical axis of the filament structure. However, providing experimental evidence to support this idea has been difficult.
A new method by Watanabe and colleagues allows the rotational movement of actin filaments to be detected as they grow from immobilized formin. This approach involves labeling actin with a fluorescent dye and then observing it using single-molecule fluorescence polarization. Thus, only favorably oriented actin molecules were detectable. In their experimental set-up, the FH2 domains of the formin mDia1 were immobilized and the actin filament was free to rotate. Their data demonstrate the continuous rotation of mDia1-bound filaments during elongation and depolymerization. This rotation was not affected by actin-bound nucleotides or profilin, an actin-binding protein, indicating that only formins exhibit rotational behaviour.
In the cell, actin in the filament is cross-linked and therefore has restricted movement. Thus, in vivo, formins must rotate, rather than the actin filaments. It is possible that actin elongation and remodeling are regulated by axial torsion on the filament.
H. Mizuno et al. Rotational movement of the formin mDia1 along the double helical strand of an actin filament.
Science 331, 80-83 (2011). doi:10.1126/science.1197692