Formin-family proteins, in the active state, form actin-based structures such as 
stress fibres. Their activation mechanisms, however, are largely unknown except 
that mDia and its closely related formins can be activated by direct binding of 
the small GTPase Rho or Cdc42. Here we show that the Rho-dependent protein 
kinase ROCK phosphorylates the C-terminal residues Ser1131, Ser1137, and Thr1141 
of formin homology domain protein 1 (FHOD1), a major endothelial formin that is 
normally autoinhibited by intramolecular interaction between the N- and 
C-terminal regions. Phosphorylation of FHOD1 at the three residues fully 
disrupts the autoinhibitory interaction, which culminates in formation of stress 
fibres. We also demonstrate that, in vascular endothelial cells, thrombin, a 
vasoactive substance leading to Rho activation, elicits both FHOD1 
phosphorylation and stress fibre formation in a ROCK-dependent manner, and that 
FHOD1 depletion by RNA interference impairs thrombin-induced stress fibre 
formation. Based on these findings we propose a novel mechanism for activation 
of formin-family proteins: ROCK, activated by G protein-coupled receptor ligands 
such as thrombin, directly phosphorylates FHOD1 at the C-terminal region, which 
renders this formin in the active form, leading to stress fibre formation.