Diaphanous related formins (DRFs) are part of the formin protein family that 
control morphogenesis, embryonic differentiation, cytokinesis, and cell 
polarity. DRFs organize the cytoskeleton in eukaryotic cells via the interaction 
with specific members of the Rho family of small GTPases including Rho, Rac, and 
Cdc42. This is best understood for Rho, which transmits signals to the actin 
cytoskeleton through the cooperation of its DRF effector mDia with ROCK 
(Rho-associated kinase). Here, we show that a constitutive active form of the 
Rac-interacting DRF FHOD1 (formin homology 2 domain containing 1) associates 
with F-actin in NIH3T3 cells, resulting in the formation of thick actin fibers. 
Cytoskeletal changes induced by FHOD1 correlated with the induction of serum 
response element transcription and were mediated by formin homology domains 1 
and 2 of FHOD1. FHOD1-induced effects required the activity of the Rho-ROCK 
cascade that is targeted at a level downstream of Rho by the DRF. However, when 
the functional interaction of FHOD1 with individual GTPases was addressed, Rac 
but not Rho or Cdc42 bound to FHOD1 in cells and induced its recruitment to 
actin filaments and lamellipodia/membrane ruffles. Furthermore, activated FHOD1 
interfered with lamellipodia formation. These results indicate that FHOD1 acts 
as an effector of Rac in actin rearrangements and transcriptional regulation and 
may provide a link for the Rac-dependent activation of the Rho cascade.