Inverted formin 2 (INF2) encodes a member of the diaphanous subfamily of formin 
proteins. Mutations in INF2 cause human kidney disease characterized by focal 
and segmental glomerulosclerosis. Disease-causing mutations occur only in the 
diaphanous inhibitory domain (DID), suggesting specific roles for this domain in 
the pathogenesis of disease. In a yeast two-hybrid screen, we identified the 
diaphanous autoregulatory domains (DADs) of the mammalian diaphanous-related 
formins (mDias) mDia1, mDia2, and mDia 3 as INF2_DID-interacting partners. The 
mDias are Rho family effectors that regulate actin dynamics. We confirmed in 
vitro INF2_DID/mDia_DAD binding by biochemical assays, confirmed the in vivo 
interaction of these protein domains by coimmunoprecipitation, and observed 
colocalization of INF2 and mDias in glomerular podocytes. We investigated the 
influence of this INF2_DID/mDia_DAD interaction on mDia mediated actin 
polymerization and on serum response factor (SRF) activation. We find that the 
interaction of INF2_DID with mDia_DAD inhibited mDia-mediated, Rho-activated 
actin polymerization, as well as SRF-responsive gene transcriptional changes. 
Similar assays using the disease-causing E184K and R218Q mutations in INF2_DID 
showed a decreased effect on SRF activation and gene transcription. The binding 
of INF2_DID to mDia_DAD may serve as a negative regulatory mechanism for mDias' 
function in actin-dependent cell processes. The effects of disease-causing INF2 
mutations suggest an important role for this protein and its interaction with 
other formins in modulating glomerular podocyte phenotype and function.