Formin proteins regulate the actin and microtubule cytoskeletons and also 
control the activity of the SRF transcription factor through depletion of the 
G-actin pool. Although the conserved formin homology 2 (FH2) domains of the 
mDia1 and Bni1 formins can nucleate actin polymerization in vitro, the activity 
of other FH2 domains and the relationship between actin polymerization and 
microtubule reorganization have been controversial. We show that, similar to the 
mDia1 FH2 domain, the FH2 domains of mDia2 and ld are sufficient for SRF 
activation in vivo. We demonstrate that an mDia1 mutant defective for 
microtubule rearrangement in vivo is also defective in SRF activation in vivo as 
well as actin polymerization in vitro and that the mDia2 FH2 domain promotes 
actin polymerization in vitro. Using co-immunoprecipitation, we show that mDia1 
is oligomeric in its inactive autoinhibited state in vivo, that the active mDia1 
and mDia2 FH2 domains form homo- but not hetero-oligomers in vivo, and that 
oligomerization is abolished by inactivating FH2 deletion and point mutations. 
Nevertheless, inactive mDia1 FH2 domain mutants retain the ability to interfere 
with cellular mDia activity. Our results show that self-oligomerization is 
essential for SRF activation in vivo and F-actin assembly in vitro and provide 
strong support for recent structural models of the FH2 domain.