Mouse Diaphanous-related formins (mDias) are members of the formin protein 
family that nucleate actin polymerization and subsequently promote filamentous 
actin (F-actin) elongation by monomer addition to fast-growing barbed ends. It 
has been suggested that mDias preferentially recruit actin complexed to profilin 
due to their proline-rich FH1 domains. During filament elongation, dimeric mDias 
remain attached to the barbed ends by their FH2 domains, which form an 
anti-parallel ring-like structure enclosing the filament barbed ends. Dimer 
formation of mDia-FH2 domains is dependent on their N-terminal lasso and linker 
subdomains (connector). Here, we investigated the effect of isolated FH2 domains 
on actin polymerization using mDia1-FH2 domain plus connector, as well as core 
mDia1, mDia2, and mDia3 missing the connector, by cosedimentation and electron 
microscopy after negative staining. Analytical ultracentrifugation showed that 
core FH2 domains of mDia1 and mDia2 exhibited a low degree of dimer formation, 
whereas mDia3-FH2 minus connector and mDia1-FH2 plus connector readily 
dimerized. Only core mDia3-FH2 was able to nucleate actin polymerization. 
However, all tested core FH2 domains decorated and bundled F-actin, as 
demonstrated by electron microscopy after negative staining. Bundling activity 
was highest for mDia3-FH2, decreased for mDia2-FH2, and further decreased for 
mDia1-FH2. The mDia1-FH2 domain plus connector induced actin polymerization also 
in the absence of profilin, but failed to induce F-actin deformation and 
bundling. We also tested whether mDia1-FH2 was able to repolymerize actin in 
complex with different proteins that stabilize globular actin. The data obtained 
demonstrated that mDia1-FH2 induced actin repolymerization only from the 
actin/cofilin-1 complex, but not when complexed to actin depolymerizing factor, 
gelsolin segment 1, vitamin D binding protein, or deoxyribonuclease I.