The Formin Inhibitor, SMIFH2, Inhibits Members of the Myosin Superfamily.

The small molecular inhibitor of formin FH2 domains, SMIFH2, is widely used in cell biological studies. It inhibits formin-driven actin polymerization in vitro, but not polymerization of pure actin. It is active against several types of formins from different species (Rizvi et al., 2009). Here, we found that SMIFH2 inhibits retrograde flow of myosin 2 filaments and contraction of stress fibers. We further checked the effect of SMIFH2 on non-muscle myosin 2A and skeletal muscle myosin 2 in vitro and found that SMIFH2 inhibits myosin ATPase activity and ability to translocate actin filaments in the in vitro motility assay. The inhibition of non-muscle myosin 2A in vitro required a higher concentration of SMIFH2 than for the inhibition of retrograde flow and stress fiber contraction in cells. We also found that SMIFH2 inhibits several other non-muscle myosin types, e.g. mammalian myosin 10, Drosophila myosin 7a and Drosophila myosin 5, more efficient than inhibition of formins. These off-target inhibitions demand additional careful analysis in each case when solely SMIFH2 is used to probe formin functions.