Actin-polymerization-driven molecular dynamics of Formin homology proteins in live cells

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In the previous research, we have discovered processive actin capping movement of mDia1 (Science 303:2007-10. 2004). mDia1 was previously identified by myself in collaboration with Dr.Shuh Narumiya in Kyoto University as an effecter of a GTPase, Rho. The current research project aimed to further extend our single-molecule imaging method to a native form of mDia1 and related molecules in order to elucidate the physiological function and regulation of Formin homology proteins, both spatially and temporally, in living cells.mDia1 belongs to the formin family proteins (formins) that share proline-rich formin homology 1 (FH1) and formin homology 2 (FH2) domains. Many actin-based cellular structures such as yeast actin cables, cytokinetic cleavage furrows and actin bundles in mammalian cells are dependent on formins. Recent studies characterized that FH2 or FH1-FH2 domains nucleate actin filaments and they processively remain associated to the growing barbed-end of filaments. However, the physiological regulation of formin-mediated actin filament formation, both temporally and spatially within the cell, is still unknown.In this project, by using single-molecule live-cell imaging, we have found that an increase in the actin monomer pool induced by actin monomer sequestering drugs rapidly activated mDia1 to initiate fast directional movement. The expression of nonpolymerizable actins was sufficient to induce frequent activation of mDia1. Rho activity was required for activation of mDia1, but the FH2 region alone can be activated by latrunculin B. These findings reveal that transient accumulation of G-actin works as a cue to activate mDia1 to execute rapid assembly of actin filaments. The current research project thus discovered that cells possess a novel acute actin polymer restoration mechanism involving mDia1 (submitted).