The Faculty


Mechanisms of actin assembly: Real-time observation of recapitulated cytoskeleton structures by single filament microscopy


B.A., Ohio Wesleyan University, 1995
Ph.D., Purdue University, 2001

BPHYS Student

Tobias Falzone, PhD

Research Summary

The focus of our research group is to determine the fundamental biochemical mechanisms that govern how cells coordinate actin filament assembly for a wide range of tasks (Figure 1). Actin filament assembly is required for processes that are necessary for proper development such as division, establishing polarity and migration, yet when unregulated can lead to devastating defects.

Mechanistically diverse nucleating proteins are required to initiate actin polymerization at the right time and place. The actin-related protein Arp2/3 complex initiates the assembly of short-branched filaments that are ideal for pushing on cell membranes and lipids (Figure 1). Recently the paradigm that the Arp2/3 complex is the principle actin filament nucleator in cells was radically shifted. Formins were identified as a second major nucleating factor that participate in a growing list of cellular processes that require long-straight bundled actin filaments including division, polarization and motility. However, many of the molecular details of how formins mediate actin filament assembly remain unclear, and it is not understood how cells separately utilize the Arp2/3 complex and multiple formin isoforms to accomplish diverse fundamental tasks (Figure 1).

Elucidating the mechanisms of complex actin-based processes requires the rigorous characterization of the participating proteins both in and out of live cells. Therefore we are combining in vitro approaches with the genetically tractable fission yeast to utilize diverse techniques from genetics, live cell fluorescence microscopy, biophysics, biochemistry, and innovative single actin filament imaging assays. Utilizing fission yeast allows rapid progress, and given the evolutionary conservation between the participating set of actin-binding proteins in fission yeast and mammals, our studies should pertain directly to healthy and diseased mammalian cells (Figure 1).

Figure 1

Figure 1. The Arp2/3 complex and Formin drive actin filament assembly for diverse cellular processes in both animal and fission yeast cells. (A) Extra- and intra-cellular signals activate nucleating proteins that rapidly assemble actin filaments. In animal cells Formins assemble actin filaments for division (Formin 1) and filopodia in migrating cells (Formin 2). The Arp2/3 complex makes filaments for the lamellipodia of migrating cells. (B) In fission yeast Formins nucleate actin filament assembly for division (Cdc12p) and polarized growth (For3p), whereas the Arp2/3 complex is required for motile endocytic actin patches.

Selected Publications

Falzone, T.T., Lenz, M., Kovar. D.R., and M.L. Gardel. 2012. Assembly kinetics determine the architecture of a-actinin crosslinked F-actin networks. Nat Commun. 3, 861. (PubMed)

Scott, B.J., Neidt, E.N., and D.R. Kovar. 2011. The functionally distinct fission yeast formin isoforms have specific actin assembly properties. Mol Biol Cell. 22, 3826-39. (PubMed)

Skau, C.T., Courson, D.S., Bestul, A.J., Winkelman, J.D., Rock, R.S., Sirotkin, V., and D.R. Kovar. 2011. Actin filament bundling by fimbrin is important for endocytosis, cytokinesis and polarization in fission yeast. J Biol Chem. 286, 26964-77. (PubMed)

Kovar, D.R., Sirotkin, V., and M. Lord. 2011. Three’s company: the fission yeast actin cytoskeleton. Trends Cell Biol. 21, 177-87. (PubMed)

Skau, C.T., and D.R. Kovar. 2010. Fimbrin and tropomyosin competition regulates endocytosis and cytokinesis kinetics in fission yeast. Curr Biol. 20, 1415-22. (PubMed)

Haglund, C.M., Choe, J.E., Skau, C.T., Kovar, D.R., and M.D. Welch. 2010. RickettsiaSca2 is a bacterial formin-like mediator of actin-based motility. Nat Cell Biol. 12, 1057-63. (PubMed)

Rizvi, S.A., Neidt, E.M., Cui, J. Feiger, Z., Skau, C.T., Gardel, M.L., Kozmin, S.A., and D.R. Kovar. 2009. Identification and characterization of a small molecule inhibitor of formin-mediated actin assembly. Chem Biol. 16, 1158-68. (PubMed)

Skau, C.T., Neidt, E.M., and D.R. Kovar. 2009. Role of tropomyosin in formin-mediated contractile ring assembly in fission yeast. Mol. Biol. Cell. 20, 2160-73. (PubMed)

Neidt, E.M., Scott, B.j., and D.R. Kovar. 2009. Formin differentially utilizes profilin isoforms to rapidly assemble actin filaments. J. Biol. Chem. 284, 673-84. (PubMed)

Neidt, E.M., Skau, c.T., and D.R. Kovar. 2008. The cytokinesis formins from the nematode worm and fission yeast differentially mediate actin filament assembly. J. Biol. Chem. 283, 23872-83. (PubMed)