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. 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

Kovar, D.R., Harris, E.S., Mahaffy, R., Higgs, H.N. and T.D. Pollard. 2006. Control of the assembly of ATP- and ADP-actin by formins and profilin. Cell. 124, 423-435. (PubMed)

Kovar, D.R. Molecular details of formin-mediated actin assembly. 2006. Curr. Opin. Cell Biol. 18, 11-17. (PubMed)

Kovar, D.R., Wu, J.-Q., and T.D. Pollard. 2005. Profilin-mediated competition between capping protein and formin Cdc12p during cytokinesis in fission yeast. Mol. Biol. Cell. 16, 2313-2324. (PubMed)

Kovar, D.R. and T.D. Pollard. Insertional assembly of actin in association with formins produces piconewton forces. 2004. Proc. Natl. Acad. Sci. USA. 41, 14725-14730. (PubMed)

Kovar, D.R., Kuhn, J.R., Tichy, A.L., and T.D. Pollard. 2003. The fission yeast cytokinesis formin Cdc12p is a barbed end actin filament capping protein gated by profilin. J. Cell Biol. 161, 875-887. (PubMed)

Wu, J.-Q., Kuhn, J.R., Kovar, D.R., and T.D. Pollard. 2003. Spatial and temporal pathway for assembly and constriction of the contractile ring in fission yeast cytokinesis. Devel. Cell. 5, 723-734. (PubMed)