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Originally published as MBC in Press, 10.1091/mbc.E08-12-1201 on February 25, 2009

Vol. 20, Issue 8, 2160-2173, April 15, 2009

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Role of Tropomyosin in Formin-mediated Contractile Ring Assembly in Fission Yeast

Colleen T. Skau*, Erin M. Neidt*, and David R. Kovar*,{dagger}

Departments of *Molecular Genetics and Cell Biology and {dagger}Biochemistry and Molecular Biology, The University of Chicago, Chicago, IL 60637

Submitted December 14, 2008; Revised February 6, 2009; Accepted February 18, 2009
Monitoring Editor: Fred Chang

Like animal cells, fission yeast divides by assembling actin filaments into a contractile ring. In addition to formin Cdc12p and profilin, the single tropomyosin isoform SpTm is required for contractile ring assembly. Cdc12p nucleates actin filaments and remains processively associated with the elongating barbed end while driving the addition of profilin-actin. SpTm is thought to stabilize mature filaments, but it is not known how SpTm localizes to the contractile ring and whether SpTm plays a direct role in Cdc12p-mediated actin polymerization. Using "bulk" and single actin filament assays, we discovered that Cdc12p can recruit SpTm to actin filaments and that SpTm has diverse effects on Cdc12p-mediated actin assembly. On its own, SpTm inhibits actin filament elongation and depolymerization. However, Cdc12p completely overcomes the combined inhibition of actin nucleation and barbed end elongation by profilin and SpTm. Furthermore, SpTm increases the length of Cdc12p-nucleated actin filaments by enhancing the elongation rate twofold and by allowing them to anneal end to end. In contrast, SpTm ultimately turns off Cdc12p-mediated elongation by "trapping" Cdc12p within annealed filaments or by dissociating Cdc12p from the barbed end. Therefore, SpTm makes multiple contributions to contractile ring assembly during and after actin polymerization.

This was published online ahead of print in MBC in Press (http://www.molbiolcell.org/cgi/doi/10.1091/mbc.E08-12-1201) on February 25, 2009.

Address correspondence to: David R. Kovar (drkovar{at}uchicago.edu)

Abbreviations used: FH, formin homology; SpTm, fission yeast tropomyosin Cdc8p; TIRF, total internal reflection fluorescence.




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