Surprisingly, we found that the major yeast tropomyosin, Tpm1, has little protective effect against cofilin in a variety of assays, however, a cofilin mutant unable to bind Tpm1 showed diminished ability to sever Tpm1-bound F-actin. variety of assays, we show that yeast cofilin can efficiently depolymerize and sever yeast actin filaments decorated with either Tpm1 or mouse tropomyosins TM1 and TM4. Our results suggest that yeast cofilin has the intrinsic ability to promote actin cable turnover, and that the severing activity may rely on Amiloride hydrochloride dihydrate its ability to bind Tpm1. == Introduction == The ability to assemble distinct and dynamic actin structures in the same cell is critical for the diverse role of actin in cell motility and morphogenesis[1]. In yeast cells that undergo polarized growth, actin cables and patches are dynamically assembled near growth sites and are required for two different cellular Amiloride hydrochloride dihydrate functions: polarized transport and endocytosis, respectively[2]. Actin filaments (F-actin) in cables are nucleated by formins and decorated by tropomyosins, whereas those Amiloride hydrochloride dihydrate in patches are nucleated by the Arp2/3 complex and decorated with cofilin and other patch specific markers[3]. Known cable components, Tpm1, Tpm2, Sac6, and Abp140, stabilize and/or bundle actin filamentsin vitroandin vivo[6][11]. The rapid turnover of actin patches has been shown to depend on cofilin[4], and a recent study found that although cofilin was not detected along actin cables under normal conditions, rapid turnover of actin cables also requires cofilin[5]. Cofilin is a member of the ADF/cofilin family of low molecular weight actin-binding proteins. These proteins can bind to actin monomers (G-actin) and filaments and stimulate depolymerization and fragmentation of F-actin[12].Every eukaryotic cell type examined to-date contains at least one member of the ADF/cofilin family, suggesting that cofilin performs a universally important function. Null mutations of cofilin inSaccharomyces cerevisiae, Drosophila melanogasterandCaenorhabditis elegansare all lethal. Cofilin-mediated Amiloride hydrochloride dihydrate F-actin severing and turnover are thought to be critical for rapid remodeling of actin organization in response to external stimuli, and for recycling of G-actin to sustain actin polymerization during cellular or intra cellular motility processes[13]. Tropomyosin has long been known to protect actin filaments from the depolymerizing and severing actions of DNaseI[14], ADF/cofilin[15],[16]and gelsolin[17]. Tropomyosin is a long protein with two -helical polypeptides that form a parallel coiled coil, which further associate through head-to-tail interactions to form a long polymer along the side of an actin filament[18]. Whereas yeast has two tropomyosins, Tpm1 and Tpm2, vertebrates have 17 tropomyosin isoforms. In general, tropomyosin inhibits the dissociation of actin subunits from the pointed end of an actin filament[19],[20], which makes actin filament stronger and less likely to bend or break[21],[22]. Tropomyosin may also be Amiloride hydrochloride dihydrate a critical factor governing the spatial segregation of Arp2/3- and formin-nucleated actin structures in the cell. Whereas tropomyosin prevents branching nucleation by Arp2/3 complex, possibly due to their overlapping binding sites on F-actin[23], a recent study also suggests that tropomyosin actively facilitates assembly of actin filaments by formin protein[24]. In order to uncover the mechanism governing the disassembly of tropomyosin-bound actin filaments, we used a proteomics approach to identify tropomyosin-binding proteins and found cofilin to be a highly ranked tropomyosin binder. This led us to test if yeast cofilin can directly sever tropomyosin-decorated F-actin. Surprisingly, we found that the major yeast tropomyosin, Tpm1, has little protective effect against cofilin in a variety of assays, however, a cofilin mutant unable to bind Tpm1 showed diminished ability to sever Tpm1-bound F-actin. These results suggest that yeast cofilin is intrinsically capable of promoting the turnover of tropomyosin-bound F-actin in yeast. == Results == Because previous studies suggest that tropomyosin inhibits actin depolymerization by cofilin[15], we hypothesized that additional proteins may be Mouse monoclonal to CD106(FITC) required to facilitate cable turnover, possibly through regulation of tropomyosin binding to actin, and set out to identify such proteins through Tpm1 affinity chromatography. For this and subsequent experiments, we used recombinant.