Cooperation between tropomyosin and α-actinin inhibits fimbrin association with actin filament networks in fission yeast

Abstract

We previously discovered that competition between fission yeast actin binding proteins (ABPs) for binding F-actin facilitates their sorting to different cellular networks. Specifically, competition between endocytic actin patch ABPs fimbrin Fim1 and cofilin Adf1 enhances their activities, and prevents tropomyosin Cdc8’s association with actin patches. However, these interactions do not explain how Fim1 is prevented from associating strongly with other F-actin networks such as the contractile ring. Here, we identified α-actinin Ain1, a contractile ring ABP, as another Fim1 competitor. Fim1 competes with Ain1 for association with F-actin, which is dependent upon their F-actin residence time. While Fim1 outcompetes both Ain1 and Cdc8 individually, Cdc8 enhances the F-actin bundling activity of Ain1, allowing Ain1 to generate F-actin bundles that Cdc8 can bind in the presence of Fim1. Therefore, the combination of contractile ring ABPs Ain1 and Cdc8 is capable of inhibiting Fim1’s association with F-actin networks.

Genetically inspired in vitro reconstitution ofSaccharomyces cerevisiaeactin cables from seven purified proteins

“…How then is tropomyosin specifically targeted to cables? Recent studies from the Kovar lab show that tropomyosin and fimbrin directly compete for binding to F-actin, which leads to their sorting to cables and patches, respectively (Skau and Kovar, 2010;Christensen et al, 2017Christensen et al, , 2019. Further, our results suggest that tropomyosin decoration allows the assembly of long, linear actin structures in a cytosolic environment where robust disassembly by cofilin and AIP1 ensures the rapid turnover of the more dense, branched networks composed of shorter actin filaments.…”

The many implications of actin filament helicity

“…Tropomyosins constitute a large family of coiled-coil proteins with as many as 40 isoforms in mammals. Today, tropomyosins appear to play a central role in regulating the binding of many other ABPs to actin filaments [103] and to drive actin network differentiation in cells [104]. Tropomyosins bind to the sides of actin filaments by spanning over several subunits (up to half a pitch per protein for the longest tropomyosin isoforms) and closely follow the filament strands.…”

Plastin 3 in X-Linked Osteoporosis: Imbalance of Ca2+-Dependent Regulation Is Equivalent to Protein Loss

“…Interestingly, plastin and myosin cooperate in cytokinesis (Leite et al, 2020) and epidermal morphogenesis/basement membrane assembly (Dor-On et al, 2017), while demonstrating mutually exclusive subcellular localization (Garbett et al, 2020), which can result from direct competition between myosin and plastin for binding sites on actin (Behrmann et al, 2012;Schwebach et al, 2020). Additional explanation for this antagonism is competition with tropomyosin, which has been demonstrated for yeast fimbrin (Christensen et al, 2019), but not yet for mammalian plastin and tropomyosin isoforms.…”

“…A role of plastins in cytokinesis has also been revealed but appears to be less universal. In Caenorhabditis elegans zygotes, plastin increases cortical contractility and helps to stabilize myosin at the equatorial cortex (Ding et al, 2017 ; Leite et al, 2020 ), whereas the role of the fission yeast ortholog fimbrin in cytokinesis is controversial (Laporte et al, 2012 ; Christensen et al, 2019 ). Interestingly, plastin and myosin cooperate in cytokinesis (Leite et al, 2020 ) and epidermal morphogenesis/basement membrane assembly (Dor-On et al, 2017 ), while demonstrating mutually exclusive subcellular localization (Garbett et al, 2020 ), which can result from direct competition between myosin and plastin for binding sites on actin (Behrmann et al, 2012 ; Schwebach et al, 2020 ).…”