Fimbrin and Tropomyosin Competition Regulates Endocytosis and Cytokinesis Kinetics in Fission Yeast

Background: Tropomyosin is an important actin filament-stabilizing protein that controls the access of other essential proteins to filaments, including myosin motors, Arp2/3 complex, formin, and cofilin. It is therefore critical to establish mechanisms for regulating the actin filament binding of tropomyosin. We examined how the actin filament crosslinking protein fimbrin Fim1p and tropomyosin Cdc8p affect each other's ability to bind filaments, localize to particular cellular structures, and regulate filament severing by cofilin Adf1p in fission yeast Schizosaccharomyces pombe. Results: We discovered a novel mechanism for regulating actin filament dynamics in fission yeast. Fim1p inhibits Cdc8p binding to actin filaments in vitro, which permits Adf1p-mediated severing in the presence of Cdc8p. In cells, the balance between Fim1p and Cdc8p is important for both endocytic actin patch kinetics and contractile ring assembly during cytokinesis. High Fim1p concentrations prevent Cdc8p from associating with actin patches, allowing rapid patch turnover and motility. In the absence of Fim1p, ectopic localization of Cdc8p to actin patches increases patch lifetime while decreasing patch motility. Fim1p and Cdc8p also play antagonistic roles during cytokinesis, in which the deletion of Fim1p rescues the contractile ring assembly defects caused by mutation of Cdc8p. Conclusion: Fimbrin Fim1p dissociates tropomyosin Cdc8p from actin filaments, permitting cofilin Adf1p-mediated severing. Therefore, we propose that in addition to actin filament crosslinking, Fim1p has a novel role as a positive actin-binding selector protein that promotes the access of other proteins to actin filaments by inhibiting Cdc8p.