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

Jenna R Christensen,Rachel R Brown,Alyssa J Harker,David R Kovar,Kaitlin E Homa,Meghan E O'Connell,Alisha N Morganthaler,Cristian Suarez

doi:10.7554/elife.47279

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.

Highlights

As in many cell types, the unicellular fission yeast assembles diverse actin filament (F-actin) networks within a crowded cytoplasm to facilitate different cellular functions such as cytokinesis, endocytosis and polarization
We previously found that fimbrin Fim1 and ADF/cofilin Adf1 synergize to displace tropomyosin Cdc8 from F-actin (Christensen et al, 2017), which helps explain why Fim1 is highly concentrated on actin patches, whereas Cdc8 is not (Skau and Kovar, 2010)
As Fim1 is highly concentrated in actin patches, we speculated that depletion of actin patches by the Arp2/3 complex inhibitor CK-666 (Burke et al, 2014; Nolen et al, 2009) would result in a rapid increase of free Fim1 in the cytoplasm that might subsequently allow Fim1 to outcompete its contractile ring actin binding proteins (ABPs) competitors

Summary

Introduction

As in many cell types, the unicellular fission yeast assembles diverse actin filament (F-actin) networks within a crowded cytoplasm to facilitate different cellular functions such as cytokinesis (contractile ring), endocytosis (actin patches) and polarization (actin cables) These F-actin networks each possess a defined set of actin-binding proteins (ABPs) that regulate the formation, organization, and dynamics of the actin filaments within the network. Synergistic activities between Fim and Adf rapidly displace Cdc from F-actin networks such as actin patches (Christensen et al, 2017; Skau and Kovar, 2010) These interactions do not explain how Fim is prevented from strongly associating with other F-actin networks such as the contractile ring.

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Conclusion