Chemical modulators are powerful tools to investigate biological processes. To identify circadian clock effectors we screened a natural product library in the model plant Arabidopsis thaliana. Two compounds, prieurianin (Pri) and prieurianin-acetate were identified as causing a shortened period of rhythms. Recently Pri was independently identified as a vesicle trafficking inhibitor and renamed endosidin 1 (ES1). Here we show that in vivo Pri primarily affects actin filament flexibility, which later results in reduced severing and filament depolymerization. This stabilization of the actin cytoskeleton subsequently causes changes in vesicle trafficking. Pri also affected microfilaments in mammalian cells, indicating that its target is highly conserved, however it failed to alter actin dynamics in vitro, suggesting that its activity requires the presence of actin-associated proteins. Furthermore, well-characterized actin inhibitors shortened period length of the Arabidopsis clock in a similar way to Pri, supporting the idea that Pri affects rhythms via altering the actin network. We conclude that actin associated processes influence the circadian system in a light-dependent manner, but their disruption does not abolish rhythmicity. In summary, we propose that the primary effect of Pri is to stabilize the actin cytoskeleton system thereby affecting endosome trafficking. Pri appears to stabilize actin filaments by a different mechanism from previously described inhibitors, and will be a useful tool to study actin-related cellular processes. © 2012 The Authors. The Plant Journal © 2012 Blackwell Publishing Ltd.
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