THRUMIN1 Is a Light-Regulated Actin-Bundling Protein Involved in Chloroplast Motility

Chloroplast movement in response to changing light conditions optimizes photosynthetic light absorption [1]. This repositioning is stimulated by blue light perceived via the phototropin photoreceptors [2-4] and is transduced to the actin cytoskeleton [5]. Some actin-based motility systems use filament reorganizations rather than myosin-based translocations [6]. Recent research favors the hypothesis that chloroplast movement is driven by actin reorganization at the plasma membrane [7, 8], but no proteins affecting chloroplast movements have been shown to associate with both the plasma membrane and actin filaments in vivo. Here we identified THRUMIN1 as, a critical link between phototropin photoreceptor activity at the plasma membrane and actin-dependent chloroplast movements. THRUMIN1 bundles filamentous actin in vitro, and it localizes to the plasma membrane and displays light- and phototropin-dependent localization to microfilaments in vivo. These results suggest that phototropin-induced actin bundling via THRUMIN1 is important for chloroplast movement. A mammalian homolog of THRUMIN1, GRXCR1, has been implicated in auditory responses and hair cell stereocilia development as a regulator of actin architecture [9, 10]. Studies of THRUMIN1 will help elucidate the function of this family of eukaryotic proteins.