{Reference Type}: Journal Article {Title}: Turn-on protein switches for controlling actin binding in cells. {Author}: Effiong UM;Khairandish H;Ramirez-Velez I;Wang Y;Belardi B; {Journal}: Nat Commun {Volume}: 15 {Issue}: 1 {Year}: 2024 Jul 11 {Factor}: 17.694 {DOI}: 10.1038/s41467-024-49934-2 {Abstract}: Within a shared cytoplasm, filamentous actin (F-actin) plays numerous and critical roles across the cell body. Cells rely on actin-binding proteins (ABPs) to organize F-actin and to integrate its polymeric characteristics into diverse cellular processes. Yet, the multitude of ABPs that engage with and shape F-actin make studying a single ABP's influence on cellular activities a significant challenge. Moreover, without a means of manipulating actin-binding subcellularly, harnessing the F-actin cytoskeleton for synthetic biology purposes remains elusive. Here, we describe a suite of designed proteins, Controllable Actin-binding Switch Tools (CASTs), whose actin-binding behavior can be controlled with external stimuli. CASTs were developed that respond to different external inputs, providing options for turn-on kinetics and enabling orthogonality and multiplexing. Being genetically encoded, we show that CASTs can be inserted into native protein sequences to control F-actin association locally and engineered into structures to control cell and tissue shape and behavior.