A light-switchable protein interaction system based on the phytochrome signaling network of Arabidopsis thaliana has been developed for precise spatiotemporal control of cellular processes. This system uses a reversible interaction between PhyB and PIF3, which can be toggled by red and infrared light. The interaction allows for the controlled translocation of target proteins to the plasma membrane with high spatial and temporal resolution. This system was optimized for use in mammalian cells and demonstrated the ability to precisely reshape and direct cell morphology by controlling the activity of Rho-family GTPases, which regulate the actin cytoskeleton. The Phy-PIF interaction was tested in various applications, including the controlled recruitment of fluorescent proteins to the membrane, the spatial control of membrane recruitment using patterned light, and the activation of signaling pathways through the translocation of GEFs. The system showed robust performance, with rapid and reversible translocation of proteins, and was able to generate precise morphological changes in mammalian cells. The system also enabled the measurement of GTPase activation using fluorescent biosensors, demonstrating the ability to monitor signaling events in real time. The Phy-PIF system offers a versatile tool for controlling a wide range of cellular processes, as it can be used to regulate diverse functions through engineered fusion proteins. It is compatible with most eukaryotic cells and can be adapted for use in genetically manipulable cells by incorporating enzymes that generate the chromophore PCB from heme or biliverdin. The system's high spatial and temporal resolution makes it a valuable tool for studying cellular processes and for developing new methods of perturbative, quantitative experiments in cell biology.A light-switchable protein interaction system based on the phytochrome signaling network of Arabidopsis thaliana has been developed for precise spatiotemporal control of cellular processes. This system uses a reversible interaction between PhyB and PIF3, which can be toggled by red and infrared light. The interaction allows for the controlled translocation of target proteins to the plasma membrane with high spatial and temporal resolution. This system was optimized for use in mammalian cells and demonstrated the ability to precisely reshape and direct cell morphology by controlling the activity of Rho-family GTPases, which regulate the actin cytoskeleton. The Phy-PIF interaction was tested in various applications, including the controlled recruitment of fluorescent proteins to the membrane, the spatial control of membrane recruitment using patterned light, and the activation of signaling pathways through the translocation of GEFs. The system showed robust performance, with rapid and reversible translocation of proteins, and was able to generate precise morphological changes in mammalian cells. The system also enabled the measurement of GTPase activation using fluorescent biosensors, demonstrating the ability to monitor signaling events in real time. The Phy-PIF system offers a versatile tool for controlling a wide range of cellular processes, as it can be used to regulate diverse functions through engineered fusion proteins. It is compatible with most eukaryotic cells and can be adapted for use in genetically manipulable cells by incorporating enzymes that generate the chromophore PCB from heme or biliverdin. The system's high spatial and temporal resolution makes it a valuable tool for studying cellular processes and for developing new methods of perturbative, quantitative experiments in cell biology.