A HaloTag-Based Multicolor Fluorogenic Sensor Visualizes and Quantifies Proteome Stress in Live Cells Using Solvatochromic and Molecular Rotor-Based Fluorophores

Protein homeostasis, or proteostasis, is essential for cellular fitness and viability. Many environmental factors compromise proteostasis, induce global proteome stress, and cause diseases. The proteome stress sensor is a powerful tool for dissecting the mechanism of cellular stress and finding therapeutics that ameliorate these diseases. In this work, we present a multicolor HaloTag-based sensor (named AgHalo) to visualize and quantify proteome stresses in live cells. The current AgHalo sensor is equipped with three fluorogenic probes that turn on fluorescence when the sensor forms either soluble oligomers or insoluble aggregates upon exposure to stress conditions, both in vitro and in cellulo. In addition, AgHalo probes can be combined with commercially available always-fluorescent HaloTag ligands to enable two-color imaging, allowing for direct visualization of the AgHalo sensor both before and after cells are subjected to stress conditions. Finally, pulse–chase experiments can be performed to discern changes in the cellular proteome in live cells by first forming the AgHalo conjugate and then either applying or removing stress at any desired time point. In summary, the AgHalo sensor can be used to visualize and quantify proteome stress in live cells, a task that is difficult to accomplish using previous always-fluorescent methods. This sensor should be suited to evaluating cellular proteostasis under various exogenous stresses, including chemical toxins, drugs, and environmental factors.