Modulating Protein Stability to Switch Toxic Protein Function On and Off in Living Cells

Toxic proteins are prime targets for molecular farming (the generation of pharmacologically active or biotechnologically usable compounds in plants) and are also efficient tools for targeted cell ablation in genetics, developmental biology, and biotechnology. However, achieving conditional activity of cytotoxins and maintaining the toxin-expressing plants as stably transformed lines remain challenging. Here, we produce a switchable version of the highly cytotoxic bacterial RNase barnase by fusing the protein to a portable protein degradation cassette, the low-temperature degron cassette. This method allows conditional genetics based on conditional protein degradation via the N-end rule or N-degron pathway and has been used to vice versa accumulate and/or deplete a diverse variety of highly active, unstable or stable target proteins in different living multicellular organisms and cell systems. Moreover, we expressed the barnase fusion under control of the trichome-specific TRIPTYCHON promoter. This enabled efficient temperature-dependent control of protein accumulation in Arabidopsis (Arabidopsis thaliana) leaf hairs (trichomes). By tuning the levels of the protein, we were able to control the fate of trichomes in vivo. The on-demand formation of trichomes through manipulating the balance between stabilization and destabilization of barnase provides proof of concept for a robust and powerful tool for conditional switchable cell arrest. We present this tool as a potential strategy for the manufacture and accumulation of cytotoxic proteins and toxic high-value products in plants or for conditional genetic cell ablation.