A Split Transcriptional Repressor That Links Protein Solubility to an Orthogonal Genetic Circuit

Monitoring the aggregation of proteins within the cellular environment is key to investigating the molecular mechanisms underlying the formation of off-pathway protein assemblies associated with the development of disease and testing therapeutic strategies to prevent the accumulation of non-native conformations. It remains challenging, however, to couple protein aggregation events underlying the cellular pathogenesis of a disease to genetic circuits and monitor their progression in a quantitative fashion using synthetic biology tools. To link the aggregation propensity of a target protein to the expression of an easily detectable reporter, we investigated the use of a transcriptional AND gate system based on complementation of a split transcription factor. We first identified two-fragment tetracycline repressor (TetR) variants that can be regulated via ligand-dependent induction and demonstrated that split TetR variants can function as transcriptional AND gates in both bacteria and mammalian cells. We then adapted split TetR for use as an aggregation sensor. Protein aggregation was detected by monitoring complementation between a larger TetR fragment that serves as a "detector" and a smaller TetR fragment expressed as a fusion to an aggregation-prone protein that serves as a "sensor" of the target protein aggregation status. This split TetR represents a novel genetic component that can be used for a wide range of applications in bacterial as well as mammalian synthetic biology and a much needed cell-based sensor for monitoring a protein's conformational status in complex cellular environments.