Development of a signal-integrating reporter to monitor mitochondria-ER contacts.

Mitochondria-ER contact sites (MERCS) serve as hotspots for important cellular processes, including calcium homeostasis, phospholipid homeostasis, mitochondria dynamics, and mitochondrial quality control. MERCS reporters based on complementation of GFP fragments have been designed to visualize MERCS in real-time, but we find that they do not accurately respond to changes in MERCS content. Here, we utilize split LacZ complementing fragments to develop the first MERCS reporter system (termed SpLacZ-MERCS) that continuously integrates the MERCS information within a cell and generates a fluorescent output. Our system exhibits good organelle targeting, no artifactual tethering, and effective, dynamic tracking of the MERCS level in single cells. The SpLacZ-MERCS reporter was validated by drug treatments and genetic perturbations known to affect mitochondria-ER contacts. The signal-integrating nature of SpLacZ-MERCS may enable systematic identification of genes and drugs that regulate mitochondria-ER interactions. Our successful application of the split LacZ complementation strategy to study MERCS may be extended to study other forms of inter-organellar crosstalk.