Bipartite Genetically Encoded Biosensors to Sense Calcium Ion Dynamics at Membrane–Membrane Contact Sites

: Self-complementing bipartite fluorescent proteins (FPs) are useful tools for the detection of protein-protein proximity and for localizing fluorophores to membrane-membrane contact sites. Here, we report versions of circularly permuted green FP (GFP), red FP (RFP), and mNeonGreen (NG), which are split into a large fragment composed of nine β-strands and a small fragment composed of two β-strands. In each case, the large and small fragments can associate in live cells to form the complete 11-stranded FP β-barrel. We further converted each of these three self-complementing FPs into bipartite calcium ion (Ca2+) biosensors. We demonstrate that appropriately targeted versions of these split FPs, and split FP-based biosensors, can be functionally assembled at membrane-membrane contact sites. We employ the bipartite NG-based Ca2+ biosensor for visualization of pharmacologically induced Ca2+ release at mitochondria-endoplasmic reticulum contact sites (MERCs).