icBTnC2, a new green calcium indicator based on troponin C and mBaoJin, with enhanced photostability

Genetically encoded green calcium indicators (GECIs) are broadly used for visualizing calcium transients in living cells. Among the diverse family of green GECIs, the Troponin C-based family offers potential advantages, including reduced calcium buffering, smaller molecular size, linear calcium response, and low cytotoxicity. However, the Troponin C-based GECIs with inverted calcium response are less developed compared to other popular GECIs, including GCaMPs and GECOs, and, as a consequence, have several drawbacks related to low dynamic range, brightness, photostability, and calcium ion sensitivity. To address these limitations, we developed a novel GECI, called icBTnC2, which incorporates Troponin C as a sensing moiety and the new bright photostable green FP mBaoJin as a reporting moiety. icBTnC2 demonstrated an inverted fluorescent response to calcium ion binding with a K_d of 62 nM. In terms of fluorescence contrast and calcium ion affinity in vitro, icBTnC2 was comparable to the best widely used calmodulin-based GECIs from the GCaMP family. icBTnC2 demonstrated superior photostability under wide-field fluorescence microscopy exhibiting 5.5-, 4.8-, 3.2-, 2.9-, and 1.3-fold higher photobleaching half-time compared to iYTnC2, mEGFP, NCaMP7, jGCaMP8f, and mBaoJin, respectively. The icBTnC2 indicator was benchmarked against other GECIs, such as jGCaMP8f, NCaMP7, iYTnC2, and R-GECO1, for visualization of calcium transients in mammalian cells and primary neuron cultures, and tested for calcium-dependent changes in fluorescence lifetime. Finally, we solved the crystal structure of the icBTnC2 indicator at 1.55 Å resolution in the calcium-bound state and, using directed mutagenesis, proposed the molecular basis of its fluorescent response to calcium ion binding.