GCaMP17 and its progeny11,16 consist of circularly permuted green fluorescent protein (cpGFP)25, the calcium-binding protein calmodulin (CaM), and CaM-interacting M13 peptide26 (Fig. 1a). The CaM/M13 complex is in proximity to the chromophore inside the cpGFP β barrel27. Calcium-dependent conformational changes in CaM/M13, including modulation of solvent access and the pKa of the chromophore, cause increased brightness with calcium binding. Despite extensive structure-guided optimization11,16, GCaMP and other protein sensors still suffer from low sensitivity and slow kinetics.
