Figure 2.

Schematic representation of the three major GECI classes. a, Schematic of the single-FP GECI sensing mechanism. Upon calcium binding, conformational changes in the CaM/M13 complex induce fluorescence changes in the circularly permuted GFP (cpEGFP). b, The cameleon family of FRET-based GECIs. A calcium-dependent increase in FRET between a cyan fluorescent protein (CFP) and yellow fluorescent protein (YFP) FRET pair is coupled to the binding of calmodulin to the M13 peptide. c, Troponin C-based FRET GECIs. Binding of calcium to troponin C induces conformational changes and an increase in FRET between CFP and YFP. d, Imaging neural activity with GCaMP3 using a two-photon microscope in awake, behaving mice. GCaMP3 expression in layer 2/3 neurons of the primary motor cortex at 72 d postinjection (scale bar, 30 mm) and ΔF/F traces of individual cells (black lines). Relative treadmill movement indicated by red line. F, forward; B, backward; eCFP, enhanced CFP; ROI, region of interest. Adapted from Tian et al., 2009.