Figure 5. Comparison of fast GCaMP probes (GCaMP3_fast and GCaMP6f_u) and their parent variants (GCaMP3 and GCaMP6f).

(a) Position of the mutations that gave rise to GCaMP3_fast and GCaMP6f_u variants, relative to GCaMP3. (b) Crystal structure of monomeric GCaMP6m in a Ca²⁺-bound form with cpEGFP (green), Ca²⁺ ions (yellow), CaM (blue) and the RS20 peptide (light brown) (adapted from Ding et al.,²⁷ PDB 3WLD). The amino acid residues highlighted in red are those that generated GCaMP6f_u relative to GCaMP3. Schematic representation of (c) Ca²⁺-bound GCaMP3/GCaMP6f showing “open” conformation of EF-hand 3 (helices V and VI); (d) Ca²⁺-bound GCaMP3_fast/GCaMP6f_u showing “closed” conformation of disabled EF-hand 3 (helices V and VI).