Figure 2.

Red GECIs structure, function, and chromophore. (A) Crystal structure of K-GECO1 in the Ca²⁺-saturated state (PDB: 5UKG (21)) is shown. The FP is colored red, CaM is cyan, the CaM-binding peptide (ckkap in K-GECO1) is yellow, and the Ca²⁺ ions are purple. (B) (Adapted from (21)). Shown is the imaging of spontaneous Ca²⁺ oscillations in dissociated rat hippocampal neurons with K-GECO1. Graph shows the change in fluorescence normalized to baseline fluorescence over time in three different regions of interest. Inset: widefield image of K-GECO1 expression in neurons (Scale bars, 30 μm). (C) Shown is the molecular structure of the typical red FP chromophore in its neutral and anionic forms. (D) Absorption spectrum illustrates the presence of both forms of the chromophore in a red GECI. Shown is R-GECO1 in the Ca²⁺-free state.