Figure 4.

Further optimization of genetically encoded hormone biosensors. A model of a ratiometric direct biosensor. The sensory domain is linked to an environmentally sensitive circularly permutated fluorescent protein (cpFP) that carries an inserted reference FP (like a Matryoshka or nested doll) (Ast et al., 2017). Ligand binding brings the reporter FP components back together and increases the fluorescence. A green–orange (GO) Matryoshka cassette composed of cpsfGFP and LSSmOrange can be used for engineering ratiometric sensors that make use of the large change in intensity caused by analyte binding in the cpFP.