Figure 9.

Mechanisms of metal ion sensing by genetically encoded and hybrid probes for Zn²⁺. (A) The Zap and Zif families consist of one or two Zn²⁺-finger domains between two FPs. Zn²⁺ binding induces a conformational change in the Zn²⁺-finger that leads to a change in FRET ratio. (B) The eCALWY family uses Zn²⁺ binding domains from Atox1 and WD4. The two FPs associate in the absence of Zn²⁺, but Zn²⁺ binding causes association of the binding domains and reduces the FRET efficiency. (C) The hybrid probe CA-FP has an FP linked to CA. When Zn²⁺ binds to CA, an exogenously added dapoxyl sulfonamide (blue hexagon) can bind to an open site on the Zn²⁺ ion, leading to a FRET response between the small-molecule fluorophore and the FP.