Figure 3.

C2A and C2B domains of Syt1 bind Cd²⁺ with higher affinity than Ca²⁺. (A) Displacement of bound Tb³⁺ from C2A by Ca²⁺ and Cd²⁺. C2A and Tb³⁺ concentrations are 15 and 240 µM, respectively. Inset: C2A–Tb³⁺ binding curve that has non-saturatable behavior due to comparable contributions of FRET and luminescence of free Tb³⁺ to the observed signal. (B) Displacement of bound Tb³⁺ from C2B by Ca²⁺ and Cd²⁺. C2B and Tb³⁺ concentrations are 15 and 65 µM, respectively. Inset: C2B–Tb³⁺ binding curve that shows saturatable behavior and produces K_d,Tb of 4.3±0.2 µM when fitted with a single-site binding model.