FIGURE 2.

Ca²⁺ decreases FRET: effect of changes in intracellular Ca²⁺ on CFP (open circles) and YFP (shaded circles) emissions (top) and corresponding YFP/CFP ratio (bottom), from HeLa cells expressing YFP-CBD-CFP (right) or CFP + YFP (left) in the presence of the muscarinic receptor M1. Cytoplasmic Ca²⁺ was increased by activation of the muscarinic receptor M1 with the agonist carbachol. The YFP/CFP ratio of cells expressing YFP-CBD-CFP significantly decreased with the induced rise in intracellular Ca²⁺ concentration, whereas the ratio from cells expressing cytoplasmic CFP and YFP remained constant. Arrows indicate the addition of 10 μM carbachol to the external Tyrode's solution.