A molecular rotor, 9-(dicyanovinyl)julolidine (DCVJ), is a fluorescent dye whose intramolecular rotation determines its fluorescence yield [Kung, C.E. & Reed, J.K. (1989) Biochemistry 28, 6678-6686]. DCVJ binds to bovine brain calmodulin and emits strong fluorescence. In fluorescence titration experiments, the dissociation constant and the number of binding sites were determined to be 20 +/- 10 microM and 0.7 +/- 0.5 in the presence of Ca2+, and 22 +/- 10 microM and 0.6 +/- 0.5 in the absence of Ca2+, respectively. The fluorescence intensity of bound DCVJ increased 10-fold in the presence of Ca2+ compared to that in the absence of Ca2+. The fluorescence titration curve of DCVJ-calmodulin showed a transition at pCa 6.5. Over the same Ca2+ range, a decrease in molecular ellipticity at 222 nm and an increase in tyrosine fluorescence of calmodulin were observed. These results mean that the conformational change of calmodulin due to the Ca2+ binding induces the microenvironmental change of the DCVJ binding site from the flexible to the rigid state, resulting in inhibition of the intramolecular rotation of DCVJ and an increase in its fluorescence.
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