Comparison of calmodulin binding to brain synaptic and coated vesicles.

Several characteristics of calmodulin association with brain synaptic and coated vesicles wer analyzed and compared. Radioimmunoassay revealed that both classes of vesicles contain approx. 1 microgram of calmodulin per mg of vesicle protein. Discontinuous sucrose gradients revealed that coated and synaptic vesicles preparations were homogeneous and had different sedimentation properties. Binding of 125I-labeled calmodulin to sympatic and coated vesicles was Ca2+ dependent and displaced by unlabeled calmodulin but not by troponin-C. Scatchard analysis revealed the presence of two binding sites. In both vesicle types there was one high-affinity, low-binding-capacity site (Kd = 1-39 nM and Bmax = 4-16 pmol/mg) and one low-affinity, high-binding-capacity site (Kd = 102-177 nM and Bmax = 151-202 pmol/mg). (Ca2+ +Mg2+)-ATPase activity was stimulated in both synaptic and coated vesicles by calmodulin. Thus synaptic and coated vesicles may possess similar calmodulin binding sites.