Interactions of calmodulin with coated vesicles from brain.

Coated vesicles purified in the presence of calcium are enriched approximately 7-fold in calmodulin content relative to standard preparations isolated in the absence of free calcium. Radioiodinated calmodulin binds specifically to coated vesicles in vitro. Binding is saturable (Kd, 10 nM) and calcium dependent. Half-maximal binding occurs at 2.4 microM free Ca2+ whereas up to 1.2 mM Mg2+ has no effect on binding. Troponin C, a protein homologous to calmodulin, competes with binding of 125I-labeled calmodulin with 1/30th the affinity of native calmodulin. Chromatography of 2 M urea-solubilized coated vesicles on a calmodulin-Sepharose column demonstrated a Ca2+-dependent interaction of coated vesicle proteins and calmodulin. The properties of calmodulin binding to coated vesicles are comparable to those of calmodulin activities in other systems.