Quantitative subcellular localization of calmodulin-dependent phosphatase in chick forebrain.

Using a radioimmunoassay, we have measured the level of calmodulin-dependent phosphatase (calcineurin) in various subcellular fractions from chick forebrain. Our results revealed high levels of the enzyme in the cytoplasm and microsomes. A considerable amount was also observed in synaptosomes, where it was found exclusively in the synaptoplasm, comprising 0.32% of the total synaptoplasmic protein. Immunocytochemical localization of the phosphatase in isolated synaptosomes supported the biochemical finding. Phosphatase was not detected in nuclei, myelin, synaptic vesicles, and mitochondria. These results suggest that myelin basic protein and histone H1, widely used in biochemical characterization studies of the phosphatase, may not be physiological substrates, and that the cytoplasm, microsomes, and synaptoplasm may prove to be useful sources for the identification of physiological substrates.