THE DOMAIN STRUCTURE OF CALCINEURIN

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This chapter discusses the domain structure of calcineurin. A calmodulin-stimulated protein phosphatase was first isolated from skeletal muscle and was also shown to be associated with calcineurin, the major calmodulin and Ca2+-binding protein of brain extracts. Calmodulin-stimulated phosphatases have been detected in many mammalian tissues. Most, like calcineurin, appear to be composed of two subunits, calcineurin A and calcineurin B, and cross react with polyclonal antibodies to bovine brain calcineurin. The antibodies also recognize calcineurin B-like proteins in several lower eukaryotes such as Paramecium tetraurelia, Drosophila melanogaster, and sea urchins but cross react more poorly with the large subunit in invertebrates and vertebrate tissues other than brain. Calmodulin-stimulated protein phosphatase has a wide tissue and species distribution, but its large subunit may exist as different isozymes. Brain, which contains 10–20 fold more of this enzyme than other tissues, is the richest source of the protein. The affinity of calcineurin for calmodulin is one of the highest reported for calmodulin stimulated enzymes. It is found that calmodulin does not compete with calcineurin B for the calcineurin B binding site on calcineurin A in reconstitution experiments.

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