Calcineurin: A Member of a Family of Calmodulin-Stimulated Protein Phosphatases

Abstract

Abstract

Calcineurin, a major calmodulin-binding protein of brain, is a heterodimer composed of a 61,000 M _r calmodulin-binding subunit, calcineurin A, and a 19,000 M _r Ca²⁺-binding subunit, calcineurin B. The discovery of a calmodulin-regulated protein phosphatase in rabbit skeletal muscle with a similar subunit structure led to the identification of calcineurin as a protein phosphatase (AA Stewart, TS Ingebritsen, A Manalan, CB Klee, P Cohen (1982) FEBS Lett 137:80–84). Using rabbit polyclonal antibodies to bovine brain calcineurin, both subunits of calcineurin can be identified in crude homogenates of bovine brain by an immunoblotting technique. In crude homogenates of bovine skeletal and cardiac muscle, a 59,000–61,000 M _r doublet and a 15,000 M _r species (the electrophoretic mobility of calcineurin B) are also detected by this technique. The cross-reactivity of these species with antibodies to brain calcineurin indicates antigenic similarity between the muscle proteins and calcineurin, and suggests the existence of a family of structurally related calmodulin-stimulated protein phosphatases. Like calcineurin, the 61,000 M _r subunits in skeletal and cardiac muscle bind calmodulin and are detected in crude tissue extracts by ¹²⁵I-calmodulin gel overlay. Thus, both the ¹²⁵I-calmodulin gel overlay method and the immunoblotting technique are useful in screening crude preparations, in which detection of calmodulin-stimulated protein phosphatase activity may be complicated by the many phosphatases present.