The calcium/calmodulin-dependent protein phosphatase calcineurin is required for 
the induction of transcriptional events that initiate and promote myogenic 
differentiation. An important effector for calcineurin in striated muscle is the 
transcription factor myocyte enhancer factor 2 (MEF2). The targeting of the 
enzyme and substrate to specific intracellular compartments by scaffold proteins 
often confers specificity in phosphatase activity. We now show that the 
scaffolding protein mAKAP organizes a calcineurin/MEF2 signaling complex in 
myocytes, regulating gene transcription. A calcineurin/mAKAP/MEF2 complex can be 
isolated from C2C12 cells and cardiac myocytes, and the calcineurin/MEF2 
association is dependent on mAKAP expression. We have identified a peptide 
comprising the calcineurin binding domain in mAKAP that can disrupt the binding 
of the phosphatase to the scaffold in vivo. Dominant interference of 
calcineurin/mAKAP binding blunts the increase in MEF2 transcriptional activity 
seen during myoblast differentiation, as well as the expression of endogenous 
MEF2-target genes. Furthermore, disruption of calcineurin binding to mAKAP in 
cardiac myocytes inhibits adrenergic-induced cellular hypertrophy. Together 
these data illustrate the importance of calcineurin anchoring by the mAKAP 
scaffold for MEF2 regulation.