Calcineurin, also called Protein Phosphatase 2B, PP2B, PPP2B, Protein Phosphatase 3, and PPP3, is an enzyme that dephosphorylates serine and threonine residues in proteins. It is a heterodimer of a 59,000 dalton catalytic A subunit and a 1,900 dalton regulatory B subunit that is activated by the binding of calcium ions and calmodulin. Calcineurin is expressed in many tissues, but its levels are highest in the brain, where it may play a role in learning and memory. It has many substrates, including NFAT, a transcription factor that is activated by dephosphorylation. Complexes of the immunosuppressants cyclosporin and FK506 with immunophilin proteins such as cyclophilin and FKBP12 are potent and specific inhibitors of calcineurin activity. Alterations in calcineurin activity are suspected to play a role in cardiac hypertrophy and graft versus host disease in organ transplantation.
Calcineurin A, also known as PP2B and PPP3CA, is the 59 kDa catalytic subunit of the calcium/calmodulin-dependent protein phosphatase. When activated by calcium in the presence of the regulatory B subunit and calmodulin, Calcineurin A selectively removes phosphates from serine and threonine residues on target proteins. Although ubiquitously expressed, Calcineurin levels are highest in brain. Calcineurin function in brain is thought to play a role in the formation and retention of memories. Calcineurin activity is inhibited by the immunosuppressant drug Cyclosporine A bound to cyclophilins.