Inhibition of calcineurin phosphatase activity by a calcineurin B homologous protein

Calcineurin, a Ca2+/calmodulin-stimulated protein phosphatase, plays a key role in T-cell activation by regulating the activity of NFAT (nuclear facto r of activated (T) under bar cells), a family of transcription factors requ ired for the synthesis of several cytokine genes. Calcineurin is the target of the immunosuppressive drugs cyclosporin A and FK506 complexed with thei r cytoplasmic receptors cyclophilin and FKBP12, respectively. In this study we report that calcineurin is also the target of a recently identified Ca2 +-binding protein, CHP (for calcineurin homologous protein), which shares a high degree of homology with the regulatory B subunit of calcineurin and w ith calmodulin, In Jurkat and HeLa cells, overexpression of CHP specificall y impaired the nuclear translocation and transcriptional activity of NFAT b ut had no effect on AP-1 transcriptional activity and only a small (<25%) i nhibitory effect on the transcriptional activity of NF kappa B. Further stu dy indicated that CHP inhibits calcineurin activity, In cells overexpressin g CHP, the phosphatase activity of immunoprecipitated calcineurin was inhib ited by similar to 50%; and in a reconstituted assay, the activity of purif ied calcineurin was inhibited up to 97% by the addition of purified recombi nant CHP in a dose-dependent manner. Moreover, pro longed activation of Jur kat cells was associated with a decreased abundance of CHP, suggesting a po ssible regulatory mechanism allowing activation of calcineurin. CHP, theref ore, is a previously unrecognized endogenous inhibitor of calcineurin activ ity.