Regulation of calcineurin phosphatase activity and interaction with the FK-506.FK-506 binding protein complex

The immunosuppressant FK-506 (tacrolimus) forms a complex with a ubiquitous intracellular receptor, FK-506 binding protein (FKBP12), and this complex inhibits the heterodimeric Ca2+/calmodulin-dependent phosphatase, calcineurin, an essential component of the T-cell receptor signal transduction pathway. Using a series of truncated calcineurin catalytic subunits, we show here that a region within the catalytic subunit that regulates phosphatase activity, the autoinhibitory domain, also regulates the Ca(2+)-dependent interaction of calcineurin with the FK-506.FKBP12 complex. Deletion of this domain produces constitutive activation of the phosphatase as demonstrated by transient transfection experiments in which expression of the truncated protein permitted Ca(2+)-independent induction of interleukin-2 transcription. Thus, deletion of the autoinhibitory domain is necessary and sufficient to constitutively activate calcineurin (CaN). Furthermore, CaN A467-492, an inhibitory peptide based on the autoinhibitory domain from calcineurin (ITSFEEAKGLDRINERMPPRRDAMP), inhibited dephosphorylation of the RII peptide substrate competitively with a Ki = 4 microM, consistent with binding of the autoinhibitory domain at the active site of the enzyme. To assess the role of the autoinhibitory domain in regulating the interaction of CaN with the FK-506.FKBP12 complex, we reconstituted wild type and mutant phosphatase heterodimers using in vitro transcribed and translated subunits. Association of the reconstituted calcineurin heterodimers with FKBP12 was dependent on FK-506. In the case of the wild type heterodimer, association with the FK-506.FKBP12 complex was also dependent upon Ca2+; however, mutant catalytic subunits, in which the autoinhibitory domains were deleted, associated with the drug-binding protein complex in the presence of 10 mM EGTA. These results indicate that the conserved autoinhibitory domain regulates both Ca(2+)-dependent phosphatase activity and association with the FK-506.FKBP12 complex.