Interaction of FKBP12-FK506 with calcineurin A at the B subunit-binding domain.

Calcineurin is a calcium-dependent protein phosphatase that plays a pivotal role in antigen-stimulated T cell activation. The complexes formed between the immunosuppressants cyclosporin A and FK506 and their respective intracellular binding proteins (immunophilins) block T cell activation by binding to calcineurin. Recent studies have shown that the immunophilin-immunosuppressant complexes interact with the latch region of the calcineurin B subunit (Milan, D., Griffith, J., Su, M., Price, E. R., and McKeon, F. (1994) Cell 79, 437-447). Mutations in the B subunit-binding domain of the calcineurin A subunit result in a reduction of calcineurin activity that correlates with B binding affinity. Calcineurin A subunit mutants D348A, F350A, W352A, S353A, and E359A lost greater than 90% of their activity to activate the transcription factor NF kappa B in Jurkat T cells. Furthermore, calcineurin A subunit mutants of residues Thr351, Leu354, and Lys360 showed NF kappa B transactivation activity and phosphatase activity with increased resistance to FKBP12-FK506 but displayed no or minimal increase in resistance for cyclosporin A inhibition. Together, these results strongly suggest that the B subunit-binding domain is required for calcineurin activity intracellulary and interacts with the FKBP12-FK506 complex.