FK-506 (Tacrolimus), Streptomyces sp. Calcineurin (PP-2B) Inhibitor - Immunosuppressant. FK 506 (Tacrolimus), a macrolide compound isolated from Streptomyces tsukubaensis, is potent immunosuppressive compounds that inhibit T-cell activation and an inhibitor of fungal growth (10 to 100 times more potent than Cyclosporin A). FK-506 binds and inhibits, in vitro the activity of immunophilin proteins known as FK 506-binding protein (FKBP), in a variety of organisms, ranging from yeast to humans. Immunosuppression activity of FK-506 seems to be due to FK-506/FKBP complex that inhibits the heterodimeric Ca2+ /calmodulin-dependent phosphatase Calcineurin. This, in turn, prevents the Calcineurin-dependent activation of interleukin-2 transcription and T-cell proliferation. FK-506 inhibits FKBP activities increases sarcoplasmic reticulum Ca2+ release and decrease the ability of Na\+\-Ca2+ exchange to extrude Ca2+ from the cell. Cell Signaling Activators and Inhibitors; Immunosuppressants. Protein Phosphatase Inhibitors; Protein Phosphatase Inhibitors (PP-2B (Calcineurin))