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Key Event Title
|Level of Biological Organization|
Key Event Components
Key Event Overview
AOPs Including This Key Event
|All life stages||Moderate|
Key Event Description
Calcineurin is a heterodimer that comprises a catalytic subunit (CnA), which handles phosphatase activity as well as calmodulin binding, and a Ca-binding regulatory subunit (CnB), which regulates intracellular calcium as well as CnA (Klee et al. 1988, Zhang et al. 1996). CnA, a 59kDa protein, has a serine-threonine phosphatase domain.
Immunophilins are a general class of proteins that exhibit peptidyl-propyl isomerase (PPIase) activity (Barik. 2006) and an immunophilin-Cn inhibitor complex such as FKBP12-FK506 and cyclophilin-CsA binds directly to CnA in the cell, causing steric hindrance of substrate binding to calcineurin, which inhibits the phosphatase activity of calcineurin without any contribution of PPIase activity (Schreiber and Crabtree 1992, Liu et al. 1993, Bierer et al. 1993, Bram et al. 1993, Rao et al. 1997, Liu et al. 1991).
How It Is Measured or Detected
Phosphatase activity can be measured using a phosphatase assay. Calcineurin, calmodulin, FK506, and FKBP are incubated together, and the phosphatase activity is measured at various concentrations of FKBP. Kinetic analysis of FKBP12 concentration-dependent phosphatase activity and calculation of Ki inhibition of calcineurin by the FKBP12-FK506 complex are conducted. (Bram et al. 1993). Phosphatase activity of calcineurin in the presence of CsA and cyclophilin can also be determined in the manner described above.
Immunophilin-calcineurin inhibitor complexes directly inhibit phosphatase activity of calcineurin, therefore, as a surrogate measurement of the calcineurin activity, the binding of cyclosporin A with cyclophilin can be detected using an ELISA kit.Microtiter plates precoated with BSA and conjugated to cyclosporin are incubated with cyclophilin. Bound cyclophilin is then revealed by incubation with anti-cyclophilin rabbit antiserum followed by incubation with anti-rabbit globulin goat IgG coupled to alkaline phosphatase (Quesniaux et al. 1987).
Domain of Applicability
Calcineurin is broadly distributed in T-cells, B‑cells, and throughout the body. The structure of CnA and CnB is highly conserved from yeasts to humans. Also highly conserved are the amino acid sequences of the catalytic and regulatory domains of calcineurin A isoforms from different organisms (Kincaid. 1996).
As for immunophilins, of which complexes inhibit the calcineurin activity, FKBP is found in a wide variety of organisms, from prokaryotes to multicellular organisms. (Siekierka et al. 1989a) Multiple subfamilies of FKBP have been reported, with at least eight types having been found in mammals. FKBP12 is reported to be expressed in B-cells, Langerhans cells, and mast cells as well as in T-cells of humans, mice, and other mammalian species.
Cyclophilins have been found in mammals, plants, insects, fungi, and bacteria. They are structurally conserved throughout evolution and all have PPIase activity (Wang P et al. 2005).
Therefore, inhibition of calcineurin phosphatase activity through immunophilin-calnineurin inhibitor complex deems to be in common among mammalian species.
 Barik, S. (2006). Immunophilins: for the love of proteins. Cellular and Molecular Life Sciences 63(24): 2889-900.
 Bierer, B.E., Holländer, G., Fruman, D. and Burakoff, S.J. (1993). Cyclosporin A and FK506: molecular mechanisms of immunosuppression and probes for transplantation biology. Current opinion in immunology 5 (5): 763-73.
 Bram, R.J., Hung, D.T., Martin, P.K., Schreiber, S.L. and Crabtree, G.R. (1993). Identification of the immunophilins capable of mediating inhibition of signal transduction by cyclosporin A and FK506: roles of calcimeurin binding and cellular location. Molecular and cellular biology 13 (8): 4760-9.
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 Kincaid, R..L. (1993). Calmodulin-dependent protein phosphatases from microorganisms to man. A study in structural conservatism and biological diversity. Adv Second Messenger Phosphoprotein Res. 1993;27:1-23.
 Klee, C. B., Draetta, G. F. and Hubbard, M. J. (1988). Calcineurin. Advances in enzymology and related areas of molecular biology. 61:149-200.
 Liu, J., Farmer, J. D. Jr., Lane, W. S., Friedman, J., Weissman, I., and Schreiber, S. L. (1991). Calcineurin is a common target of cyclophilin-cyclosporin A and FKBP-FK506 complexes. Cell. 66(4): 807-815.
 Liu, J. (1993). FK506 and cyclosporin, molecular probes for studying intracellular signal transduction. Immunology today. 14(6): 290-305.
 Panhans-Gross, A., Novak, N., Kraft, S., and Bieber, T. (2001). Human epidermal Langerhans’ cells are targets for the immunosuppressive macrolide tacrolimus (FK506). Journal of Allergy and Clinical Immunology 107(2): 345-52.
 Rao, A., Luo, C., and Hogan, PG. (1997). Transcription factors of the NFAT family: regulation and function. Annual Review of Immunology 15: 707-47.
 Schreiber, SL., and Crabtree, GR. (1992). The mechanism of action of cyclosporin A and FK506. Immunology Today 13(4): 136-42.
 Siekierka, JJ., Hung, SH., Poe, M., Lin, CS., and Sigal, NH. (1989a). A cytosolic binding protein for the immunosuppressant FK506 has peptidyl-prolyl isomerase activity but is distinct from cyclophilin. Nature 341(6244): 755-57.
 Siekierka, JJ., Wiederrecht, G., Greulich, H., Boulton, D., Hung, SH., Cryan, J., Hodges, PJ., and Sigal, NH. (1990). The cytosolic-binding protein for the immunosuppressant FK-506 is both a ubiquitous an highly conserved peptidyl-prolyl cis-trans isomerase. Journal of Biological Chemistry 265(34): 21011-5.
 Zhang, B.W., Zimmer, G., Chen, J., Ladd, D., Li, E., Alt, F.W., Wiederrecht, G., Cryan, J., O'Neill, E.A., Seidman, C.E., Abbas, A.K. and Seidman, J.G.. (1996). T cell responses in calcineurin A alpha-deficient mice. Journal of experimental medicine 183(2): 413-20.