Regulation of the Calmodulin-stimulated Protein Phosphatase, Calcineurin

Abstract

The role of protein phosphatases in the regulation of cellular processes is now well established (1Shenolikar S. Annu. Rev. Cell Biol. 1994; 10: 55-86Crossref PubMed Scopus (403) Google Scholar, 2Cohen P.T. Chen M.X. Armstrong C.G. Adv. Pharmacol. 1996; 36: 67-89Crossref PubMed Scopus (37) Google Scholar). Calcineurin (also called protein phosphatase 2B), a major calmodulin-binding protein in brain and the only serine/threonine protein phosphatase under the control of Ca2+/calmodulin, plays a critical role in the coupling of Ca2+ signals to cellular responses (3Klee C.B. Crouch T.H. Krinks M.H. Proc. Natl. Acad. Sci. U. S. A. 1980; 76: 6270-6273Crossref Scopus (610) Google Scholar, 4Stewart A.A. Ingebritsen T.S. Manalan A. Klee C.B. Cohen P. FEBS Lett. 1982; 137: 80-84Crossref PubMed Scopus (339) Google Scholar, 5Klee C.B. Draetta G.F. Hubbard M.J. Adv. Enzymol. Relat. Areas Mol. Biol. 1988; 61: 149-200PubMed Google Scholar, 6Kincaid R.L. Adv. Second Messenger Phosphoprotein Res. 1993; 27: 1-25PubMed Google Scholar). Its stimulation by the multifunctional protein, calmodulin, ensures the coordinated regulation of its protein phosphatase activity with the activities of the many other enzymes, including a large number of protein kinases, under Ca2+ and calmodulin control. Despite its special abundance in neural tissues, calcineurin is broadly distributed, and its structure is highly conserved from yeast to man (6Kincaid R.L. Adv. Second Messenger Phosphoprotein Res. 1993; 27: 1-25PubMed Google Scholar). Its resistance to the endogenous phosphatase inhibitor 1 and inhibitor 2 and to the potent inhibitors of protein phosphatase 1 and 2A, okadaic acid, calyculin, and microcystin (1Shenolikar S. Annu. Rev. Cell Biol. 1994; 10: 55-86Crossref PubMed Scopus (403) Google Scholar, 2Cohen P.T. Chen M.X. Armstrong C.G. Adv. Pharmacol. 1996; 36: 67-89Crossref PubMed Scopus (37) Google Scholar) made it difficult to identify its functions until it was identified as the target of the immunosuppressive drugs, FK506 and cyclosporin A (CsA). 1The abbreviations used are: CsA, cyclosporin A; IP3, inositol trisphosphate; FKBP, FK506-binding protein; NMDA, N-methyl-d-aspartate.Calcineurin was thus shown to play an essential role in T cell activation (7Liu J. Farmer J.D. Lane W.S. Friedman J. Weissman I. Schreiber S.L. Cell. 1991; 66: 807-815Abstract Full Text PDF PubMed Scopus (3620) Google Scholar). The demonstration that FK506 and CsA, when bound to their respective binding proteins, FKBP12 and cyclophilin A, are specific inhibitors of calcineurin provided the tools needed to reveal its many other roles in the transduction of Ca2+ signals (8Liu J. Albers M.W. Wandless T.J. Luan S. Alberg D.G. Belshaw P.J. Cohen P. MacKintosh C. Klee C.B. Schreiber S.L. Biochemistry. 1992; 31: 3896-3901Crossref PubMed Scopus (502) Google Scholar). Its calmodulin dependence distinguishes it from two other known Ca2+-regulated protein phosphatases, the insulin-sensitive pyruvate dehydrogenase phosphatase of mitochondria (9Denton R.M. McCormack J.G. Rutter G.A. Burnett P. Edgell N.J. Moule S.K. Diggle T.A. Adv. Enzyme Regul. 1996; 36: 183-198Crossref PubMed Scopus (54) Google Scholar) and a family of protein phosphatases homologous to the product of the Drosophila retinal degeneration C (rdgC) gene (10Steele F.R. Washburn T. Rieger R. O'Tousa J.E. Cell. 1992; 69: 669-676Abstract Full Text PDF PubMed Scopus (134) Google Scholar, 11Vinos J. Jalink K. Hardy R.W. Britt S.G. Zuker C.S. Science. 1997; 277: 687-690Crossref PubMed Scopus (103) Google Scholar, 12Sherman P.M. Sun H. Macke J.P. Williams J. Smallwood P.M. Nathans J. Proc. Natl. Acad. Sci. U. S. A. 1997; 94: 11639-11644Crossref PubMed Scopus (50) Google Scholar). Calcineurin has a relatively narrow substrate specificity. Phosphoproteins listed in Table I are preferentially dephosphorylated by calcineurin whereas others such as casein, synapsin 1, and calmodulin kinase II are dephosphorylated at much slower rates or not at all (5Klee C.B. Draetta G.F. Hubbard M.J. Adv. Enzymol. Relat. Areas Mol. Biol. 1988; 61: 149-200PubMed Google Scholar). Other potentially physiological substrates, whose kinetic characteristics have not been determined, include NO synthase, a GTPase involved in endocytosis (dynamin, previously called dephosphin), the transcription factor Elk-1, and the heat shock protein, hsp25 (13Dawson T.M. Steiner J.P. Dawson V.L. Dinerman J.L. Uhl G.R. Snyder S.H. Proc. Natl. Acad. Sci. U. S. A. 1993; 90: 9808-9812Crossref PubMed Scopus (512) Google Scholar, 14Herskovits J.S. Burgess C.C. Obar R.A. Vallee R.B. J. Cell Biol. 1993; 122: 565-578Crossref PubMed Scopus (398) Google Scholar, 15Sugimoto T. Stewart S. Guan K.-L. J. Biol. Chem. 1997; 272: 29415-29418Abstract Full Text Full Text PDF PubMed Scopus (115) Google Scholar, 16Gaestel M. Benndorf R. Hayess K. Priemer E. Engel K. J. Biol. Chem. 1992; 267: 21607-21611Abstract Full Text PDF PubMed Google Scholar). The substrate specificity of