Dual calcium ion regulation of calcineurin by calmodulin and calcineurin B

Preliminary crystallization studies of calmodulin-dependent protein phosphatase (calcineurin) from bovine brain

Calcineurin is a serine/threonine protein phosphatase which catalyzes the hydrolysis of both phosphoseryl/phosphothreonyl and phosphotyrosyl proteins as well as low molecular weight compounds such as p-nitrophenyl phosphate. It is a hetero-dimeric protein consisting of a 60 kDa A chain and 19 kDa B chain. Calcineurin A is organized into functionally distinct domains such as a catalytic domain, a calcineurin B binding domain, a calmodulin-binding domain, and an inhibitory domain. Calcineurin B has four EF-hand calcium binding domains with a secondary structure that is homologous to calmodulin but its metal binding properties are more similar to troponin-C. The N-terminal myristoyl group of calcineurin B might play a role in the interaction between subunits A and B during phosphorylation/dephosphorylation processes. Crystals of size 0.125 x 0.07 x 0.03 mm and 0.7 x 0.03 x 0.02 mm have been obtained for calcineurin and the A subunit respectively. Crystals of calcineurin show strong diffraction to 5.3 A and weak diffraction to 3.0 A on rotating anode operated at 50 kV and 100 mA. Further work is in progress to improve the X-ray diffraction quality of these crystals and to obtain well diffracting crystals of calcineurin B.

Calcineurin subunit interactions: mapping the calcineurin B binding domain on calcineurin A

Recombinant forms of the A and B subunits of the protein phosphatase calcineurin were produced in Escherichia coli, reconstituted into a heterodimer and purified to homogeneity. The reconstituted heterodimer exhibited properties like that of bovine brain calcineurin. This included calmodulin-stimulated activity and a subunit stoichiometry and Stokes radius consistent with native-like structure. In order to map the region on the A subunit where calcineurin B binds, a series of overlapping 20-residue peptides corresponding to this putative domain were synthesized. Using isolated calcineurin A and B subunits, an assay that relied upon peptide inhibition of calcineurin B stimulation of calcineurin A activity was developed. All five peptides, but not a control peptide, inhibited calcineurin B-dependent stimulation of calcineurin A although with different potencies. The three most effective inhibitory peptides spanned calcineurin A residues 338-377. These three peptides also altered the electrophoretic mobility of the isolated calcineurin B subunit during native polyacrylamide gel electrophoresis indicating a direct interaction between these peptides and calcineurin B. The peptide corresponding to residues 348-367 was also able to block binding of calcineurin B to the catalytic subunit.

Characterization of a mutant calcineurin A alpha gene expressed by EL4 lymphoma cells

The calmodulin-stimulated phosphatase calcineurin plays a critical role in calcium-dependent T-lymphocyte activation pathways. Here, we report the identification of a missense mutation in the calcineurin A alpha gene expressed by EL4 T-lymphoma cells. This mutation changes an evolutionarily conserved aspartic acid to asparagine within the autoinhibitory domain of the calcineurin A alpha protein. A comparison of wild-type and mutant autoinhibitory peptides indicates that this amino acid substitution greatly reduces inhibition of calcineurin phosphatase activity. Additional peptide inhibition studies support a pseudosubstrate model of autoinhibitory function, in which the conserved aspartic acid residue may serve as a molecular mimic of either phosphoserine or phosphothreonine. Expression of the mutant calcineurin appears to affect cellular signal transduction pathways, as EL4 cells can be activated by suboptimal concentrations of calcium ionophore in the presence of phorbol esters. Moreover, this phenotype can be transferred to Jurkat T cells by transfection of the mutated calcineurin gene. These findings implicate a conserved aspartic acid in the mechanism of calcineurin autoinhibition and suggest that mutation of this residue is associated with aberrant calcium-dependent signaling in vivo.

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.

Characterization of the calcium-binding sites of calcineurin B

Calcineurin (CaN) is a calcium- and calmodulin-dependent serine/threonine phosphatase whose inhibition by the immunosuppressant-immunophilin complexes (cyclosporin-cyclophilin and FK506-FKBP) is considered key to the mechanism of immunosuppression. CaN is a heterodimer, consisting of a 59 kDa catalytic subunit (A) and a 19 kDa calcium-binding regulatory subunit (B). The latter is postulated to harbor four calcium binding domains of the EF hand type. The titration of the CaN B apoprotein with the isomorphic Cd2+ was followed by 113Cd NMR and these data support one high-affinity metal binding site and three lower-affinity ones. Flow dialysis data with Ca2+ indicate one high affinity calcium binding site with Kd approximately 2.4 x 10(-8) M and three other sites with Kd approximately 1.5 x 10(-5) M. The chemical shifts of all four 113Cd resonances (-75, -93, -106 and -119 ppm) are in the same range as found in other 113Cd substituted calcium-binding proteins, and are indicative of all-oxygen coordination of pentagonal bipyramidal geometry.

Kinetics and localization of the phosphorylation of rhodopsin by protein kinase C

Protein kinase C isolated from retina catalyzes the stoichiometric phosphorylation of bovine rhodopsin. Enzymological studies using receptor in rod outer segment membranes stripped of peripheral proteins reveal that the phosphorylation is independent of receptor conformation or liganded state; the half-time for phosphorylation of unbleached (dark-adapted) rhodopsin, bleached (light-activated) rhodopsin, and opsin (chromophore removed) is the same. The phosphorylation by protein kinase C is Ca2+ and lipid regulated; the Km for Ca2+ decreases with increasing concentrations of membrane, consistent with known properties of Ca(2+)-regulated protein kinase Cs. The Km for ATP is 27 microM, with an optimal concentration for MgCl2 of approximately 1 mM. The phosphorylation of rhodopsin by protein kinase C is inhibited by the protein kinase C-selective inhibitor sangivamycin. Proteolysis by Asp-N reveals that all the protein kinase C phosphorylation sites are on the carboxyl terminus of the receptor. Cleavage with trypsin indicates that Ser338, the primary phosphorylation site of rhodopsin kinase, is not phosphorylated significantly; rather, the primary phosphorylation site of protein kinase C is on the membrane proximal half of the carboxyl terminus. The protein kinase C-catalyzed phosphorylation of rhodopsin is analogous to the ligand-independent phosphorylation of other G protein-coupled receptors that is catalyzed by second messenger-regulated kinases.

The interaction between the catalytic A subunit of calcineurin and its autoinhibitory domain, in the yeast two-hybrid system, is disrupted by cyclosporin A and FK506

The Ca(2+)-calmodulin dependent protein phosphatase, calcineurin, is thought to mediate the action of the two immunosuppressants, cyclosporin A (CsA) and FK506. Calcineurin from all species consists of a catalytic A subunit and a regulatory peptide B, which plays an essential role in catalysis. The enzymatic function is probably also regulated by an autoinhibitory domain (AID) present in the catalytic subunit. We have used the yeast two-hybrid system to show that the putative AID of the yeast catalytic subunit Cna1 binds only to truncated Cna1, devoid of AID. Although deletion of the genes encoding the yeast catalytic subunits of calcineurin (CNA1 and CNA2) maintain the interaction, absence of the regulatory subunit Cnb1 prevents binding. Interestingly, both CsA and FK506 disrupt this interaction, whereas binding of Cna1 to calmodulin remains unaffected. This indicates that a simple cellular system, developed in yeast, could provide further insight into an understanding of calcineurin inhibition.