Differences between Mg(2+) and transition metal ions in the activation of calcineurin

UVA1 radiation inhibits calcineurin through oxidative damage mediated by photosensitization

The protein phosphatase calcineurin has been gradually revealing itself as the central controller of our immune response, although it is involved in a wide array of signaling pathways related to cellular development and cell cycle progression. As such, calcineurin is an attractive, yet delicate, therapeutic target for the prevention of allograft rejection and treatment of several inflammatory skin conditions. However, calcineurin activity is not only sensitive to immunosuppressants such as cyclosporin A and tacrolimus, but also subject to modulation by reactive oxygen species. We have recently shown, both in vivo and in vitro, that UVA1 radiation suppresses calcineurin activity. In this paper, we present evidence that this activity loss is due to singlet oxygen and superoxide generated by photosensitization and show that a closely related phosphatase, PP2A, is not affected. Furthermore, a survey of this damage reveals oxidation of several Met and Cys residues as well as an overall conformational change. These findings provide a mechanistic basis for the hypothesis that UVA1 and calcineurin inhibitors both affect the same signal transduction pathway in skin.

Mechanism of activation of Saccharomyces cerevisiae calcineurin by Mn2+

Saccharomyces cerevisiae calcineurin (CN) consists of a catalytic subunit CNA1 or CNA2 and a regulatory subunit CNB1. The kinetics of activation of yeast CN holoenzymes and their catalytic domains by Mn2+ were investigated. We report that the in vitro phosphatase reaction activated by Mn2+ typically has a pronounced initial lag phase caused by slow conformational rearrangement of the holoenzyme-Mn2+. A similar lag phase was detected using just the catalytic domain of yeast CN, indicating that the slowness of Mn2+-induced conformational change of CN results from a rearrangement within the catalytic domain. The Mn2+-activation of CN was reversible. The dissociation constant of the CN heterodimer containing the CNA2 subunit in the presence of Mn2+ was 3-fold higher than that of CN containing the CNA1 subunit and that of the catalytic domains of CNA1 and CNA2, pointing to differences between the residues surrounding the Mn2+-binding sites of CNA1 and CNA2.

The effect of surface attachment on ligand binding: studying the association of Mg2+, Ca2+and Sr2+by 1-thioglycerol and 1,4-dithiothreitol monolayers

The difference in the heterogeneous binding of Mg(2+), Ca(2+) and Sr(2+) ions by 1-thioglycerol (TG) and 1,4-dithiothreitol (DTT) spontaneously adsorbed monolayers on Au has been studied following the changes in the double layer capacity. A mathematical treatment, based on calculating the electrochemical potential difference at the monolayer-electrolyte interface, has followed our recent work which dealt with the acid-base equilibrium at the interface as a means of calculating the pK of ionizable SAMs and their binding with Cd(2+). Experimentally, spontaneously adsorbed monolayers of TG and DTT were assembled on Au surfaces and studied by impedance spectroscopy and alternating current voltammetry (ACV). The capacity was measured for each of the modified surfaces at increasing concentrations of the divalent metal ions separately. The goal of this study has been to examine the effect of metal ion binding by similar ligands that are differently attached onto the surface. TG and DTT monolayers differ in their flexibility, which is a result of their attachment to the surface through one and two arms, respectively. The general trend of the apparent heterogeneous association constants of the divalent metal ions, which were calculated from the capacity measurements, was substantially different from the classical Irving-Williams series that is applicable to homogeneous systems. This difference could be nicely explained by the reduction of the degree of freedom and flexibility of the attached ligands.

Non-catalytic domains of subunit A negatively regulate the activity of calcineurin

Calcineurin is composed of a catalytic subunit A (CNA) and a regulatory subunit B (CNB). In addition to the catalytic core, CNA further contains three non-catalytic domains--CNB binding domain (BBH), calmodulin binding domain (CBD), and autoinhibitory domain (AI). To investigate the effect of these three domains on the activity of CNA, we have constructed domain deletion mutants CNAa (catalytic domain only), CNAac (CNAa and CBD), and CNAaci (CNAa, CBD and AI). By using p-nitrophenylphosphate and (32)P-labeled R(II) peptide as substrates, we have systematically examined the phosphatase activities, kinetics, and regulatory effects of Mn(2+)/Ni(2+) and Mg(2+). The results show that the catalytic core has the highest activity and the order of activity of the remaining constructs is CNAac>CNAaci>CNA. Sequential removal of the non-catalytic domains corresponds to concurrent increases of the phosphatase activity assayed under several conditions. This observation clearly demonstrates that non-catalytic domains negatively regulate the enzyme activity and act as intra-molecular inhibitors, possibly through restraining the conformation elasticity of the catalytic core required for optimal catalysis or interfering with substrate access. The sequential domain deletion favors activation of the enzyme by Mn(2+)/Ni(2+) but not by Mg(2+) (except for CNAa), suggesting that enzyme activation by Mn(2+)/Ni(2+) is mainly mediated via the catalytic domain, whereas activation by Mg(2+) is via both the catalytic core and non-catalytic domains.

Response of recombinant calcineurin to metal ions, reduction-oxidation agents, and enzymatic modification

Recombinant calcineurin heterodimer with the full length delta-isoform of the catalytic subunit (CaN(500)) was expressed in insect cells using the baculovirus system and compared to native bovine brain enzyme in its response to divalent metal ions, redox reagents, and enzymatic modification of arginine residues. The response to various metal ions showed essentially the same profile as bovine brain calcineurin, although Co2+ and Zn2+ did not support recombinant activity as well. Kinetic analysis showed that metal ion and substrate binding were not independent, as found for the bovine brain calcineurin. Incubation with DTT or ascorbate alone caused similar effects on the activity of both enzymes, but different responses were observed when incubated with both DTT and ascorbate; only the recombinant enzyme showed activation. Arginine deimination of recombinant calcineurin by peptidylarginine deiminase resulted in the loss of 60-80% of its phosphatase activity with protection observed if calmodulin was present. Recombinant calcineurin was reactivated by treatment with the protease clostripain, suggesting that deimination of an arginine in the carboxyl terminal domain may be responsible for the loss of phosphatase activity and decreased calmodulin binding [Arch. Biochem. Biophys. 318 (1995) 370]. Supporting this conclusion, a truncated variant of the catalytic subunit lacking the carboxyl terminus showed no loss of phosphatase activity compared to full length calcineurin subunit and contained lower amounts of citrulline than the full length subunit after deimination. These different responses of recombinant calcineurin are consistent with conformational differences compared to bovine brain calcineurin and raise questions about its utility for studying the mechanism of calcineurin.