Interaction amongst calcineurin subunits. Stimulatory and inhibitory effects of subunit B on calmodulin stimulation of subunit A phosphatase activity depend on Mn2+ exposure of the holoenzyme prior to its dissociation by urea

Calcineurin was dissociated into subunits A and B by 6 M urea in the presence (method A) and absence (method B) of MnCl2 and dissociated subunits were isolated by gel filtration in urea in the absence (method B) or presence (method A) of MnCl2. Phosphatase activity was associated with the A subunit isolated by either method. The phosphatase activity (nmol/mg) of subunit A isolated by method A was greater (2-5-fold) than by method B. Mn2+ increased subunit A phosphatase and calmodulin further increased the enzyme activity. Subunit B isolated by method A or B increased Mn2+ + calmodulin stimulated subunit A phosphatase prepared by method B but interestingly and unexpectedly inhibited such stimulated activity of the subunit A prepared by method A. These results imply the tightly bound cation (in our case, most likely Mn2+) with subunit A dramatically and differentially influences the effects of two Ca2+-binding proteins, calmodulin and subunit B, on the subunit A phosphatase.

Resolution of bovine brain calcineurin subunits: stimulatory effect of subunit B on subunit A phosphatase activity

Calcineurin was dissociated into subunits A and B by SDS and the dissociated subunits were separated by Sephadex G-100 column chromatography in SDS. The phosphatase activity was associated with the A subunit and was detected only in the presence of MnCl2 of the various divalent cations tested. The Mn2+-dependent phosphatase of A subunit was stimulated (4-5-fold) by calmodulin. The subunit B increased only modestly Mn2+ stimulated phosphatase activity of subunit A but markedly increased it when assay also contained calmodulin. These results support the view that subunit B plays an important role in Mn2+/calmodulin regulation of subunit A phosphatase activity. They also lend further support to our earlier postulate ([1984] FEBS Lett. 169, 251-255) that Mn2+ is a powerful regulator of calcineurin phosphatase.

EGTA-sensitive and -insensitive forms of particulate adenylate cyclase in rat cerebral cortex: regulation by divalent cations and GTP

Interactions of several divalent cations (Mn2+, Ca2+, Co2+, Sr2+, and Zn2+) with EGTA-inhibitable adenylate cyclase were investigated in washed membranes (particles) isolated from the gray matter of rat cerebral cortex. The EGTA-inhibitable (called sensitive) enzyme activity was assayed in the presence of Triton X-100 since this detergent caused a marked increase (up to 20-fold) in the enzyme activity. The effects of various divalent metals (all added as chloride salt) indicated the presence of two distinct sites called site I and site II. At low concentrations (less than micromolar) Mn2+, Co2+, and Ca2+ increased (up to 10-fold) the enzyme activity to the same extent and appeared to act via binding to site I (high affinity site). The rank order of affinity was Mn2+ greater than or equal to Co2+ greater than Ca2+. Zn2+ showed the highest affinity and Sr2+ the lowest towards binding to site I; both these metals increased the enzyme activity to lesser extents than Mn2+, Co2+, or Ca2+. GTP was not required for the stimulation of this enzyme by low concentrations of Ca2+. The interaction of Mn2+ with site II (low affinity site) caused further increase in the enzyme activity, whereas Co2+, Ca2+, and Sr2+ were inhibitory at concentrations greater than 10 microM. Isolated fraction contained loosely and tightly associated pools of calmodulin. Myelin basic protein, but not calcineurin, inhibited the EGTA-sensitive adenylate cyclase activity. The EGTA-insensitive enzyme activity was increased by norepinephrine by mechanisms that depended on GTP and was inhibited by Ca2+. The stimulation of the EGTA-insensitive enzyme modulated the Mg2+ requirement such that Mg2+ binding to the low affinity site (site II) apparently occurred with higher affinity. The likely significance of these results is discussed with regard to (i) the presence of two classes of adenylate cyclase in rat cerebral cortex gray matter and (ii) the regulation of their activities by calmodulin-requiring and GTP-requiring mechanisms.

Isolation and characterization of calcineurin from bovine brain

Calcineurin was isolated from bovine cerebrum extracts by sequential chromatography on Affi-Gel blue and calmodulin affinity columns. Calcineurin so isolated was approximately 90% pure and was composed of equimolar amounts of subunit A (Mr = 61 000-63 000) and subunit B (Mr = 15 000-17 000) when examined by sodium dodecyl sulfate gel electrophoresis. A polypeptide (less than 10%) with Mr = 71 000 whose function and role remains to be investigated, was routinely detected in the calcineurin preparation. Both inhibitory activity (towards calmodulin-dependent cAMP phosphodiesterase) and phosphatase activity (with 32P-labelled myelin basic protein as substrate) were associated with calcineurin as evidenced by (i) coelution from Affi-Gel blue, Affi-Gel calmodulin, diethythaminoethyl-Sepharose, and Sephacryl S-200 chromatography columns; (ii) association with the same protein band on nondenaturing gels; (iii) similar stability upon storage at 4 degrees C and with repeated freezing and thawing; and (iv) parallel heat inactivation. Phosphatase activity of calcineurin was maximal with 32P-labelled myelin basic protein as the substrate. Using this substrate, enzyme activity was generally stimulated 5- to 10-fold in the presence of Ca2+ and calmodulin; half-maximal activation (A0.5) was observed with 25 nM calmodulin. Calmodulin increased the Vmax of the reaction without affecting the Km for the substrate. Optimum temperature and pH for the reaction were 45 degrees C and 7, respectively, in both the absence and presence of Ca2+ and calmodulin.(ABSTRACT TRUNCATED AT 250 WORDS)

Phorbol ester inhibits myoblast fusion and activates beta-adrenergic receptor coupled adenylate cyclase

Primary cultures of myoblasts, derived from embryonic chick pectoral muscle, were treated with phorbol ester (TPA) for 8-96 h. TPA treatment blocked the fusion of myoblasts along with the expression of the MM form of creatine kinase. Interestingly, TPA treatment markedly increased the activity of beta-adrenergic receptor coupled adenylate cyclase (AC) activity. The study suggests that TPA treatment augments the functional interaction between a coupling Ns protein and catalytic unit of AC. The likely significance of these results is briefly presented.

Behaviour of cardiac microsomal Ca2+ pump under conditions that may simulate pathological situations

The behaviour of Ca2+ ATPase activity in relation to Ca2+ transport process was studied under different experimental conditions in canine cardiac microsomal fraction predominantly containing sarcoplasmic reticulum. The total Ca2+ concentration required for half maximal activation (Ka) of microsomal Ca2+ ATPase and Ca2+ uptake did not differ significantly, unless 0.1 mmol/l EGTA was present in the incubation media. Pretreatment of cardiac microsomes with membrane disruptive agents like phospholipase A, trypsin as well as deoxycholate strongly increased (2-3 fold) Ca2+ ATPase activity but uptake rate of Ca2+ declined. Only in phospholipase C and beta-glucuronidase pretreatment, a parallel decrease of Ca2+ ATPase and uptake was observed. In presence of excess (free)Ca2+ (greater than 10 mumol/l) both Ca2+ ATPase as well as Ca2+ uptake were inhibited, however, Ca2+ binding process remained unaltered. Likewise, low pH completely altered the relation between Ca2+ binding and ATPase activity; whereas Ca2+ ATPase was inhibited, Ca2+ binding did not change. Our present data provide evidence for some cellular factors that may be involved in producing uncoupling of microsomal Ca2+ ATPase from Ca2+ accumulation process that was previously observed in various pathological situations.

Comparison of cholinergic inhibition and beta-adrenergic stimulation of adenylate cyclase from rat and guinea-pig hearts: effects of guanine nucleotides and monovalent cations

Adenylate cyclase activities and the effects of isoproterenol (a beta-adrenergic receptor agonist), carbachol (a cholinergic receptor agonist) and forskolin (a plant diterpene) were determined in homogenates and washed particulate fractions prepared from rat and guinea-pig hearts. Many interesting differences were noted in the stimulatory and inhibitory effects of isoproterenol and carbachol respectively on the heart enzyme. The likely significance of these results is presented.

Intrinsic phosphatase activity of bovine brain calcineurin requires a tightly bound trace metal

The divalent metal requirement of intrinsic phosphatase activity was investigated using native and trypsinized calcineurin. This was assessed by examining (1) the stimulation of the enzyme by various metals, (2) the inhibition of the enzyme activity by metal chelators (EDTA and EGTA), and (3) the restoration by various metals of the activity of the EDTA-inhibited calcineurin phosphatase. The results supported the view that a tightly bound trace metal is necessary for expression of the phosphatase activity of calcineurin and implicate Mn2+ as the tightly bound metal.

Interaction of EGTA with a hydrophobic region inhibits particulate adenylate cyclase from rat cerebral cortex: a study of an EGTA-inhibitable enzyme by using alamethicin

Washed membranes isolated from rat cerebral cortex (gray matter) showed the presence of EGTA-inhibitable and EGTA-insensitive forms of adenylate cyclase activity. The former activity was stimulated by low concentrations (microM) of various divalent cations (Mn2+, Ca2+, Co2+ and Sr2+) assayed with MgATP2- and MgCl2. At higher concentrations (mM), only Mn2+ stimulated this enzyme whereas Ca2+, Co2+ and Sr2+ were inhibitory. Alamethicin markedly (up to 30-fold) increased the activity of EGTA-inhibitable form and only moderately of EGTA-insensitive form of the enzyme. The increased activity due to alamethicin does not result from solubilization of the enzyme from membranes. Our results suggest the presence of two distinct metal binding sites--one of high (Site I) and other of low (Site II) affinity. Divalent metals via interacting with these produce divergent effects on the enzyme. Site I appears to be located in the hydrophobic region of catalytic unit of the enzyme or of membrane-associated calmodulin. The likely significance of these results is briefly presented.

Characterization of Ca2+ release from the cardiac sarcoplasmic reticulum

The characteristics of Ca2+ release in relation to Ca2+ binding were studied in sarcoplasmic reticulum vesicles isolated from canine myocardium. The Ca2+ binding appeared to be dependent on ATP as a 4 fold increase in Ca2+ binding was observed upon the addition of ATP. In the presence of a suboptimal ATP concentration (20 mumol/l; without ATP regenerating system) a rapid release of Ca2+ started within 2 min. The rate of Ca2+ release was increased by increasing the concentration of Ca2+ in the preincubation medium when studied by diluting preloaded vesicles in medium free of Ca2+ and ATP; an apparent saturation was reached at 5 mmol/l Ca2+ but Ca2+ release again increased between 5 and 10 mmol/l Ca2+. High pH (8.0) enhanced the Ca2+ release process. When Ca2+ loaded vesicles were treated with various phospholipases and proteases, an enhanced Ca2+ release was observed in comparison to the control values. The release of Ca2+ was also increased by pharmacological agents like caffeine, ether and halothane. The Ca2+ release rate was stimulated by the p-chloromercurybenzoate treatment, which decreased ATP dependent Ca2+ binding and Ca2+-stimulated ATPase activities of the sarcoplasmic reticulum vesicles. The effect of temperature when evaluated by Arrhenius plots showed a higher energy of activation of Ca2+ release (66.15 kJ/mol) in comparison to that for Ca2+ binding (41.03 kJ/mol). These results indicate that, although Ca2+ release and Ca2+ binding activities of the cardiac sarcoplasmic reticulum appears to be related, Ca2+ release is probably a distinct process and is controlled differently. It seems that the Ca2+ release site in sarcoplasmic reticulum membranes is lipoprotein in nature.

Lowering temperature increases cardiac muscarinic receptor site affinity towards agonists and antagonists

1. The effect of temperature on the binding of agonists and antagonists to cardiac muscarinic receptor sites was investigated. 2. Compared to 37 degrees C, carbachol-[3H]QNB and atropine-[3H]QNB competition curves were shifted to the left at lower temperatures. This was also observed with other agonists and antagonists. 3. Such an effect of temperature persisted in the N-ethylmaleimide (NEM)-and dithiothreitol (DTT)-treated membranes. 4. Dissociation of the membrane-bound [3H]QNB was higher at 37 degrees C 40% at 60 min) compared to 18 degrees C (approximately 20%). Both carbachol and atropine increased the dissociation at 37 degrees C to a similar maximum (to about 60%). 5. The results show marked temperature-dependent alterations in the cardiac muscarinic receptor sites and suggest that these occur either by direct action of the temperature on receptor conformation or its indirect action via membrane fluidity. The likely significance of these results is discussed.

Calcium ion stimulated endogenous protein kinase catalyzed phosphorylation of basic proteins in myelin subfractions and myelin-like membrane fraction from rat brain

Polypeptide composition and endogenous phosphorylation were investigated in the subfractions of rat brain myelin isolated by either discontinuous or continuous sucrose density gradient centrifugation of myelin. Similarly, a myelin-like membrane fraction (SN4) was also studied. Observations were made that strongly indicated the presence of a calcium-stimulated protein kinase in a highly purified myelin preparation and which exclusively phosphorylated myelin basic proteins of the membrane preparation. Adenosine cyclic 3',5'-phosphate (cAMP) stimulated kinase on the other hand was found to be considerably enriched in the myelin-like membrane fraction. Although this latter enzyme is also capable of phosphorylating the basic proteins, its effect was at least 5 times weaker compared to the calcium-stimulated myelin protein kinase. Within the gradient subfractions there appeared a close relation between the amount of basic proteins and their calcium-stimulated phosphorylation; a similar relationship, however, was not obtained in the case of cAMP-dependent phosphorylation of myelin basic proteins. The former (i.e., Ca2+-stimulated phosphorylation) was found to require a protein factor that functionally resembled calmodulin. The results thus raises an interesting possibility of the presence of calmodulin-like proteins and a calcium-stimulated protein kinase in adult myelin membrane from mammalian brain, both of which have been hitherto unrecognized constituents of myelin membranes.

Requirement for sulfhydryl groups in the differential effects of magnesium ion and GTP on agonist binding of muscarinic cholinergic receptor sites in rat atrial membrane fraction

1. The binding of agonist (carbachol) and antagonist (atropine) to the rat heart atrial muscarinic cholinergic receptor sites was investigated. 2. Divalent cations (Mg2+ or Ca2+), in low concentrations, modestly increased the agonist binding affinity of the receptor site without any effect on the antagonist binding affinity. 3. Guanine nucleotides (e.g. GTP), on the other hand, decreased the agonist binding affinity (but not the antagonist binding affinity), and the extent of GTP effect depended on the absence or presence of divalent cation (Mg2+) in the binding assay. 4. Pretreatment of atrial membranes with N-ethylmaleimide (NEM) altered the agonist binding curve (obtained with varying concentrations of carbachol) such that the Hill coefficient (nH) became very close to 1.0, whereas the corresponding nH values for control (untreated) or dithiothreitol (DTT)-treated membranes were much less than 1.0; NEM or DTT treatments failed to show any effect of antagonist binding curve. 5. NEM treatment abolished both divalent cation-induced and guanine nucleotide-induced alterations in the agonist binding affinity of the receptor site. 6. Monovalent cations in low concentrations did not mimic the effects of Mg2+ or Ca2+ on agonist binding. Instead, concentration dependent decreases in both agonist and antagonist binding affinities and abilities were observed. Neither NEM nor DTT treatments failed to alter the monovalent cation effects on carbachol and atropine binding. 7. These observations indicate a likely involvement of -SH groups in the opposing effects of Mg2+ and guanine nucleotides (GTP) on cardiac muscarinic receptor-agonist interaction. The results further suggest some subtle in vitro differences in the brain and heart muscarinic receptor sites with regard to the influence by divalent cations and guanine nucleotides on the receptor-agonist interaction.

Subcellular Ca2+ transport in different areas of dog heart

Calcium transport and ATPase activities were determined in the heavy and mitochondrial fractions isolated from the left and right atria as well as ventricles of dogs. Ultrastructural distribution of these organelles in different areas of the myocardium was also examined. Calcium binding, calcium uptake, and calcium ATPase activities of the atrial microsomes were lower than those of the ventricles. On the other hand, mitochondrial calcium binding and uptake activities in the right atrium were higher than those in other areas. The mitochondrial total ATPase activities in the atria were also higher than those in the ventricles. Mitochondrial as well as microsomal yields from ventricles were significantly higher. Size and number of mitochondria in the ventricles were greater whereas no striking difference in the distribution of sarcoplasmic reticulum was apparent in different areas of the heart. Poorly developed calcium transport functions in the atrial microsomes may be one of the factors responsible for the generation of lower contractile force in this tissue in comparison with the ventricle.

Calcium ion-stimulated phosphorylation of myelin proteins

Myelin isolated from the central and peripheral nervous system contains a Mg2+-dependent protein kinase that catalyses phosphorylation of myelin-specific proteins. This phosphorylation is markedly stimulated by Ca2+ but not by cyclic AMP. Evidence was obtained that suggested an involvement of calmodulin-like protein in the stimulatory effects of Ca2+ on myelin phosphorylation.