Purification and characterization of an inhibitor protein of brain adenylate cyclase and cyclic nucleotide phosphodiesterase

De Novo Assembly of the Genome of the Sea Urchin Paracentrotus lividus (Lamarck 1816)

The Mediterranean purple sea urchin Paracentrotus lividus (Lamarck 1816) is a remarkable model system for molecular, evolutionary and cell biology studies, particularly in the field of developmental biology. We sequenced the genome, performed a de novo assembly, and analysed the assembly content. The genome of P. lividus was sequenced using Illumina NextSeq 500 System (Illumina) in a 2 × 150 paired-end format. More than 30,000 open reading frames (ORFs), (more than 8000 are unique), were identified and analysed to provide molecular tools accessible for the scientific community. In particular, several genes involved in complex innate immune responses, oxidative metabolism, signal transduction, and kinome, as well as genes regulating the membrane receptors, were identified in the P. lividus genome. In this way, the employment of the Mediterranean sea urchin for investigations and comparative analyses was empowered, leading to the explanation of cis-regulatory networks and their evolution in a key developmental model occupying an important evolutionary position with respect to vertebrates and humans.

A Review of Calcineurin Biophysics with Implications for Cardiac Physiology

Calcineurin, also known as protein phosphatase 2B, is a heterodimeric serine threonine phosphatase involved in numerous signaling pathways. During the past 50 years, calcineurin has been the subject of extensive investigation. Many of its cellular and physiological functions have been described, and the underlying biophysical mechanisms are the subject of active investigation. With the abundance of techniques and experimental designs utilized to study calcineurin and its numerous substrates, it is difficult to reconcile the available information. There have been a plethora of reports describing the role of calcineurin in cardiac disease. However, a physiological role of calcineurin in healthy cardiomyocyte function requires clarification. Here, we review the seminal biophysical and structural details that are responsible for the molecular function and inhibition of calcineurin. We then focus on literature describing the roles of calcineurin in cardiomyocyte physiology and disease.

A Friend or Foe: Calcineurin across the Gamut of Neurological Disorders

The serine/threonine phosphatase calcineurin (CaN) is a unique but confounding calcium/calmodulin-mediated enzyme. CaN has shown to play essential roles from regulating calcium homeostasis to being an intricate part of learning and memory formation. Neurological disorders, despite differing in their etiology, share similar pathological outcomes, such as mitochondrial dysfunction and apoptotic signaling brought about by excitotoxic elements. CaN, being deeply integrated in vital neuronal functions, may be implicated in various neurological disorders. Understanding the enzyme and its physiological niche in the nervous system is vital in uncovering its roles in the spectrum of brain disorders. By reviewing the crosstalk in different neurological pathologies, a possible grasp of CaN's complex signaling may lead to forming better neurotherapy. This Outlook attempts to explore the various neuronal functions of CaN and investigate its pervasive role through the gamut of neurological disorders.

Identification of two calcineurin B-binding proteins: tubulin and heat shock protein 60

Calcineurin (CaN) is a Ca++/calmodulin-dependent protein phosphatase with two subunits: a catalytic subunit (CaNA) and a regulatory subunit (CaNB). With four Ca(++)-binding sites and a sequence homology to calmodulin, CaNB has been defined as the regulatory subunit for CaNA. However, we have shown that mitochondrial expression of CaNB far exceeds that of CaNA. To investigate the role of this excess CaNB, we have generated glutathione-S-transferase-CaNB (GST-CaNB) fusion protein and demonstrated that the fusion protein predominantly bound to alpha-tubulin, a 57 kDa protein in bovine brain extracts, and heat shock protein 60 (Hsp60) in bovine kidney extracts. Their Ca(++)-dependent interactions with CaNB were verified by immunoprecipitation. The binding of CaNB could be demonstrated with purified alpha/beta tubulins and Hsp60, but not GroEL, a bacterial Hsp60 analog. The interaction of CaNB and Hsp60 was not disrupted by the incubation with Hsp10, ATP and Mg++, suggesting that CaNB was not associated with Hsp60 as a misfolded substrate, and may serve as a regulatory protein. Thus, CaNB may play other regulatory roles in Ca(++)-dependent events in addition to its interaction with CaNA, and may be important for Ca(++)-dependent processes in mitochondria.

Calcineurin: form and function

Calcineurin is a eukaryotic Ca(2+)- and calmodulin-dependent serine/threonine protein phosphatase. It is a heterodimeric protein consisting of a catalytic subunit calcineurin A, which contains an active site dinuclear metal center, and a tightly associated, myristoylated, Ca(2+)-binding subunit, calcineurin B. The primary sequence of both subunits and heterodimeric quaternary structure is highly conserved from yeast to mammals. As a serine/threonine protein phosphatase, calcineurin participates in a number of cellular processes and Ca(2+)-dependent signal transduction pathways. Calcineurin is potently inhibited by immunosuppressant drugs, cyclosporin A and FK506, in the presence of their respective cytoplasmic immunophilin proteins, cyclophilin and FK506-binding protein. Many studies have used these immunosuppressant drugs and/or modern genetic techniques to disrupt calcineurin in model organisms such as yeast, filamentous fungi, plants, vertebrates, and mammals to explore its biological function. Recent advances regarding calcineurin structure include the determination of its three-dimensional structure. In addition, biochemical and spectroscopic studies are beginning to unravel aspects of the mechanism of phosphate ester hydrolysis including the importance of the dinuclear metal ion cofactor and metal ion redox chemistry, studies which may lead to new calcineurin inhibitors. This review provides a comprehensive examination of the biological roles of calcineurin and reviews aspects related to its structure and catalytic mechanism.

Potential role of calcineurin for brain ischemia and traumatic injury

Calcineurin belongs to the family of Ca2+/calmodulin-dependent protein phosphatase, protein phosphatase 2B. Calcineurin is the only protein phosphatase which is regulated by a second messenger, Ca2+. Furthermore, calcineurin is highly localized in the central nervous system, especially in those neurons vulnerable to ischemic and traumatic insults. For these reasons, calcineurin is considered to play important roles in neuron-specific functions. Recently, on the basis of the finding that FK506 and cyclosporin A serve as calcineurin-specific inhibitors, this enzyme has become the subject of much study. It is clear that calcineurin is involved in many neuronal (or non-neuronal) functions such as neurotransmitter release, regulation of receptor functions, signal transduction systems, neurite outgrowth, gene expression and neuronal cell death. In this review, we describe the calcineurin functions, functions of the substrates, and the pathogenesis of traumatic and ischemic insults, and we discuss the potential role of calcineurin. There are many similarities in traumatic and ischemic pathogenesis of the brain in which the release of excessive glutamate is followed by an intracellular Ca2+ increase. However, the intracellular cascade which leads to neuronal cell death after the release of excess Ca2+ is unclear. Although calcineurin is thought to be a key toxic enzyme on the basis of studies using immunosuppressants (FK506 or cyclosporin A), many of the functions of the substrates for calcineurin protect against neuronal cell death. We concluded that calcineurin is a bi-directional enzyme for neuronal cell death, having protective and toxic actions, and the balance of the bi-directional effects may be important in ischemic and traumatic pathogenesis.

Calcineurin. Structure, function, and inhibition

Calcineurin is a serine-threonine specific Ca(2+)-calmodulin-activated protein phosphatase that is conserved from yeast to humans. Remarkably, this enzyme is the common target for two novel and structurally unrelated immunosuppressive antifungal drugs, cyclosporin A and FK506. Both drugs form complexes with abundant intracellular binding proteins, cyclosporin A with cyclophilin A and FK506 with FKBP 12, which bind to and inhibit calcineurin. The X-ray structure of an FKPB12-FK506-calcineurin AB ternary complex reveals that FKBP12-FK506 binds in a hydophobic groove between the calcineurin A catalytic and the regulatory B subunit, in accord with biochemical and genetic studies on inhibitor action. Calcineurin plays a key role in regulating the transcription factor NF-AT during T-cell activation, and in mediating responses of microorganisms to cation stress. These findings highlight the potential of yeast genetic studies to define novel drug targets and elucidate conserved elements of signal transduction cascades.

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.

Immunological approach to identify calmodulin-stimulated phosphatase isozymes from bovine brain

Molecular cloning of human, mouse and rat brain CaM-stimulated phosphatase has suggested the existence of two genes for the alpha subunit of the enzymes. A alpha and A beta fragments of A alpha and A beta from rat brain library have been expressed in bacteria to produce specific anti-calcineurin A alpha and anti-calcineurin A beta antibodies (Kuno et al., J Neurochem 58: 1643-1651, 1992). Alternative mRNA splicing gives rise to additional calcineurin isozymes with some containing an insertion sequence of ATVEAIEADE. Antibody against synthetic peptide of this insertion sequence has been raised in this study. Three CaM-stimulated phosphatase isozymes previously purified from bovine brain (BPI, BPII, BPIII) (Yokoyama & Wang, J Biol Chem 266: 14822-14829, 1991), along with the bacterially expressed rat A alpha and A beta fragments, were analyzed by two calcineurin alpha subunit monoclonal antibodies VJ6 and VD3, the rat anti-calcineurin A alpha and anti-calcineurin A beta specific polyclonal antibodies, and the insertion peptide antibody. The bovine brain CaM-stimulated phosphatase isozymes BPI and BPIII reacted with both anti-calcineurin A alpha and anti-calcineurin A beta antibodies. While BPII reacted with anti-calcineurin A alpha but not anti-calcineurin A beta antibody, it differed from the expressed A alpha fragment in immunoreactivity towards the monoclonal antibodies. The results show that the bovine brain CaM-stimulated phosphatase isozymes cannot be simply categorized as derived from A alpha or A beta genes products.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of the autoinhibitory domain in differential metal ion activation of calmodulin-stimulated phosphatase

Metal ion activators, Ni2+ and Mn2+, have been suggested to induce different conformations of calmodulin (CaM)-stimulated phosphatase. In the present study, an autoinhibitory domain previously implicated in the conformation transition of CaM stimulation of the phosphatase is shown to participate in defining the differential metal ion activation. A proteolytic derivative of the phosphatase deleted from the autoinhibitory domain displayed CaM-independent Mn(2+)-stimulated activity which was about 4-times that of the CaM-stimulated activity of the native enzyme. The Ni(2+)-stimulated activity of the derivative, on the other hand, retained slight CaM-dependence, and the CaM-stimulated activity was 90% of that of the native enzyme. A synthetic peptide corresponding to the autoinhibitory domain could inhibit the Mn(2+)-stimulated activity of the phosphatase derivative by 80%, but had little effect on the Ni(2+)-stimulated activity.

The in vitro phosphorylation of calmodulin by the insulin receptor tyrosine kinase

Calmodulin, a ubiquitous Ca2+-binding regulatory protein, is phosphorylated exclusively on tyrosine-99 in an insulin-dependent manner by wheat germ lectin-purified preparations of insulin receptors from rat adipocyte plasma membranes. Calmodulin is phosphorylated in the presence of polylysine, histone Hf2b, and protamine sulfate, but not in the absence of these cofactors or in the presence of other basic compounds known to interact with calmodulin, such as mellitin, myelin basic protein, chlorpromazine, trifluoperazine, substance P, glucagon, polyarginine, mastoparin, beta-endorphin, spermine, spermidine, and putrescine. The incorporation of 32P into calmodulin, expressed in terms of moles of phosphate per moles of calmodulin and assayed at calmodulin concentrations of 1.2 and 0.06 microM, is 0.023 + 0.002 and 0.046 + 0.006, respectively. This low stoichiometry is likely due to the relative impurity of the receptor preparation, as similar studies not shown here, using highly purified human insulin receptors, yield a stoichiometry of 1 mol phosphate/mol calmodulin. The time course of phosphorylation is characterized by a short initial lag phase of approximately 5 min, a rapid linear rate from approximately 5 to 40 min, with a steady state of 32P incorporation being approached at approximately 60 min. The K0.5 for ATP is 104 + 18 microM. Phosphorylated calmodulin is partially purified by HPLC on a C4 column using a trifluoroacetic acid/acetonitrile gradient solvent system. Phosphoamino acid analysis and limited thrombin digestion were used to determine that the site of insulin-induced phosphorylation of calmodulin is exclusively on tyrosine-99 regardless of the basic protein cofactor used. Phosphorylated calmodulin does not exhibit the characteristic Ca2+ shift normally observed with calmodulin in electrophoretic gels, an observation that is consistent with this modification affecting the biological activity of the molecule. Thus, the tyrosine phosphorylation of calmodulin represents a potentially important post-translational modification altering calmodulin's ability to regulate a variety of enzymes involved in growth, differentiation, and metabolic regulation.

Dephosphorylation of microtubule proteins by brain protein phosphatases 1 and 2A, and its effect on microtubule assembly

Protein phosphatase C was purified 140-fold from bovine brain with 8% yield using histone H1 phosphorylated by the catalytic subunit of cyclic AMP-dependent protein kinase (cyclic AMP-kinase). Brain protein phosphatase C was considered to consist of 10 and 90%, respectively, of the catalytic subunits of protein phosphatases 1 and 2A on the basis of the effects of ATP and inhibitor-2. Protein phosphatase C dephosphorylated microtubule-associated protein 2 (MAP2), tau factor, and tubulin phosphorylated by a multifunctional Ca2+/calmodulin-dependent protein kinase (calmodulin-kinase) and the catalytic subunit of cyclic AMP-kinase. The properties of dephosphorylation of MAP2, tau factor, and tubulin were compared. The Km values were in the ranges of 1.6-2.7 microM for MAP2 and tau factor. The Km value for tubulin decreased from 25 to 10-12.5 microM in the presence of 1.0 mM Mn2+. No difference in kinetic properties of dephosphorylation was observed between the substrates phosphorylated by the two kinases. Protein phosphatase C did not dephosphorylate the native tubulin, but universally dephosphorylated tubulin phosphorylated by the two kinases. The holoenzyme of protein phosphatase 2A from porcine brain could also dephosphorylate MAP2, tau factor, and tubulin phosphorylated by the two kinases. The phosphorylation of MAP2 and tau factor by calmodulin-kinase separately induced the inhibition of microtubule assembly, and the dephosphorylation by protein phosphatase C removed its inhibitory effect. These data suggest that brain protein phosphatases 1 and 2A are involved in the switch-off mechanism of both Ca2+/calmodulin-dependent and cyclic AMP-dependent regulation of microtubule formation.

Purification of calmodulin-stimulated cyclic nucleotide phosphodiesterase by monoclonal antibody affinity chromatography

Immobilized ACC-1 and ACAP-1 antibodies are effective tools for the purification of active calmodulin-dependent phosphodiesterases. ACC-1 antibody binds all bovine and rat brain isozymes in a Ca2+-dependent manner and has been used for their purification. Since ACC-1 binds both bovine brain isozymes (61- and 63-kDa forms) and ACAP-1 recognizes only the 61-kDa isozyme, ACAP-1 can be used to separate and purify the two brain isozymes. The procedures described here for phosphodiesterase isolation from brain are rapid and require few enzymatic assays, resulting in preparations of good purity, specific activity, and yield (Tables II, III). The procedures for brain tissue can be easily adapted for use with larger amounts of tissue. The cross-reactivity of ACP-1 for rat brain phosphodiesterase suggests that this antibody may recognize isozymes from other mammalian tissues.

Cardiac calmodulin-stimulated protein phosphatase: purification and identification of specific sarcolemmal substrates

A calmodulin-stimulated protein phosphatase has been purified from bovine myocardium. The purification procedure involves sequential DEAE-Sephacel ion exchange chromatography, calmodulin-Sepharose affinity chromatography, and high performance liquid chromatography using a Spherogel TSK DEAE 5PW column. By SDS polyacrylamide gel electrophoresis, the purified cardiac phosphatase consists of two subunits of Mr 61,000 and 19,000, similar to the brain enzyme, calcineurin. Protein phosphatase activity of the cardiac enzyme is stimulated by Ca2+-calmodulin and inhibited by the calmodulin antagonist drug, calmidazolium. Effects of a series of divalent cations on catalytic activity of the cardiac calmodulin-stimulated protein phosphatase are similar to those observed with calcineurin, when the two enzymes are assayed under identical conditions. Highly enriched preparations of bovine cardiac sarcolemma contain substrates of cAMP-dependent protein kinase of Mr 166 K, 133 K, 108 K, 79 K, 39 K, and 14 K, which are specifically dephosphorylated by the calmodulin-stimulated phosphatase with pseudofirst-order rate constants of 0.23, 0.46, 0.69, 0.35, 0.69, and 0.115 min-1, respectively. These substrates are not present in purified preparations of cardiac sarcoplasmic reticulum. These results support a role of the calmodulin-stimulated phosphatase in the Ca2+-regulation of specific sarcolemmal processes by protein dephosphorylation.

The distribution of calcineurin in rat brain by light and electron microscopic immunohistochemistry and enzyme-immunoassay

Calcineurin is the calcium (divalent cations)-dependent calmodulin-stimulated phosphoprotein phosphatase which is capable of dephosphorylating various substrate proteins. The subcellular and regional distribution of calcineurin in the rat brain has been studied by light and electron microscopic immunohistochemistry using antiserum against calcineurin. Immunoreactivity was observed in many neurons but was not detected in glial cells, such as astrocytes, oligodendrocytes and ependymal cells by the PAP method. Light microscopy demonstrates strong immunoreactivity in neuronal somata and neurites. By electron microscopy, calcineurin immunoreactivity was found to be present in dendrites including postsynaptic densities, somata, spines, axons and terminals. Calcineurin immunoreactivity was present in neurons throughout the brain, but a marked regional variation in strength of the immunoreactivity was observed. The caudatoputamen, hippocampal formation, and substantia nigra were strongly stained. Cerebral and cerebellar neocortex showed moderate immunoreactivity. In substantia nigra and globus pallidus, only neurites were stained, but neuronal somata not. The staining of the substantia nigra was thought to be due to that of the nerve terminals originating from the caudatoputamen, in view of the findings by cerebral hemitransection and electron microscopic immunohistochemistry. We developed an enzyme-immunoassay (EIA) for calcineurin. The sensitivity of the EIA was 1 ng (13 fmol) of calcineurin. We determined the level of calcineurin in various regions of the rat brain. The caudate nucleus, putamen and hippocampal formation showed a high concentration of calcineurin. The results are consistent with those obtained by immunohistochemistry.

Novel interactions of cations with dihydropyridine calcium antagonist binding sites in brain

The effects of monovalent (Na+, Li+, K+, Rb+) and divalent (Ca2+, Mg2+, Mn2+) cations on dihydropyridine calcium antagonist binding sites in brain and cardiac membranes were investigated using a low ionic strength buffer (5 mM Tris-HCl, pH 7.4), and the dihydropyridine, [3H]-nitrendipine. At 25 degrees C, the monovalent cations Na+, Li+, and K+ (100 mM) but not Rb+ significantly decreased the apparent dissociation constant (KD) but had no effect on the maximum binding site capacity (Bmax) of [3H]-nitrendipine in brain. The divalent cations Ca2+, Mg2+, and Mn2+ (2 mM) significantly increased the Bmax, but did not affect the KD of [3H]-nitrendipine. The effects of cations were concentration-dependent (EC50 monovalent cations 10-25 mM; EC50 divalent cations 50-200 microM) and demonstrated brain region selectivity. The effect of Ca2+, but not Mg2+ or Mn2+ on [3H]-nitrendipine binding was described by a two-site model. At 25 degrees C, neither mono- nor divalent cations altered the characteristics of [3H]-nitrendipine binding to rat cardiac membranes. At 37 degrees C, Na+ (100 mM) but not K+ (100 mM) significantly increased the Bmax of [3H]-nitrendipine in rat brain membranes. Ca2+ (2 mM) significantly increased the Bmax of [3H]-nitrendipine binding to rat brain membranes to a greater extent than at 25 degrees C. Both Na+ and K+ had no effect on [3H]-nitrendipine binding to cardiac membranes, while Ca2+ (2 mM) significantly decreased the KD of [3H]-nitrendipine. It is suggested that the selective effects of mono- and divalent cations on [3H]-nitrendipine binding to rat brain and cardiac membranes may be associated with differences in the calcium current blocking activity of dihydropyridine calcium antagonists in brain and cardiac tissues.

Calcineurin as a neuronal marker of human brain tumors

Calcineurin is one of the calmodulin binding proteins and a Ca2+-dependent and calmodulin-stimulated phosphoprotein phosphatase. We used antisera to the calcineurin as a cell-type-specific marker in order to identify neuronal cells in the rat brain and human neoplasms. In normal rat brain slices, basal ganglia were stained macroscopically, and other areas such as cerebral cortex, corpus callosum, cerebellar cortex, granular layer and pyramidal tract of the spinal cord were lightly identified as well. Under the light microscope, it was found that only the neuronal cells were stained, and astrocytes, oligodendrocytes, ependymal cells and vessels were not. Intracellular distribution of the staining showed various patterns and staining intensity of varying degree. Using the PAP method, localization of the calcineurin in formalin-fixed, paraffin-embedded tissues were studied in 65 human intracranial neoplasms, and in 11 human extracranial neoplasms. The neuronal elements of neuroblastoma, ganglioglioma, ganglioneuroma and retinoblastoma were clearly stained. In contrast, glioblastoma, astrocytoma, oligodendroglioma, ependymoma, meningioma, neurinoma, pituitary adenoma, craniopharyngioma, hemangioblastoma, hamartoma, lymphoma and mesenchymal tumor were all negative. Two cases out of 5 medulloblastomas were stained, but others were not. Although positive tumors disclosed various staining patterns and intensities, these results indicated that calcineurin could be a new neuronal marker in human brain tumors.

Protein phosphorylation and oocyte maturation. II. Inhibition of starfish oocyte maturation by intracellular microinjection of protein phosphatases 1 and 2A and alkaline phosphatase

Oocyte maturation (meiosis re-initiation) in starfish is induced by the natural hormone 1-methyladenine (1-MeAde). Following hormonal stimulation of the oocyte, an intracellular Maturation Promoting Factor (MPF) appears in the cytoplasm which triggers nuclear envelope breakdown and maturation divisions. Microinjection of pure preparations of the catalytic subunits of protein phosphatases 1 and 2A inhibits 1-MeAde-induced maturation in a dose-dependent manner. Calmodulin-dependent protein phosphatase 2B is inefficient. Maturation induced by mimetics of 1-MeAde, such as dithiothreitol (DTT), methylglyoxal-bis(guanylhydrazone) (MGBG), 8-hydroxyeicosatetraenoic acid (8 HETE) and arachidonic acid (AA) is also inhibited by these protein phosphatases. In all cases inhibition can be reversed by increasing the concentration of 1-Me-Ade or of mimetic. Alkaline phosphatase also inhibits maturation in a dose-dependent way and in a reversible manner. Microinjection of protein phosphatase is still effective when preformed long after the end of the hormone-dependent period, and can even be effective a few minutes before the breakdown of the nuclear envelope. No detectable MPF activity is found in 1-MeAde-treated phosphatase-injected oocytes. However, microinjection of phosphatase 2A simultaneously with MPF (obtained from 1-MeAde-treated donors) does not result in inhibition. These results constitute direct evidence for the necessity of an elevated level of phosphorylated proteins for MPF activity and maturation. The mode of action of 1-MeAde in inducing starfish oocyte maturation is discussed in relation to protein phosphorylation.

A protein inhibitor of calmodulin-regulated cyclic nucleotide phosphodiesterase in amphibian ovaries

The cytosol fraction of an extract of Xenopus laevis ovaries contains a protein inhibitor that can specifically block the activation of calmodulin-sensitive cyclic nucleotide phosphodiesterase (PDE I) found in that tissue. This inhibitor was purified by DEAE-cellulose chromatography, gel filtration on Sephacryl S-200, and affinity chromatography on calmodulin-Sepharose. It has a molecular weight of approximately 90,000, and is heat-labile and susceptible to inactivation by chymotrypsin. The inhibitor blocks calmodulin activation of cyclic nucleotide phosphodiesterases from amphibian ovary and bovine brain and of the myosin light chain kinase from rabbit smooth muscle, but does not affect the activity of a calmodulin-insensitive cyclic nucleotide phosphodiesterase. The inhibitor not only affects the activation of Xenopus PDE I and of the bovine brain phosphodiesterase by calmodulin, but also inhibits the stimulation of these enzymes by lysophosphatidylcholine. The inhibitor also acts on PDE I activated by partial tryptic proteolysis, but the enzyme fully activated by trypsin is only slightly susceptible to inhibition by this protein. The inhibition of PDE I activation caused by this ovarian factor can be reversed by adding excess amounts of calmodulin or lysophosphatidylcholine. The presence of this inhibitor provides a possible explanation for the previously observed inactivity of PDE I in vivo.

Dephosphorylation of microtubule-associated protein 2, tau factor, and tubulin by calcineurin

Calcineurin dephosphorylated microtubule-associated protein 2 (MAP2) and tau factor phosphorylated by cyclic AMP-dependent and Ca2+, calmodulin-dependent protein kinases from the brain. Tubulin, only phosphorylated by the Ca2+, calmodulin-dependent protein kinase, served as substrate for calcineurin. The concentrations of calmodulin required to give half-maximal activation of calcineurin were 21 and 16 nM with MAP2 and tau factor as substrates, respectively. The Km and Vmax values were in ranges of 1-3 microM and 0.4-1.7 mumol/mg/min, respectively, for MAP2 and tau factor. The Km value for tubulin was in a similar range, but the Vmax value was lower. The peptide map analysis revealed that calcineurin dephosphorylated MAP2 and tau factor universally, but not in a site-specific manner. The autophosphorylated Ca2+, calmodulin-dependent protein kinase was not dephosphorylated by calcineurin. These results suggest that calcineurin plays an important role in the functions of microtubules via dephosphorylation.

Changes in the subcellular distribution of calmodulin-kinase II during brain development

Subcellular fractions prepared from rodent forebrain at different postnatal ages were examined for calmodulin-binding proteins using [125I]calmodulin and a gel overlay technique. Synaptic junction (SJ) fractions from newborn brain, which display purity comparable to adult SJ fractions, contain low but detectable amounts of 60 and 50 kdalton calmodulin-binding polypeptides; the latter being the major postsynaptic density protein. These polypeptides have recently been shown to be the calmodulin-binding protein subunits of calmodulin-dependent protein kinase II (CaM-kinase II). CaM-kinase II polypeptides represented the predominent calmodulin-binding proteins in nearly every subcellular fraction examined, regardless of postnatal age. Large increases were observed in the CaM-kinase II content of every subcellular fraction throughout postnatal development. During development, a striking shift in the subcellular distribution of CaM-kinase Ii was observed. Over 4 times as much CaM-kinase II was cytosolic relative to particulate in newborn brain while this ratio was completely reversed in adult brain. Large age-dependent increases in particulate-associated CaM-kinase II were observed in highly purified synaptic plasma membrane (5-fold) and SJ (14-fold) fractions. The CaM-kinase II content of SJ fractions increased approximately 70% between days 24 and 90, a period in development that follows the most active stages of synapse formation in situ. In adult brain, approximately 60% of CaM-kinase II in crude synaptosomal fractions (P2-INT) was recovered in SJ fractions. The CaM-kinase II in SPM fractions from all developmental ages resists solubilization in Triton X-100 and greater than 90% is recovered in SJ fractions. These studies indicate that during brain development the accumulation of SJ-associated CaM-kinase II represents an important process in the molecular and enzymatic maturation of CNS postsynaptic structures.

Activation of brain calcineurin towards proteins containing Thr(P) and Ser(P) by Ca2+, calmodulin, Mg2+ and transition metal ions

Using Thr(P)-inhibitor-1 and Ser(P)-casein as substrates, studies on the activation of calcineurin purified from bovine brain have been carried out. The phosphatase requires the synergistic action of Ca2+, calmodulin and another divalent cation (Mg2+, Mn2+, Co2+ or Ni2+, but not Zn2+) for full expression of its activity. Ca2+ and Ca2+ X calmodulin act as allosteric activators to transform the phosphatase to a relaxed conformation, while Mg2+ acts solely as a cofactor for the catalytic action of the enzyme. In addition to their function as cofactors for catalysis, transition metal ions can also substitute for Ca2+ as allosteric activators. Ca2+ and calmodulin exert their activating effects mainly by increasing the Vm of the phosphatase reaction with little effect on the Km values for the substrates or on the KA values for the divalent cation cofactors. The predominant factor in dictating the catalytic properties of calcineurin is the divalent cation cofactor. For example, with Mg2+ as a cofactor, the phosphatase exhibits an optimum around pH 8.0-8.5; while with a transition metal ion as a cofactor, the optimum is around pH 7.0-7.5, regardless of whether Thr(P)-inhibitor-1 or Ser(P)-casein serves as a substrate, in the absence or the presence of Ca2+ X calmodulin.

Initiation of DNA synthesis in quiescent Swiss 3T3 and 3T6 cells by lanthanum

La3+ stimulated quiescent Swiss 3T3 and 3T6 cells to enter the DNA-synthesizing S phase of the cell cycle. La3+ and insulin interacted synergistically to increase DNA synthesis. A brief exposure of the cells to soluble LaCl3 optimally stimulated entry into S. La3+ was similar to A13+ in its mitogenic properties (J. Cell. Physiol. 118, 298, 1984), but La3+ was 10 times more potent than A13+.

Calmodulin-binding proteins: visualization by 125I-calmodulin overlay on blots quenched with Tween 20 or bovine serum albumin and poly(ethylene oxide)

To streamline detection of calmodulin-binding proteins, blotting techniques for the electrophoretic transfer of proteins onto nitrocellulose filters, followed by overlay with 125I-calmodulin, have been adapted. Autoradiography of the 125I-calmodulin-labeled blots allows the identification and quantitation of proteins that possess affinity for calmodulin. Five protocols for suppressing nonspecific binding and for enhancing specific interactions of 125I-calmodulin with electrophoretically separated proteins were investigated. Tween 20 and bovine serum albumin alone, as well as combinations of bovine serum albumin and poly(ethylene oxide) or hemoglobin and gelatin, were evaluated as quenching and enhancing agents. Tween 20 proved highly effective for quenching nonspecific binding and for enhancing specific 125I-calmodulin binding of a 61,000-Mr rat brain protein, which was only faintly observed on blots quenched with proteins alone. However, Tween 20 dissociated 50% of 68,000-Mr proteins and 80% of 21,000-Mr 125I-labeled protein standards from the nitrocellulose filter. An alternative, the combination of bovine serum albumin followed by incubation with 15,000- to 20,000-Mr poly(ethylene oxide), proved satisfactory for the recovery of 61,000-Mr calmodulin-binding activity and for the detection of calmodulin-binding peptides (50,000 to 14,000 Mr) produced by limited proteolysis of rat brain 51,000-Mr calmodulin-binding protein. These blotting procedures for detection of calmodulin-binding proteins are compatible with a variety of one-dimensional and two-dimensional electrophoresis systems, including a two-dimensional electrophoresis system utilizing urea and sodium dodecyl sulfate in the first dimension and nonurea sodium dodecyl sulfate electrophoresis in the second, a system which proved useful for resolving calmodulin-binding proteins displaying anomalous electrophoretic migration in the presence of urea.

Characterization of bovine brain calmodulin-dependent protein phosphatase

Calmodulin-dependent protein phosphatase of bovine brain exhibited a pH optimum of 7 and appeared to require sulfhydryl groups for activity. Phosphatase activity was inhibited by both NaF and ZnCl2, but was stimulated approximately 2-fold by MnCl2. The enzyme exhibited broad substrate specificity, dephosphorylating casein, troponin I, protamine, histone, and phosvitin, and was not phosphorylated by cAMP-dependent protein kinase. With 32P-labeled casein as a substrate, phosphatase was activated 15-fold by calmodulin; the dissociation constant of phosphatase for calmodulin was 30 nM. Activation of the enzyme by calmodulin as a function of Ca2+ was highly cooperative; the Hill coefficient was 4.9. At a saturating concentration of calmodulin, half-maximal activation of phosphatase was obtained at 0.3 microM Ca2+. Calmodulin increased the Vmax from 1.7 to 41 nmol mg protein-1 min-1 with no significant change in its Km. Formation of a Ca2+-dependent complex between calmodulin and the phosphatase was demonstrated by a calmodulin-Sepharose affinity column, gel-filtration chromatography, and sedimentation on a sucrose density gradient. The rate of formation and dissociation of the calmodulin X phosphatase complex was rapid and readily reversible in response to changes in Ca2+ concentration. The calmodulin X phosphatase complex consists of 1 mol of calmodulin and 1 mol of phosphatase.

Characterization of phosphotyrosyl-protein phosphatase activity associated with calcineurin

Calcineurin purified from bovine brain is shown to possess phosphotyrosyl -protein phosphatase activity towards proteins phosphorylated by the epidermal growth factor receptor/kinase. The phosphatase activity is augmented by Ca2+/calmodulin or divalent cation (Ni2+ greater than Mn2+ greater than Mg2+ greater than Co2+). In the simultaneous presence of all three effectors, the enzymatic activity is synergistically increased. Ca2+/calmodulin activates the Mg2+-supported activity by decreasing the Km value for phosphotyrosyl -casein from 2.2 to 0.6 microM, and increasing the Vmax from 0.4 to 4.6 nmol/min/mg. These results represent the first demonstration that calcineurin can dephosphorylate phosphotyrosyl -proteins and suggest a novel mechanism of activation of this enzyme.

Calmodulin binding proteins of the cholinergic electromotor synapse: synaptosomes, synaptic vesicles, receptor-enriched membranes, and cytoskeleton

Calmodulin binding proteins (CBPs) have been identified using a gel overlay technique for fractions isolated from Torpedo electromotor nerve endings. Different fractions possessed characteristic patterns of CBPs. Synaptosomes showed five major CBPs--Mr 220,000, 160,000, 125,000, 55,000, and 51,000. Polypeptides of Mr 55,000 and 51,000 were found in the cytoplasm and the others are membrane-associated. The Triton X-100-insoluble cytoskeleton of synaptosomes was isolated in the presence or absence of calcium. The major CBPs had Mr of 19,000, 18,000, and 16,000. In the presence of calcium, no other CBPs were seen. In the absence of calcium, an Mr 160,000 polypeptide was present in the Triton cytoskeleton. Synaptic vesicles showed CBPs of Mr 160,000, 25,000, and 20,000. Membrane fragments enriched in acetylcholine receptors contained two major CBPs, Mr 160,000 and 125,000, together with a less prominent protein at Mr 26,000. A protein of Mr similar to that of fodrin was present in synaptosomes and acetylcholine receptor membrane fragments, but only in small amounts relative to the other polypeptides observed. The heavy and light chains of clathrin-coated vesicles from pig brain did not bind calmodulin, although strong labelling of an Mr 47,000 polypeptide was found. Results showed that calelectrin does not bind calmodulin. The possible identity of the calmodulin binding proteins is discussed.

Calcineurin is a calcium ion-dependent, calmodulin-stimulated protein phosphatase

A Ca2+-dependent, calmodulin-stimulated protein phosphatase (EC 3.1.3.16) is known to be associated with calcineurin, a major calmodulin binding protein in brain. The protein phosphatase activity has now been shown to be retained by a substrate affinity column (thiophosphorylated myosin P-light chain Sepharose) in the presence of Ca2+, and to be eluted specifically with EGTA. Calcineurin behaved identically. This establishes that calcineurin is the Ca2+-dependent protein phosphatase, and that interaction of Ca2+ with the B-subunit is essential for substrate binding.