8 Phosphoprotein Phosphatases

Tyrosine phosphatases and their possible interplay with tyrosine kinases

Protein tyrosine phosphatases represent a new family of intracellular and receptor-linked enzymes. They are totally specific toward tyrosyl residues in proteins, and, with specific activities 10-1000-fold greater than those of the protein tyrosine kinases, they can be expected to tightly control the level of phosphotyrosine within the cell. Most transmembrane forms contain two conserved intracellular catalytic domains, as displayed by the leukocyte common antigen CD45, but highly variable external segments. Some are related to the neuronal cell adhesion molecules (NCAMs) or fasciclin II and others contain fibronectin III repeats; this suggests that these enzymes might be involved in cell-cell interaction. The intercellular enzymes appear to contain a highly conserved catalytic core linked to a regulatory segment. Deletion of the regulatory domain alters both substrate specificity and cellular localization. Likewise, overexpression of the full-length and truncated enzymes affects cell cycle progression and actin filament stability, respectively. The interplay between tyrosine kinases and phosphatases is considered. A hypothesis is presented suggesting that in some systems phosphatases might act synergistically with the kinases and elicit a physiological response, irrespective of the state of phosphorylation of the target protein.

FLR-4, a novel serine/threonine protein kinase, regulates defecation rhythm in Caenorhabditis elegans

The defecation behavior of the nematode Caenorhabditis elegans is controlled by a 45-s ultradian rhythm. An essential component of the clock that regulates the rhythm is the inositol trisphosphate receptor in the intestine, but other components remain to be discovered. Here, we show that the flr-4 gene, whose mutants exhibit very short defecation cycle periods, encodes a novel serine/threonine protein kinase with a carboxyl terminal hydrophobic region. The expression of functional flr-4::GFP was detected in the intestine, part of pharyngeal muscles and a pair of neurons, but expression of flr-4 in the intestine was sufficient for the wild-type phenotype. Furthermore, laser killing of the flr-4-expressing neurons did not change the defecation phenotypes of wild-type and flr-4 mutant animals. Temperature-shift experiments with a temperature-sensitive flr-4 mutant suggested that FLR-4 acts in a cell-functional rather than developmental aspect in the regulation of defecation rhythms. The function of FLR-4 was impaired by missense mutations in the kinase domain and near the hydrophobic region, where the latter allele seemed to be a weak antimorph. Thus, a novel protein kinase with a unique structural feature acts in the intestine to increase the length of defecation cycle periods.

Susceptibility of the prion protein to enzymic phosphorylation

Ten protein kinases have been assayed for their ability to phosphorylate in vitro the recombinant bovine PrP (25-242) (rbPrP). Substantial phosphorylation was observed with PKC, CK2, and two tyrosine kinases, Lyn and c-Fgr. With regard to CK2, phosphorylation occurs at Ser 154 with a stoichiometry of about 0.1 mol phosphate/mol rbPrP, which is doubled by mild heat treatment of rbPrP. Heat also reduces the overall protein ellipticity, suggesting that reversibly unfolded conformers are more susceptible to phosphorylation. Our data disclose the possibility that phosphorylation might modulate PrP biological activity.

Spatial gradients of cellular phospho-proteins

If a protein is rapidly phosphorylated and dephosphorylated at separate cellular locations and protein diffusion is slow, then a spatial gradient of the phosphorylated form of the protein may develop within the cell. We have estimated the potential size of such gradients using measured values of protein diffusion coefficients and protein kinase and phosphatase activities. We analysed two different cellular geometries: (1) where the kinases is located on the plasma membrane of a spherical cell and the phospatase is distributed homogenously in the cytoplasm and (2) where the kinase is located on one planar membrane and the phosphatase on a second parallel planar membrane. The estimated gradients of phospho-proteins were potentially very large, which has important implications for cellular signalling.

Structure and function of the low Mr phosphotyrosine protein phosphatases

Phosphotyrosine protein phosphatases (PTPases) catalyse the hydrolysis of phosphotyrosine residues in proteins and are hence implicated in the complex mechanism of the control of cell proliferation and differentiation. The low Mr PTPases are a group of soluble PTPases displaying a reduced molecular mass; in addition, a group of low molecular mass dual specificity (ds)PTPases which hydrolyse phosphotyrosine and phosphoserine/threonine residues in proteins are known. The enzymes belonging to the two groups are unrelated to each other and to other PTPase classes except for the presence of a CXXXXXRS/T sequence motif containing some of the catalytic residues (active site signature) and for the common catalytic mechanism, clearly indicating convergent evolution. The low Mr PTPases have a long evolutionary history since microbial (prokaryotic and eukaryotic) counterparts of both tyrosine-specific and dsPTPases have been described. Despite the relevant number of data reported on the structural and catalytic features of a number of low Mr PTPases, only limited information is presently available on the substrate specificity and the true biological roles of these enzymes, in prokaryotic, yeast and eukaryotic cells.

Stimulation of phosphorylase phosphatase activity of protein phosphatase 2A1 by protamine is ionic strength dependent and involves interaction of protamine with both substrate and enzyme

The effects of protamine on the phosphorylase phosphatase activity of porcine cardiac protein phosphatase 2A1 (PP2A1) were complex and ionic strength dependent. Under ionic strength conditions that protamine activation was optimal, activation of PP2A1 by either dilution or heparin was prevented. A time-dependent deactivation of the protamine-stimulated phosphatase activity was observed when PP2A1 was preincubated with protamine. Protamine forms a very tight association with phosphorylase a, which is optimal at a 1:1 protamine:phosphorylase a monomer molar ratio. Protamine activation of PP2A1 activity, however, is not substrate-directed since the basic polypeptide did not stimulate either the activity of the catalytic subunit or trypsinolysis of [32P]phosphorylase a. The interaction of protamine with phosphorylase a does not apparently involve the phosphorylation site in the protein substrate (ser 14). The activation of PP2A1 by protamine is proposed to involve part of the basic polypeptide, not associated with phosphorylase a monomer, interacting with the regulatory and/or the catalytic subunit(s) of the phosphatase. A minimal model for the activation of PP2A1 by protamine was tested kinetically. In this model, free PP2A1 binds with decreasing affinities to the protamine:phosphorylase a complex, free phosphorylase a, and free protamine. Protamine decreases the K(m) of PP2A1 for the phosphorylase a monomer 5-fold and increases the Vmax 17-fold. Interaction of free protamine with PP2A1 inhibits the phosphatase activity.

Inhibition of protein phosphatase-2B (calcineurin) activity towards Alzheimer abnormally phosphorylated tau by neuroleptics

Abnormally hyperphosphorylated tau is the major protein component of neurofibrillary tangles, the characteristic lesion of Alzheimer's disease (AD). Protein phosphatases (PP) type 1 (PP-1), type 2A (PP-2A) and type 2B (PP-2B) appear to be involved in the regulation of tau phosphorylation. The incidence of neurofibrillary tangles is higher in brains of schizophrenic patients treated with neuroleptics than in those without this treatment. We have found that the commonly used neuroleptics chlorpromazine, trifluoperazine and clozapine inhibit PP-2B but not PP-1 or PP-2A activity towards [32P]phosphorylase kinase as a substrate. When AD abnormally hyperphosphorylated tau is used as a substrate, PP-2B activity is inhibited by trifluoperazine > chlorpromazine > clozapine. Using phosphorylation-dependent monoclonal antibodies, tau-1, AT8 and PHF-1, we have found that the dephosphorylation of the abnormal tau by PP-2B is inhibited at all the sites recognized by these antibodies. The IC50 of the inhibition of dephosphorylation at tau-1 site is approximately 20 microM for trifluoperazine and approximately 120 microM for chlorpromazine. These two neuroleptics inhibit tau dephosphorylation by PP-2B through antagonizing calmodulin as well as directly interacting with PP-2B. The inhibition of the dephosphorylation of abnormally hyperphosphorylated tau by neuroleptics raises an intriguing possibility that the chronic use of these drugs might contribute to neurofibrillary degeneration in schizophrenic and AD patients.

Degradation of a calcium influx factor (CIF) can be blocked by phosphatase inhibitors or chelation of Ca2+

In many cell types, depletion of Ca2+ stores causes activation of Ca2+ influx by a mechanism whose molecular basis remains unclear. We recently described a new messenger that is released by empty Ca2+ stores and that activates Ca2+ influx in heterologous cells (Randriamampita, C. & Tsien, R. Y. (1993) Nature 364, 809-814). This factor, provisionally named CIF (for Ca2+ influx factor), seems to be a small nonprotein factor possessing a phosphate group. Meanwhile Parekh et al. reported that okadaic acid, an inhibitor of protein phosphatases 1 and 2A, potentiates Ca2+ influx in Xenopus oocytes (Parekh, A. B., Terlau, H. & Stühmer, W. (1993) Nature 364, 814-818). A link between these two observations is presented in this paper. We show that in astrocytoma cells, okadaic acid and cyclosporin A (an inhibitor of calcineurin) both potentiate the Ca2+ elevations due to low doses of CIF, thapsigargin, or carbachol. In lymphocytes, okadaic acid potentiates the Ca2+ elevations due to low doses of phytohemagglutinin and increases the amount of extractable CIF. CIF degradation can be observed in cell-free homogenates of lymphocytes and is prevented by the above phosphatase inhibitors, an effect that can at least partly explain their potentiation of Ca2+ influx. CIF degradation is also prevented by lowering free Ca2+ concentrations, which could be a feedback mechanism to enhance Ca2+ influx when cells are depleted of Ca2+.

Dephosphorylation of Alzheimer's disease abnormally phosphorylated tau by protein phosphatase-2A

Microtubule-associated protein tau is abnormally hyperphosphorylated in the brain of patients with Alzheimer's disease, and is the major protein subunit of paired helical filaments. There is also a significant pool of non-paired helical filament abnormally phosphorylated tau in Alzheimer's disease brain. In the present study, the site-specific dephosphorylation of this Alzheimer's disease abnormally phosphorylated tau by protein phosphatase-2A was studied and compared with that by protein phosphatase-2B. The dephosphorylation was detected by its interaction with several phosphorylation-dependent antibodies to various abnormal phosphorylation sites. Protein phosphatase-2A was able to dephosphorylate the abnormally phosphorylated tau at Ser-46, Ser-199, Ser-202, Ser-396 and Ser-404, but not at Ser-235 (the amino acids are numbered according to the largest isoform of human tau, tau441). Two major types of protein phosphatase-2A, protein phosphatase-2A1 and -2A2, dephosphorylated the abnormally phosphorylated tau at approximately the same rate. After the abnormally phosphorylated tau was dephosphorylated by protein phosphatase-2A, its relative mobility on sodium dodecyl sulfate-polyacrylamide gel electrophoresis increased. The dephosphorylation of the abnormal tau by protein phosphatase-2A1 and -2A2 was markedly stimulated by Mn2+. These results suggest that tau dephosphorylation is catalysed by protein phosphatase-2A in addition to protein phosphatase-2B. A deficiency of either protein phosphatase-2A or -2B, or both, may be involved in abnormal phosphorylation of tau in Alzheimer's disease.

Dephosphorylation of microtubule-associated protein tau by protein phosphatase-1 and -2C and its implication in Alzheimer disease

Microtubule-associated protein tau is abnormally hyperphosphorylated and forms the major protein subunit of paired helical filaments (PHF) in Alzheimer disease brains. The abnormally phosphorylated sites Ser-199, Ser-202, Ser-396 and Ser-404 but not Ser-46 and Ser-235 of Alzheimer tau were found to be dephosphorylated by protein phosphatase-1 and this dephosphorylation was activated by Mn2+. In contrast, protein phosphatase-2C did not dephosphorylate any of these sites. Both protein phosphatase-1 and -2C had high activities towards [32P]tau phosphorylated by cAMP-dependent protein kinase. These results suggest that both protein phosphatase-1 and -2C might be associated with normal phosphorylation state of tau, but only the former and not the latter phosphatase is involved in its abnormal phosphorylation in Alzheimer disease.

Phosphatase 2A associated with polyomavirus small-T or middle-T antigen is an okadaic acid-sensitive tyrosyl phosphatase

Papovavirus tumor antigens have been shown to associate with the cellular phosphoserine/threonine-specific protein phosphatase 2A (PP2A). We were interested in the consequences that T-antigen association might have on PP2A activity and so studies of the phosphatase activity in immunoprecipitates, prepared from polyoma virus-transformed or polyoma virus-infected mouse 3T3 fibroblasts, were performed. The phosphoserine/threonine phosphatase activity, measured with phosphorylase a as the substrate, showed all the characteristics of PP2A. It was stimulated by polycations, inhibited by fluoride or p-nitrophenyl phosphate, sensitive to okadaic acid and microcystin and insensitive to inhibitor-1 and inhibitor-2. Phosphotyrosyl phosphatase (PTPase) activity was associated with the middle-T/small-T-associated complex when reduced, carboxamidomethylated and maleylated lysozyme, phosphorylated exclusively on tyrosyl residues, was used as the substrate. This PTPase activity was as sensitive to okadaic acid as was the phosphorylase phosphatase activity; it could be inhibited by phosphorylase a and did not dephosphorylate poly(Glu80Tyr20). The level of middle-T/small-T-associated PTPase activity relative to the phosphorylase phosphatase activity was tenfold higher than that of the purified dimeric PP2A. A similar activity ratio was observed with the purified phosphatase after stimulation with a cellular protein, designated phosphotyrosyl phosphatase activator. These results suggest that the same enzyme may possess dual specificity. In contrast to the cellular trimeric PP2A, containing the 55-kDa putative regulatory subunit, the middle-T/small-T-associated enzyme had low activity towards a retinoblastoma peptide phosphorylated by p34cdc2. These results indicate how middle-T/small-T might effect the activity of PP2A in polyoma virus-transformed cells.

Reactive oxygen species are involved in the activation of cellular phospholipase A2

Vanadate (V) potentiated (4- to 10-fold) the activation of cellular phospholipase A2 (PLA2) induced by H2O2 (H), a phorbol ester (T), a Ca(2+)-ionophore (A) and opsonized zymosan in macrophages. V+H induced in intact cells the activation and translocation of PLA2 and protein kinase C (PKC) to the plasma membrane. V+H and V+T+A induced strong chemiluminescence (CL) which was abrogated by a specific NADPH oxidase inhibitor diphenylene iodonium (DPI). DPI markedly suppressed the stimulation of PLA2 by V+T+A and V+OZ. The results suggest that the formation of endogenous reactive oxygen species (ROS) is important for PLA2 activation.

Purification and characterization of porcine heart type 2A protein phosphatases

Protein phosphatases 2A1 and 2A2 were isolated from porcine heart tissue extracts by precipitation at pH 5.0 and separated by chromatography on DEAE-Sephacel. Phosphatase 2A1 was then purified to apparent homogeneity by chromatography on phenyl-Sepharose, aminohexyl-Sepharose, Sephacryl S-300, and L-tyrosine-agarose. Phosphatase 2A2 was purified to apparent homogeneity by chromatography on phenyl-Sepharose, DEAE-Sephacel, aminohexyl-Sepharose and L-tyrosine-agarose. Purified phosphatases 2A1 and 2A2 had specific activities of 2200 and 2710 nanomoles of phosphate released from phosphorylase a/mg protein, respectively. The apparent molecular weights of phosphatases 2A1 and 2A2 on gel filtration were 155 and 105 kDa, respectively. Both enzymes contain 70 and 37 kDa subunits and 2A1 also contains a 57 kDa subunit. The 37 kDa catalytic subunit (2Ac) was obtained from the purified phosphatases by treatment with room temperature ethanol followed by sucrose density gradient centrifugation or gel filtration chromatography.

Purification of porcine heart latent protein phosphatase Fc.M

Latent protein phosphatase, Fc.M, was purified from porcine heart extracts by a procedure involving precipitation at pH 5.0, DEAE-Sephacel chromatography, ammonium sulfate fractionation, chromatography on phenyl-Sepharose, Biogel-A 0.5m and poly-L-lysine-agarose. The purified enzyme had a specific activity of 12,200 nanomoles of phosphate released from phosphorylase a/mg protein when assayed following activation by pretreatment with Mn++ and trypsin in the presence of 0.2 M NaCl. The enzyme is a heterodimer of 66 kDa composed of a catalytic (37 kDa) and a modulator (31 kDa) subunit.

Protein tyrosine phosphatases: a diverse family of intracellular and transmembrane enzymes

Protein tyrosine phosphatases (PTPs) represent a diverse family of enzymes that exist as integral membrane and nonreceptor forms. The PTPs, with specific activities in vitro 10 to 1000 times greater than those of the protein tyrosine kinases would be expected to effectively control the amount of phosphotyrosine in the cell. They dephosphorylate tyrosyl residues in vivo and take part in signal transduction and cell cycle regulation. Most of the transmembrane forms, such as the leukocyte common antigen (CD45), contain two conserved intracellular catalytic domains; but their external segments are highly variable. The structural features of the transmembrane forms suggest that these receptor-linked PTPs are capable of transducing external signals; however, the ligands remain unidentified. A hypothesis is proposed explaining how phosphatases might act synergistically with the kinases to elicit a full physiological response, without regard to the state of phosphorylation of the target proteins.

Characterization of a membrane-associated phosphotyrosyl protein phosphatase from the A431 human epidermoid carcinoma cell line

The A431 human epidermoid carcinoma cell line exhibits a 30-100-fold overexpression of the epidermal growth factor (EGF) receptor. We have characterized a membrane-associated phosphotyrosyl-protein phosphatase (PTPase) in these cells since it seemed reasonable that overexpression of the EGF-receptor tyrosine kinase will be matched by high PTPase activity. Indeed, of 12 cell lines tested, the A431 cells had the highest specific PTPase activity. About 70% of the total cellular PTPase activity was found associated with membranes after cell fractionation. The membrane-associated PTPase was hydrophobic as judged by its behaviour in Triton X-114 phase partitioning. High-performance liquid chromatography (HPLC) on a DEAE column revealed a single, homogeneous species of membrane-associated PTPase with an apparent molecular mass of 43 kDa as determined by HPLC on a gel permeation column in the presence of Triton X-100. Comparison of this PTPase with the membrane-associated PTPase activities present in rat spleen and in the human chronic myelogenous leukemia cell line K562 revealed additional species resolvable by DEAE-HPLC. These findings suggest that cells may possess different PTPase activities depending on their growth and differentiation states.

Dephosphorylation of the human T lymphocyte CD3 antigen

Previous studies demonstrated that activation of T lymphocytes by phorbol ester or mitogenic lectin leads to phosphorylation of Ser 126 of the CD3 antigen gamma chain, whereas treatment with ionomycin results in phosphorylation of both Ser 123 and 126 [Davies, A. A. et al. (1987) J. Biol. Chem. 262, 10918-10921]. In the present study, the dephosphorylation of Ser 123 and Ser 126 of the gamma chain was investigated. Phorbol-ester-induced phosphorylation of the gamma-chain Ser 126 in vivo was reversed following removal of phorbol ester. Dephosphorylation of both Ser 123 and 126 was also observed in vitro using the microsome fraction of T lymphocytes. In order to identify the phosphatases acting at these two sites, the immunoprecipitated gamma chain was used as substrate either following treatment with protein kinase C in vitro, in which case phosphorylation occurs mainly at Ser 123, or following in vivo phosphorylation of Ser 126. Purified oligomeric forms of the polycation-stimulated phosphatases were more effective in dephosphorylating both phosphorylated forms of the gamma chain compared with equivalent amounts of ATP,Mg2+-dependent phosphatases or calcineurin. By using phosphopeptide analogues of the CD3 gamma chain containing Ser 123 or Ser 126 as substrates (A3 and A6), it was shown that polycation-stimulated phosphatases selectively dephosphorylated Ser 123 compared to Ser 126. In order to determine which phosphatases dephosphorylate the gamma chain in microsomes, A3 and A6 were used as substrates for characterising phosphatases in microsomes from human T leukaemia Jurkat 6 cells. Three phosphopeptide phosphatases (250-400 kDa) co-eluted through five purification steps with three forms of polycation-stimulated phosphorylase phosphatase. The partially purified A3/A6 phosphopeptide phosphatases were insensitive to Ca2+, calmodulin and inhibitor-1, and dephosphorylated A3 preferentially compared with A6. A latent form of microsomal ATP,Mg2+-dependent phosphorylase phosphatase was stimulated 10-fold by trypsinisation, but did not dephosphorylate phosphopeptides A3 and A6. The results show that high-Mr forms of polycation-stimulated phosphatases are the only enzymes in human T leukaemia cell microsomes which dephosphorylate gamma chain phosphopeptides. The data point to an important role for polycation-stimulated phosphatases in regulating the phosphorylation state, and so function(s), of the CD3 antigen.

Evidence for a latent form of protein phosphatase 1 associated with cardiac myofibrils

Detergent-purified myofibrils from bovine heart contained very little spontaneously active protein phosphatase 1 activity. Phosphatase 1, extracted from the myofibrils by freeze-thawing in the presence of 500 mM KCl, was markedly activated by cobalt/trypsin treatment. Myofibril phosphatase 1 was separated from phosphatase 2A by chromatography on heparin-Sepharose. The phosphatase 1 was isolated in a latent form. Pretreatment with trypsin released free catalytic subunit and increased activity about 25-fold. Addition of cobalt with the trypsin increased activity another 2-fold. The latent myofibril phosphatase 1 did not appear to be the same as previously characterized forms of protein phosphatase 1. We suggest that cardiac myofibril phosphatase 1 contains a unique inhibitory subunit which directs the enzyme to the myofibril and regulates the dephosphorylation of myofibril phosphoproteins.

Tissue-specific expression of mRNAs encoding alpha- and beta-catalytic subunits of protein phosphatase 2A

The mRNAs encoding the catalytic subunits of the type 1 phosphatase and the alpha and beta forms of phosphatase 2A (termed 2A alpha and 2A beta) were detected in all the porcine tissues examined (brain, heart, muscle, liver, kidney and ovaries). Total phosphatase 2A transcripts were about 10 times more abundant in brain and heart than in the other tissues while the distribution of phosphatase 1 mRNA showed significantly less variation. Phosphatase 2A alpha mRNA was about 10 times more abundant than phosphatase 2A beta mRNA in most tissues. Similarly, both the alpha and beta forms of the mRNA of the catalytic subunit of the phosphatase 2A were present in all the rat tissues examined. In addition to the 2 kb mRNA generally expressed, rat tissues also express a 2.8 kb mRNA related to 2A alpha and a 1.3 kb mRNA related to 2A beta.

Protein phosphorylation and hormone action

Many key regulatory proteins exist in cells as either a phosphorylated or a dephosphorylated form, their steady-state levels of phosphorylation reflecting the relative activities of the protein kinases and protein phosphatases that catalyse the interconversion process. Phosphorylation of seryl or threonyl (and occasionally tyrosyl) residues triggers small conformational changes in these proteins that alter their biological properties. Hormones and other extracellular signals transmit information to the interior of the cell by activating transmembrane signalling systems that control the production of a relatively small number of chemical mediators, termed 'second messengers'. These substances regulate the activities of protein kinases and phosphatases, and so alter the phosphorylation states of many intracellular proteins, accounting for the diversity of action of hormones. In this lecture I review recent work which demonstrates that a wide variety of cellular processes are controlled by relatively few protein kinases and protein phosphatases with pleiotropic actions. These enzymes provide the basis of an interlocking network that allows extracellular signals to coordinate biochemical functions.

Modulation of the substrate specificity of the polycation-stimulated protein phosphatase from Xenopus laevis oocytes

A polycation-stimulated (PCS) protein phosphatase was isolated in high yield (280 micrograms/100 g ovaries) from Xenopus laevis oocytes through a procedure involving a tyrosine-agarose hydrophobic chromatography. The 220-kDa enzyme contains a 35-kDa and a 62-kDa subunit. It was identified as the low-Mr polycation-stimulated (PCSL) protein phosphatase. The labile p-nitrophenyl phosphatase activity, copurifying with the phosphorylase phosphatase activity, can be increased severalfold by preincubating the purified enzyme with ATP, its analogues or PPi. This activation is time-dependent and accompanied by a parallel decrease of the phosphorylase phosphatase activity. Although the stimulation was antagonized by metal ions during the preincubation, the basal and ATP-stimulated p-nitrophenyl phosphatase requires Mg2+ or Mn2+ in the assay, with pH optima of 8.5-9 and 7.5 respectively.

Effects of intracellular free magnesium on calcium current in isolated cardiac myocytes

Magnesium ions play a fundamental role in cellular function, but the effects of changes in the concentration of intracellular ionized magnesium ([Mg2+]i) on cell physiology have only recently received experimental attention. Increasing [Mg2+]i from 0.3 to 3.0 mM in cardiac cells by internal perfusion has only small effects on the basal voltage-gated calcium current (ICa) or on ICa elevated by dihydropyridine calcium channel agonists. In contrast, ICa elevated by cyclic adenosine monophosphate (cAMP)-dependent phosphorylation decreases by more than 50 percent. The effect of [Mg2+]i is not due to changes in the concentration of cAMP or in the velocity of phosphorylation but rather appears to be a direct effect on the phosphorylated channel or on channel dephosphorylation.