The protein phosphatases involved in cellular regulation. Evidence that dephosphorylation of glycogen phosphorylase and glycogen synthase in the glycogen and microsomal fractions of rat liver are catalysed by the same enzyme: protein phosphatase-1

Identification of a third form of protein phosphatase 1 in rabbit skeletal muscle that is associated with myosin

A third form of protein phosphatase 1 has been identified in skeletal muscle which is distinct from the species composed of the catalytic subunit complexed to the glycogen-binding subunit (protein phosphatase 1G) or inhibitor-2 (protein phosphatase 1I). The third form has an apparent molecular mass of 110 kDa, is not immunoprecipitated by antibody prepared against the glycogen-binding subunit, does not interact with glycogen and is devoid of inhibitor-2. It is tightly bound to myosin and is therefore termed protein phosphatase 1M.

Regulation of the dephosphorylation of phosphorylase A by glucose, AMP and polyamines

1. The effect of glucose, caffeine, AMP and polyamines was investigated on the dephosphorylation of phosphorylase a by the catalytic subunits of protein phosphatase-1 and -2A. 2. Caffeine at 1-20 microM inhibited the dephosphorylation of the dimeric phosphorylase a at 37 degrees C using skeletal muscle enzymes; 0.1-10 mM of caffeine enhanced the rate of dephosphorylation greatly at 13 degrees C and slightly at 37 degrees C. 3. alpha-D-Glucose was more effective in accelerating both the dephosphorylation and the tryptic digestion of phosphorylase a than the beta-anomer. 4. Polyamines were found to moderate the inhibitory effect of AMP at concentrations which may occur in the tissues. In the presence of 5 mM glucose polyamines could cancel the AMP inhibition of the dephosphorylation of liver phosphorylase a by hepatic protein phosphatase-1 and -2A.

Phosphorylase phosphatase from skeletal muscle membranes

Microsomes containing 12-15 U/mg phosphorylase phosphatase were obtained from skeletal muscle glycogen particles following glycogen digestion and differential centrifugation. The phosphatase associated with the membranes is in an inhibited state; dilution induces dissociation and deinhibition of the enzyme. Phosphatase-depleted membranes can rebind purified phosphatase catalytic subunit but not the complex between catalytic subunit and inhibitor 2. Binding involves a receptor, deduced from saturation phenomena, which is responsible for inhibition of the bound enzyme and which is a protein, since trypsin treatment releases all bound enzyme and prevents rebinding. The phosphatase extracted from the membranes is of type 1 and is a mixture of complexes, the major ones displaying a Mr of 300,000 and 70,000. From these complexes the 35-kDa catalytic subunit can be obtained either by trypsin treatment or by acetone precipitation. Purification to homogeneity involves chromatography on polylysine and FPLC chromatography on Mono Q and Polyanion SI columns. The purified enzyme exhibits a specific activity of 26,800 U/mg (27,900 U/mg after trypsin treatment) and consists of a major protein of 38 kDa (SDS gel electrophoresis). A minor component of 33 kDa, which may represent either a proteolytic product or an isozyme, can be separated. Both 38-kDa and 33-kDa catalytic subunits form a 70-kDa inactive complex with inhibitor 2 and upon incubation of the complexes the catalytic subunit is slowly converted to the inactive conformation which can then be reactivated by either the kinase FA or trypsin and Mn2+. Alternatively the inactive catalytic subunit is reactivated by Mn2+ alone once it has been isolated by FPLC chromatography on SI. The observation that the same catalytic subunit is present at various cell locations (namely cytosol, glycogen particles and microsomes), though in different conformations, is in favour of the hypothesis that displacement of the catalytic subunit from one cell site to the other may represent a new mechanism for phosphatase regulation in skeletal muscle.

The ATP,Mg-dependent protein phosphatase: regulation by casein kinase-1

The free modulator subunit of the ATP,Mg-dependent phosphatase is phosphorylated up to 1 mol per mol by casein kinase-1, up to 1.85 mol per mol after dephosphorylation by the PCSH1 phosphatase, but 10-fold less when purified in the presence of NaF, suggesting an in vivo phosphorylation of the casein kinase-1 sites. Peptide mapping of 32P-modulator labeled by casein kinase-1 or -2 shows a different phosphorylation pattern. Phosphorylation of the inactive phosphatase by casein kinase-1 prevents the subsequent kinase FA-mediated activation, while it does not impair the activated phosphatase.

Protein phosphatases of the guinea-pig parotid gland

The nature of protein phosphatases of the guinea-pig parotid gland was investigated. The protein phosphatases were characterized by (a) the use of five different 32P-labelled substrate proteins (phosphorylase a, histone H2B, casein, and the alpha and beta subunits of phosphorylase kinase), (b) their behaviour during ion-exchange chromatography, (c) their relative molecular mass distribution during gel filtration, (d) their sensitivity towards inhibition by inhibitor 2, (e) their ability to be stimulated by protamine and (f) by their behaviour during freezing and thawing in the presence of 2-mercaptoethanol. The following results were obtained. 1. The 'cytosol' (100,000 X g supernatant) contains protein phosphatases of the types 1, 2A and 2B. 2. On the basis of inhibition with inhibitor 2 (1.2 micrograms/ml) the 'cytosolic' phosphorylase phosphatase activity consists to about 40% of protein phosphatase 1 and to about 60% of protein phosphatase 2A. 3. In the cytosol about 80-90% of the protein phosphatases 1 and 2A exist in an inactive state. 4. A 5-10-fold activation can be achieved by ethanol precipitation, which results in the generation of a mixture of forms of low apparent molecular mass of about 30 kDa. 5. Microsome-associated phosphorylase phosphatase activities can be extracted in a highly active state by detergent (1% Triton X-100) or by 0.8 M NaCl. 6. Activity measurements in the presence of inhibitor 2 (1.2 micrograms/ml) indicate that the microsomal activities consist to about 75% of protein phosphatase 1 and to about 25% of protein phosphatase 2A. Activities corresponding to protein phosphatases 2B and 2C could not be detected. 7. The 'microsomal' protein phosphatase activities exhibit lower apparent molecular masses (70 kDa and 30 kDa) than the 'cytosolic' protein phosphatases (about 260 kDa). 8. After ethanol treatment of the microsomal protein phosphatases only activities with apparent molecular masses of about 30 kDa can be detected. These share several similarities with the ethanol-treated cytosolic protein phosphatases. 9. Both cytosolic and microsomal protein phosphatases display activity towards histone H2B and casein.

High-affinity binding of glycogen-synthase phosphatase to glycogen particles in the liver. Role of glycogen in the inhibition of synthase phosphatase by phosphorylase a

1. Post-mitochondrial supernatants were prepared from the livers of 24 h-fasted rats. Upon centrifugation at high speed, the major part of the glycogen-synthase phosphatase activity sedimented with the microsomal fraction. However, two approaches showed that the enzyme was associated with residual glycogen rather than with vesicles of the endoplasmic reticulum. Indeed, the activity was entirely solubilized when the remaining glycogen was degraded either by glucagon treatment in vivo or by alpha-amylolysis in vitro. No evidence could be found for an association of glycogen-synthase phosphatase with the smooth endoplasmic reticulum, as isolated with the use of discontinuous sucrose gradients. 2. After solubilization by glucagon treatment in vivo, synthase phosphatase could be transferred to glycogen particles with very high affinity. Half-maximal binding occurred at a glycogen concentration of about 0.25 mg/ml, whereas glycogen synthase and phosphorylase required 1.5-2 mg/ml. 3. In gel-filtered extracts prepared from glycogen-depleted livers, the activation of glycogen synthase was not inhibited at all by phosphorylase alpha. The inhibition was restored when the liver homogenates were prepared in a glycogen-containing buffer. The effect was half-maximal at a glycogen concentration of about 0.25 mg/ml, and virtually complete at 1 mg/ml. These findings explain long-standing observations that in fasted animals the liver contains appreciable amounts of both synthase and phosphorylase in the active form.

Identification of protein phosphatases-1 and 2A and inhibitor-2 in oocytes of the starfish Asterias rubens and Marthasterias glacialis

Protein phosphatases present in the particulate and soluble fractions of oocytes of the starfish Asterias rubens and Marthasterias glacialis have been classified according to the criteria used for these enzymes from mammalian cells. The major protein phosphatase activity in the particulate fraction had very similar properties to protein phosphatase-1 from mammalian tissues, including preferential dephosphorylation of the beta subunit of phosphorylase kinase, sensitivity to inhibitor-1 and inhibitor-2, inhibition of phosphorylase phosphatase activity by protamine and heparin, and retention by heparin-Sepharose. The major protein phosphatase in the soluble fraction had very similar properties to mammalian protein phosphatase-2A, including preferential dephosphorylation of the alpha subunit of phosphorylase kinase, insensitivity to inhibitors-1 and 2, activation by protamine and heparin, and exclusion from heparin-Sepharose. An acid-stable and heat-stable protein was detected in the soluble fraction of starfish oocytes, whose properties were indistinguishable from those of inhibitor-2 from mammalian tissues. It inhibited protein phosphatase-1 specifically, and its apparent molecular mass on SDS polyacrylamide gels was 31 kDa. Furthermore, an inactive hybrid formed between the starfish oocyte inhibitor and the catalytic subunit of mammalian protein phosphatase-1 could be reactivated by preincubation with MgATP and mammalian glycogen synthase kinase-3. The remarkable similarities between starfish oocyte protein phosphatases and their mammalian counterparts are indicative of strict phylogenetic conservation of these enzymes. The results will facilitate further analysis of the role of protein phosphorylation in the control of starfish oocyte maturation by the hormone 1-methyladenine.

The protein phosphatases of Drosophila melanogaster and their inhibitors

Protein phosphatases-1, 2A and 2B have been identified in membrane and soluble fractions of Drosophila melanogaster heads. Similarities between Drosophila and mammalian protein phosphatase-1 included specificity for the beta subunit of phosphorylase kinase, sensitivity to inhibitor-1 and inhibitor-2, inhibition by protamine, retention by heparin-Sepharose and selective interaction with membranes. In addition, an inactive form of protein phosphatase-1, termed protein phosphatase-1I, was detected in the soluble fraction that could be activated by preincubation with MgATP and mammalian glycogen synthase kinase-3. Inhibitor-2 partially purified from Drosophila had an identical molecular mass to its mammalian counterpart, and recombined with mammalian protein phosphatase-1 to form a hybrid protein phosphatase-1I. Similarities between Drosophila and mammalian protein phosphatase-2A included preferential dephosphorylation of the alpha subunit of phosphorylase kinase, insensitivity to inhibitors-1 and -2, activation by protamine, exclusion from heparin-Sepharose and apparent molecular mass. A Ca2+-dependent calmodulin-stimulated protein phosphatase (protein phosphatase-2B) that was inhibited by trifluoperazine was identified in the soluble fraction. The remarkable similarities between Drosophila protein phosphatases and their mammalian counterparts are indicative of strict phylogenetic conservation and demonstrate that the procedures used to classify mammalian protein phosphatases have a wider application. Characterisation of the Drosophila phosphatases will facilitate genetic analysis of dephosphorylation systems and their possible roles in neuronal and behavioural plasticity in Drosophila.

Increase in liver protein phosphatase-1 in spontaneously diabetic Chinese hamsters

Two broad-specifically protein phosphatases, termed protein phosphatase-1 (PrP-1) and protein phosphatase-2A (PrP-2A), accounting for all the hepatic activity regulating glycogen phosphorylase, were measured in spontaneously diabetic Chinese hamsters exhibiting persistent glycosuria. When compared with genetically related inbred sublines free of glycosuria, diabetic animals demonstrated approximately 25% increase in PrP-1 activity measured either in crude tissue extracts or in cytosols fractionated by ion-exchange chromatography. No significant alteration in total PrP-2A activity was observed in the diabetic animals. These findings indicate that a specific change in hepatic PrP-1 is associated with genetically acquired diabetes in Chinese hamsters. In contrast to reported data using animals with experimentally induced diabetes mellitus, hepatic PrP-1 was increased in the spontaneously diabetic Chinese hamsters. The data suggests that distinct alterations in PrP-1 and associated metabolic consequences are exhibited by different types of diabetes.

Control of glycogen synthesis in health and disease

Investigations in our laboratory have shown that the activity of glycogen synthase phosphatase in the liver is shared by at least two functionally distinct proteins: a G-component, which is tightly associated with glycogen particles, and a soluble S-component. Most preparations of glycogen synthase-b that are isolated from the liver of fed glucagon-treated animals require the presence of both components in order to be converted to synthase-a. The G-component is subject to control mechanisms that do not affect the S-component. Its activity is strongly inhibited by phosphorylase-a. This feature explains why glycogen synthesis and glycogenolysis do not normally occur simultaneously, except in the glycogen-depleted liver, where a futile cycle may occur. Experiments in vitro have shown that a minimal glycogen concentration is required to ensure the interaction between the G-component and phosphorylase-a. The G-component is also selectively inhibited by Ca2+, and the magnitude of this inhibition depends markedly on the glycogen concentration. The latter inhibition is probably one of the mechanisms by which cyclic adenosine monophosphate (cAMP)-independent glycogenolytic agents achieve the inactivation of glycogen synthase in the liver. Glucocorticoid hormones and insulin are required for the induction and/or maintenance of the G-component in the liver. During the development of the fetal rat, glucocorticoids induce the G-component in the liver. This is an essential event in the glucocorticoid-triggered deposition of glycogen in the fetal liver. A functional adrenal cortex is also required in the adult animal to prevent a loss of the capacity for hepatic glycogen storage during starvation. The latter capacity depends on the concentration of functional G-component in the liver. Chronic diabetes causes a similar functional loss. However, the effect of glucocorticoids is not mediated by a putative secretion of insulin.

Identification of high levels of protein phosphatase-1 in rat liver nuclei

Rat liver nuclei contain a protein phosphatase that is indistinguishable from the catalytic subunit of protein phosphatase-1 in its molecular mass, sensitivity to inhibitor-1 and inhibitor-2 and specificity for the beta-subunit of phosphorylase kinase. This activity is not bound to the outer nuclear membrane, but located within the nucleus. The average level of protein phosphatase-1 activity in nuclei is at least 5-fold higher than its average extranuclear concentration.