Two structures of the catalytic domain of phosphorylase kinase: an active protein kinase complexed with substrate analogue and product

BACKGROUND

Control of intracellular events by protein phosphorylation is promoted by specific protein kinases. All the known protein kinase possess a common structure that defines a catalytically competent entity termed the 'kinase catalytic core'. Within this common structural framework each kinase displays its own unique substrate specificity, and a regulatory mechanism that may be modulated by association with other proteins. Structural studies of phosphorylase kinase (Phk), the major substrate of which is glycogen phosphorylase, may be expected to shed light on its regulation.

RESULTS

We report two crystal structures of the catalytic core (residues 1-298; Phk gamma trnc) of the gamma-subunit of rabbit muscle phosphorylase kinase: the binary complex with Mn2+/beta-gamma-imidoadenosine 5'-triphosphate (AMPPNP) to a resolution of 2.6 A and the binary complex with Mg2+/ADP to a resolution of 3.0 A. The structures were solved by molecular replacement using the cAMP-dependent protein kinase (cAPK) as a model.

CONCLUSIONS

The overall structure of Phk gamma trnc is similar to that of the catalytic core of other protein kinases. It consists of two domians joined on one edge by a 'hinge', with the catalytic site located in the cleft between the domains. Phk gamma trnc is constitutively active, and lacks the need for an activatory phosphorylation event that is essential for many kinases. The structure exhibits an essentially 'closed' conformation of the domains which is similar to that of cAPK complexed with substrates. The phosphorylated residue that is located at the domain interface in many protein kinases and that is believed to stabilize an active conformation is substituted by a glutamate in Phk gamma trnc. The glutamate, in a similar manner to the phosphorylated residue in other protein kinases, interacts with an arginine adjacent to the catalytic aspartate but does not participate in interdomain contacts. The interactions between the enzyme and the nucleotide product of its activity, Mg2+/ADP, explain the inhibitory properties of the nucleotides that are observed in kinetic studies.

Does phosphorylase kinase control glycogen biosynthesis in skeletal muscle?

Immunoblotting as well as enzyme assays demonstrate the presence of the self-glucosylating protein, glycogenin, in the protein-glycogen complex, in the sarcoplasmic reticulum and in phosphorylase kinase. In all three compartments glycogenin occurs in different, albeit, defined glucosylated forms, which upon deglucosylation are converted into a 42 kDa form. We suggest that phosphorylase kinase might have a dual function in glycogen biogenesis: firstly, control of glycogen degradation in the protein-glycogen complex via phosphorylation of glycogen phosphorylase b; secondly, regulation of glycogen biosynthesis on the sarcoplasmic reticular membranes via phosphorylation and thereby inhibition of glycogen synthase.

Identification of the substrate and pseudosubstrate binding sites of phosphorylase kinase gamma-subunit

Using site-directed mutagenesis, we proposed that an autoinhibitory domain(s) is located at the C-terminal region (301-386) of the phosphorylase kinase gamma-subunit (Huang, C.-Y.F., Yuan C.-J., Livanova, N.B., and Graves, D.J. (1993) Mol. Cell. Biochem. 127/128, 7-18). Removal of the putative inhibitory domain(s) by truncation results in the generation of a constitutively active and calmodulin-independent form, gamma 1-300. To probe the structural basis of autoinhibition of gamma-subunit activity, two synthetic peptides, PhK13 (gamma 303-327) and PhK5 (gamma 343-367), corresponding to the two calmodulin-binding regions, were assayed for their ability to inhibit gamma 1-300. Competitive inhibition of gamma 1-300 by PhK13 was found versus phosphorylase b (Ki = 1.8 microM) and noncompetitive inhibition versus ATP. PhK5 showed noncompetitive inhibition with respect to both phosphorylase b and ATP. Calmodulin released the inhibition caused by both peptides. These results indicate that there are two distinct auto-inhibitory domains within the C terminus of the gamma-subunit and that these two domains overlap with the calmodulin-binding regions. Two mutant forms of gamma 1-300, E111K and E154R, were used to probe the enzyme-substrate-binding region using peptide substrate analogs corresponding to residues 9-18 of phosphorylase b (KRK11Q12ISVRGL). The data suggest that Glu111 interacts with the P-3 position of the substrate (Lys11) and Glu154 interacts with the P-2 site (Gln12). Both E111K and E154R were competitively inhibited with respect to phosphorylase b by PhK13, with 14- and 8-fold higher Ki values, respectively, than that observed with the wild-type enzyme. These data are consistent with a model for the regulation of the gamma-subunit of phosphorylase kinase in which PhK13 acts as a competitive pseudosubstrate that directly binds the substrate binding site of the gamma-subunit (Glu111 and Glu154).

Expression, purification and crystallisation of phosphorylase kinase catalytic domain

The catalytic subunit of phosphorylase kinase is composed of a kinase catalytic core domain (residues 1 to 298), which has a 33% identity with the kinase core of the cyclic AMP-dependent protein kinase, and a C-terminal calmodulin binding domain. The kinase domain of the catalytic subunit has been expressed in Escherichia coli, purified and crystallised in the presence of ATP and magnesium from 5% (w/v) polyethylene glycol 8000, 10% (v/v) glycerol, 50 mM Hepes/NaOH (pH 6.9). A three-fold excess of magnesium to ATP was used for crystal growth. The inclusion of glycerol in the crystallization medium produced a marked reduction in mosaic spread of the diffraction spots from greater than 1 degree to 0.3 degree. The crystals are orthorhombic, space group P2(1)2(1)2(1) with unit cell dimensions a = 47.1 A, b = 69.1 A, c = 112.9 A and one molecule per asymmetric unit. Data to 3 A resolution have been collected and structure determination is in progress.

Interaction of smooth muscle caldesmon with calmodulin mutants

The interaction of avian smooth muscle caldesmon with calmodulin (CaM) was investigated by studying the ability of selected mutant calmodulins to induce fluorescence changes in caldesmon. Different types of CaM mutants were used including point charge mutants, cluster mutations, and mutations which alter the calcium binding of CaM. The caldesmon binding properties were only slightly affected by E84K-CaM or by the double mutation E84Q/E120Q-CaM. Affinity of calmodulin to caldesmon was decreased 2-4 times by point mutation G33V-CaM, double mutation E84K/E120K-CaM, deletion of residues 82-84, and by cluster mutations DEE118-120-->KKK or EEE82-84-->KKK. Mutations of the first (E31A-CaM) and the second (E67A-CaM) calcium binding sites reduced the affinity of calmodulin to caldesmon by at least 5-fold; in addition these calmodulin mutants exhibited smaller changes in the fluorescence spectra of caldesmon. Simultaneous mutation of the two negatively charged clusters of calmodulin EEE82-84-->KKK and DEE118-120-->KKK resulted in a more than 15-fold decrease in the affinity of calmodulin for caldesmon. The data indicate that charged and uncharged amino acids in both halves of CaM play an important role in the binding of calmodulin to caldesmon, and that Ca2+ binding must be maintained in the amino-terminal sites for maximal interaction with caldesmon.

Phosphorylation/activation of phosphorylase b kinase by cAMP/Ca2(+)-independent, autophosphorylation-dependent protein kinase

Phosphorylase b kinase from rabbit skeletal muscle can be phosphorylated and activated by a cyclic nucleotide- and Ca2(+)-independent protein kinase previously identified as an autophosphorylation-dependent multifunctional protein kinase (auto-kinase) from brain and liver (Yang et al., J. Biol. Chem. 262, 7034-7040 (1987) and Yang et al. J. Biol. Chem. 262, 9421-9427 (1987)). This independent kinase phosphorylates both alpha and beta subunits of phosphorylase b kinase and results in a approximately 5-fold activation of the kinase when 0.55 and 0.5 mol of phosphate are incorporated into the alpha and beta subunits, respectively. Activation of phosphorylase b kinase catalyzed by auto-kinase is about 70% of that observed with cAMP-dependent protein kinase. Analysis of phosphopeptide maps of alpha and beta subunits further reveals that both kinases phosphorylate almost the same sites on both alpha and beta subunits, suggesting that activation of phosphorylase b kinase by the two kinases may be through a common molecular action mechanism. Taken together with the previous result that auto-kinase can inactivate glycogen synthase, the present study provides initial evidence that a coordinate control mechanism for simultaneous regulation of glycogenolysis and glycogenesis can be modulated by autophosphorylation-dependent protein kinase in a cAMP- and Ca2(+)-independent pathway, representing a new mode of control mechanism for the regulation of glycogen metabolism in cells.

[Kinetics of action of phosphorylase kinase in an enzymatic cascade system. II. Assessment of the initial rate of the enzymatic reaction catalyzed by phosphorylase kinase]

The turbidimetric method for determining the phosphorylase kinase activity has been developed. The reaction mixture contained, alongside with other components of the kinase reaction, also the substrates of glycogen phosphorylase a, the final product of the kinase reaction-glycogen and glucose 1-phosphate (or inorganic phosphate). The kinetics of the cascade enzymatic system were followed by the increment (decrement) of absorbance of the glycogen solution at 360 nm (delta A). The initial rate of the phosphorylase kinase-catalyzed enzymatic reaction, nu 0, can be calculated according to the formula: nu 0 = 2tg alpha/a2, where tg alpha is the initial slope of the kinetic curve in the coordinates: delta A-(time)2 and a2 is the specific enzymatic activity of phosphorylase a. The latter was estimated from the initial rates of the phosphorylase reaction measured by the addition of glycogen to the reaction mixture after the completion of the kinase reaction. The reliability of the turbidimetric method for determining the phosphorylase kinase activity was proved by comparison with a direct method based on the measurement of the amount of incorporated 32P.

Effects of vanadium treatment on the alterations of cardiac glycogen phosphorylase and phosphorylase kinase in streptozotocin-induced chronic diabetic rats

Supersensitivity to isoproterenol (ISO) induced activation of cardiac phosphorylase in diabetic rat heart has been previously demonstrated and was also reproduced in this study. To explore further the nature of this supersensitivity, we examined the activity of phosphorylase kinase and the level of cyclic AMP (cAMP) in this tissue. We observed a significantly enhanced activation of phosphorylase kinase but no increase in cAMP levels in response to ISO stimulation in diabetic rat heart, suggesting that the supersensitivity of phosphorylase activation in diabetic heart may result from an enhanced activation of phosphorylase kinase that does not involve the cAMP pathway. On the other hand, perfusion of diabetic rat heart with verapamil (5 x 10(-8) M) prior to ISO stimulation abolished the enhanced cardiac phosphorylase activation, suggesting a role for calcium in the supersensitivity of phosphorylase activation. Furthermore, treatment of the diabetic rats with an insulin-like compound, vanadyl sulphate, completely abolished the enhanced cardiac phosphorylase activation and restored the increase in ISO-induced cAMP elevation in diabetic heart. The present study has provided further information on the changes of phosphorylase activation in the diabetic rat heart and demonstrated beneficial effects of vanadyl sulphate on the pathway leading to phosphorylase activation in diabetic rat heart.

Zero length conformation-dependent cross-linking of phosphorylase kinase subunits by transglutaminase

Transglutaminase, a zero length cross-linker that catalyzes the formation of isopeptide bonds between proximal Gln and Lys side chains, was used as a structural and conformational probe of the hexadecameric phosphorylase kinase molecule (alpha beta gamma delta)4. Brief cross-linking of nonactivated kinase caused formation of alpha-beta dimers, with no cross-linking involving the gamma- and delta-subunits. When the kinase was first activated by autophosphorylation, significant amounts of alpha-alpha dimers were also observed in addition to the alpha-beta, demonstrating the occurrence of a conformational change in the alpha-subunits concomitant with activation. Both dimers resulted from intramolecular cross-linking. Because the COOH-terminal regions of the alpha-subunits are at the lobe tips of this bilobal kinase (Wilkinson D. A. Marion, T. N., Tillman, D. M., Norcum, M. T., Hainfeld, J. F., Seyer, J. M., and Carlson, G. M. (1994) J. Mol. Biol. 235, 974-982), the formation of zero length cross-linked alpha-alpha dimers indicates that the polypeptide backbones of these subunits must stretch from the lobe tips to a more central location where they abut each other. Excess putrescine, as the amine substrate in place of endogenous Lys, was incorporated by transglutaminase predominately into the alpha-subunits of the kinase, with only slight modification of the beta- and gamma-subunits. Exogenous calmodulin (delta'), an activator of the kinase with a binding site on the alpha-subunits (James, P., Cohen, P., and Carafoli, E. (1991) J. Biol. Chem. 266, 7087-7091), was a potent inhibitor of cross-linking. It also inhibited incorporation of putrescine into the alpha-subunits but stimulated incorporation into the beta- and gamma-subunits. Heparin, another activator of the kinase, had the same effects as exogenous calmodulin on cross-linking and putrescine incorporation, suggesting a commonality in the mechanism through which these two effectors activate the holoenzyme, including promoting a conformational change that increases the surface accessibility of target Gln residues on the catalytic gamma-subunit.

Electrostatic effects in the control of glycogen phosphorylase by phosphorylation

Electrostatic effects are important in the initial activation mechanism of glycogen phosphorylase by phosphorylation. Analysis of the electrostatic surface potential of glycogen phosphorylase with the program GRASP shows that in the unphosphorylated state, the N-terminal 20 residues, which include a number of basic amino acids, are located close to a position on the surface of the molecule that is highly acidic. Upon phosphorylation by phosphorylase kinase at Ser 14, the N-terminal residues change their position and conformation so that the Ser-P is directed away from the acidic patch and to an intersubunit site where 2 arginines bind the phosphate. This recognition site is created through tertiary and quaternary structural changes that accompany the activation mechanism.

Oxidation and site-directed mutagenesis of the sulfhydryl groups of a truncated gamma catalytic subunit of phosphorylase kinase. Functional and structural effects

A truncated form of the gamma subunit of phosphorylase kinase is inactivated by Cu2+ with the formation of two intra-molecular disulfide bonds. The formation of a disulfide bond between Cys-36 and Cys-172 (semioxidized form) results in approximately 50% loss of specific activity because the Km for MgATP is about 10-fold higher. The second disulfide bond is between Cys-184 and Cys-197 and causes further loss of activity. Eight Cys mutants, i.e. C36S, C36A, C42S, C138S, C172S, C184S, C184A, and C197S, were expressed and purified. Kinetic studies suggest that Cys-36 is important for interaction at the nucleotide site because of its hydrophobicity. With Cys-184 mutants, C184S and C184A, tyrosyl phosphorylation of angiotensin II is affected much more than serine kinase activity. The loss of tyrosine kinase activity is related to a lowered activity with Mn2+. With Mn2+, angiotensin II is a competitive inhibitor with respect to seryl kinase activity of C184S. With Mg2+, however, angiotensin II is a noncompetitive inhibitor. We suggest that metal ions influence the conformation of truncated gamma and that the protein substrate binding region containing Cys-184 is important for the dual specificity of this kinase.

Regional distribution of glycogen, glucose and phosphorylated sugars in rat brain after intoxicating doses of ethanol

Ethanol and anaesthetics increase glycogen levels in the brain. However, no data have been reported about the effect of ethanol on glycogen and glucose metabolism in specific brain regions. We have studied the concentrations of glycogen, glucose, glucose 6-P, glucose 1,6-P2 and fructose 2,6-P2 and the activities of glycogen synthase, glycogen phosphorylase and glycogen phosphorylase kinase in seven brain regions of starved rats following treatment with a single dose or several doses of ethanol. Our results show that: (1) the effect of ethanol on glucose metabolism depends on whether it is given in one single dose or in a series of doses; (2) glycogen concentration increases after a single dose of ethanol but not after long exposure; (3) glucose, glucose 6-P in some areas, and the bisphosphorylated sugar, fructose 2,6-P2 significantly increase after prolonged exposure to ethanol; and (4) the enzymatic activities of glycogen metabolism are not modified after a long exposure to ethanol. In summary, these data show that ethanol may modify the use of glycogen, glucose and derivatives in brain. Moreover, the changes produced depend on the pattern of ethanol intake and the brain area considered.

[The properties of glycogenolytic enzymes and their activation characteristics in the muscle tissue of vertebrates with different ecologies]

The supramolecular structure of glycogenolytic enzymes on intracellular organelles, their properties, and specific features of their regulation largely depend on ecological factors. Development of an ordered supramolecular structure on a glycogen granule and strength of the bond between its components are determined by temperature and do not depend on the mobility of the species in question. Specific features of activation of the glycogenolytic enzymes are determined by functional properties of the muscle tissue. Studies on a hydrodynamic device have shown relationship between activation of glycogen phosphorylase (EC 2.4.1.1) and glycogen phosphorylase kinase (EC 2.7.1.38) and the mobile activity of fish.

[Kinetics of kinase phosphorylase action in a cascade enzymatic system. I. Theoretical basis of a method for determining phosphorylase kinase activity]

Based on a theoretical analysis of functioning of a monocascade enzymatic system, a method for continuous monitoring of the phosphorylase kinase-catalyzed enzymatic reaction has been developed. The method is based on the ability of the kinase reaction product--the phosphorylated form of glycogen phosphorylase (form a)-to catalyze glycogen phosphorolysis (with inorganic phosphate as the low molecular weight substrate) or synthesis (with glucose 1-phosphate) in the absence of AMP. A turbidimetric method may be used for the monitoring of the reaction of glycogen degradation (or synthesis) by phosphorylase a formed in the course of the kinase reaction. A method to calculate the initial rate of the kinase reaction from an absorbance versus (time)2 plot has been theoretically substantiated.

Mutational analyses of the metal ion and substrate binding sites of phosphorylase kinase gamma subunit

Phosphorylase kinase (PhK) and truncated gamma subunit, denoted gamma 1-300, can phosphorylate seryl and tyrosyl residues dependent on the metal ion [Yuan, C.-J., Huang, C. F., & Graves, D. J. (1993) J. Biol. Chem. 268, 17683-17686]. Recombinant gamma 1-300 was used to explore its dual specificity and the location of the metal ion binding sites by using site-directed mutagenesis. Two approaches were taken to generate 26 mutants. First, on the basis of the crystal structure of cAMP-dependent protein kinase (cAPK), the invariant Asn155 and highly conserved Asp168-Phe169-Gly170 residues were mutated. Changes included production of N155H, D168E, D168N, F169R, G170V, G170I, G170L (less than 1% of enzymatic activities were found in these mutants), F169W, and G170A mutants. Second, charge to alanine and charge reversal scanning mutations were used to probe the metal ion binding sites. Two mutants, E111K and E154R, showed very different metal ion response compared to wild-type gamma and were further characterized. The mutants F169W, G170A, E111K, and E154R had 15%, 5%, 8%, and 25% specific activity relative to wild-type gamma, respectively. The folding pattern of wild-type and mutated enzyme forms of gamma was determined by photoacoustic infrared spectroscopy. Conformational disruptions were found in G170V, G170I, and G170L mutants, but the conformation of the rest of the mutants was similar to that of wild-type gamma, suggesting that the loss of enzymatic activities of these mutants was not because of incorrect refolding.(ABSTRACT TRUNCATED AT 250 WORDS)

Baculovirus-directed expression of the gamma-subunit of phosphorylase kinase: purification and calmodulin dependence

A recombinant baculovirus containing a cDNA encoding the gamma-subunit of phosphorylase kinase from mouse skeletal muscle was constructed. Cultures of Sf-9 insect cells infected with the gamma-baculovirus produce an intact and soluble gamma-protein. A purification procedure is presented that yields a sample of gamma-protein which is devoid of interfering enzyme activity and which is not associated with calmodulin from the insect cells. The isolated gamma sample has a Km for phosphorylase b of 36 (+/- 6, S.E.M) microM at pH 8.2 and 140 (+/- 25) microM at pH 6.8. These values are similar to those reported for the activated phosphorylase kinase holoenzyme isolated from skeletal muscle tissue. However, the Vmax. of the baculovirus-expressed gamma is 65 and 80% of that of the activated holoenzyme at pH 6.8 and 8.2 respectively. These results indicate that one or more of the regulatory subunits alpha, beta, or calmodulin stimulate the activity of the catalytic subunit gamma in the activated holoenzyme. Addition of calmodulin to the baculovirus-expressed gamma stimulates its activity 1.5-2.0 fold at pH 6.8 in both the presence and absence of calcium. At pH 8.2, calmodulin has only minor stimulatory affects. The stimulation by calmodulin at pH 6.8 results from an increase in the Vmax of gamma with little effect on its Km. This result is unlike that for most calmodulin-stimulated kinases which bind calmodulin only in the presence of calcium and exhibit a decrease in their Km upon binding calmodulin. The change in Vmax. of gamma in the presence of calmodulin and in the absence of calcium presents a novel mechanism for the regulation of a calmodulin-stimulated kinase.

Identification of the glycogenic compound 5-iodotubercidin as a general protein kinase inhibitor

Addition of micromolar concentrations of the adenosine derivative 5-iodotubercidin (Itu) initiates glycogen synthesis in isolated hepatocytes by causing inactivation of phosphorylase and activation of glycogen synthase [Flückiger-Isler and Walter (1993) Biochem. J. 292, 85-91]. We report here that Itu also antagonizes the effects of saturating concentrations of glucagon and vasopressin on these enzymes. The Itu-induced activation of glycogen synthase could not be explained by the removal of phosphorylase a (a potent inhibitor of the glycogen-associated synthase phosphatase). When tested on purified enzymes, Itu did not affect the activities of the major Ser/Thr-specific protein phosphatases (PP-1, PP-2A, PP-2B and PP-2C), but it inhibited various Ser/Thr-specific protein kinases as well as the tyrosine kinase activity of the insulin receptor (IC50 between 0.4 and 28 microM at 10-15 microM ATP). Tubercidin, which did not affect glycogen synthase or phosphorylase in liver cells, was 300 times less potent as a protein kinase inhibitor. Kinetic analysis of the inhibition of casein kinase-1 and protein kinase A showed that Itu acts as a competitive inhibitor with respect to ATP, and as a mixed-type inhibitor with respect to the protein substrate. We propose that Itu inactivates phosphorylase and activates glycogen synthase by inhibiting phosphorylase kinase and various glycogen synthase kinases. Consistent with the broad specificity of Itu in vitro, this compound decreased the phosphorylation level of numerous phosphopolypeptides in intact liver cells. Our data suggest that at least some of the biological effects of Itu can be explained by an inhibition of protein kinases.

Purification of two forms of bovine liver glycogen phosphorylase b with distinct subunit composition

The procedures for the purification of two forms of bovine liver glycogen phosphorylase b are described. Both forms showed a single band in nondenaturing gel electrophoresis. Gel electrophoresis in the presence of sodium dodecyl sulfate produced a single-band pattern for one of the enzyme forms (phosphorylase b1) and a triple-band pattern for the other (phosphorylase b3). Molecular weights associated with these bands were 97 kDa in the first case and 97, 55, and 40 kDa in the second. The yield from 1 kg of liver was approximately 10 mg for phosphorylase b1 and 140 mg for phosphorylase b3. The specific activity was 40-44 U/mg in both cases. As phosphorylase b1 is composed of just one kind of monomer, it is a novel bovine liver phosphorylase b structure.

Elevated phosphorylase kinase activity in psoriatic epidermis: correlation with increased phosphorylation and psoriatic activity

To determine whether abnormal activity of a calmodulin-containing enzyme which catalyses phosphorylation reactions may play a pathogenetic role in psoriasis, the presence and activity of phosphorylase kinase (PK) in human epidermis were determined in patients with untreated/active psoriasis (n = 10), treated/resolving psoriasis (n = 10), and non-psoriatic controls (n = 10). Biopsies were taken from involved and uninvolved skin for PK, organic phosphorus, and inorganic phosphate estimation, and light and electron microscopy. The enzyme was present in involved and uninvolved skin of every patient in the study. PK activity (units/mg protein) was significantly higher in active psoriasis than in resolving psoriasis and controls. PK activity correlated directly with organic phosphorus levels, and inversely with the extent of cellular glycogenolysis measured by the depletion of glycogen granules within the keratinocytes. The study demonstrates that PK is present in both psoriatic and normal epidermis, with significantly higher levels in active psoriasis. Furthermore, higher levels of PK activity, glycogenolysis and phosphorylation are associated with increased clinical psoriatic activity. We conclude that PK, a calmodulin-containing enzyme, is involved in regulating calcium-dependent phosphorylation events in human epidermis, and disturbance of its activity may play a key role in the clinical manifestations of psoriasis.

The interaction of calmodulin with regulatory peptides of phosphorylase kinase

The regulatory peptides Phk13 (301-327) and Phk5 (342-367) have been synthesized and their interaction with calmodulin studied. In the case of Phk13 modified forms were also synthesized in which a tryptophan group was placed at position 4 or 21, as well as a form with tryptophan at position 4 and nitrotyrosine at position 21. From tryptic digestion, circular dichroism, and radiationless energy transfer measurements, it appears that Phk13 forms an elongated complex with calmodulin in which the peptide is in a non-helical conformation, probably bent into a hairpin-shaped structure, the connecting strand of calmodulin is extended and exposed to the action of proteolytic enzymes, and the peptide makes contact with both the N- and C-terminal half-molecules of calmodulin. In contrast, the Phk5 peptide has an alpha-helical conformation in the complex, which is relatively compact in shape.

Use of the synthetic peptide neurogranin(28-43) as a selective protein kinase C substrate in assays of tissue homogenates

The synthetic peptide neurogranin(28-43), the sequence of which is homologous to the phosphorylation site of the brain specific protein kinase C (PKC) substrates neurogranin and neuromodulin, was tested for its utility as a PKC substrate in crude tissue homogenates. The phosphorylation of neurogranin(28-43) shows time- and protein concentration-dependency. In prolonged incubations, the addition of the protein phosphatase inhibitor sodium pyrophosphate results in increased phosphorylation of neurogranin(28-43). The phosphorylation of neurogranin(28-43) was compared to that of another widely used PKC substrate, S6(229-249). Neurogranin(28-43) is as potent as S6(229-249) and more selective than S6(229-249) as a PKC substrate. Greater than 95% of phosphate incorporation into neurogranin(28-43) can be inhibited by a selective PKC inhibitor, PKC(19-36). Kinetic analysis of neurogranin(28-43) phosphorylation in hippocampal homogenate revealed an apparent Km of 147 nM, virtually identical to previously published Km observed for phosphorylation of the substrate by purified PKC. In addition, we assayed several neuronal and nonneuronal tissues using neurogranin(28-43) as substrate in the presence or absence of detergent. We show that the relative PKC activity assayed with neurogranin(28-43) correlates well to the relative amount of PKC known to be present in various neuronal and nonneuronal tissues. Overall, this report shows that neurogranin(28-43) can be used to selectively assay PKC, even in tissue containing low PKC activity.

Expression, purification, characterization, and deletion mutations of phosphorylase kinase gamma subunit: identification of an inhibitory domain in the gamma subunit

A catalytic fragment, gamma 1-298, derived from limited chymotryptic digestion of phosphorylase b kinase (Harris, W.R. et al., J. Biol. Chem., 265: 11740-11745, 1990), is reported to have about six-fold greater specific activity than does the gamma subunit-calmodulin complex. To test whether there is an inhibitory domain located outside the catalytic core of the gamma subunit, full-length wild-type and seven truncated forms of gamma were expressed in E. coli. Recombinant proteins accumulate in the inclusion bodies and can be isolated, solubilized, renatured, and purified further by ammonium sulfate precipitation and Q-Sepharose column. Four out of seven truncated mutants show similar (gamma 1-353 and gamma 1-341) or less (gamma 1-331 and gamma 1-276) specific activity than does the full-length wild-type gamma, gamma 1-386. Three truncated forms, gamma 1-316, gamma 1-300, and gamma 1-290 have molar specific activities approximately twice as great as those of the full-length wild-type gamma and the nonactivated holoenzyme. All recombinant gamma s exhibit similar Km values for both substrates, i.e., about 18 microM for phosphorylase b and about 75 microM for MgATP. Three truncated gamma s, gamma 1-316, gamma 1-300, and gamma 1-290, have a 1.9- to 2.5-fold greater catalytic efficiency (Vmax/Km) than that of the full-length wild-type gamma and a 3.5- to 4.5-fold greater efficiency than that of the truncated gamma 1-331. This evidence suggests that there is at least one inhibitory domain in the C-terminal region of gamma, which is located at gamma 301-331. gamma 1-290, but not gamma 1-276, which contains the highly conserved kinase domain, is the minimum sequence required for the gamma subunit to exhibit phosphotransferase activity. Both gamma 1-290 and gamma 1-300 have several properties similar to full-length wild-type gamma, including metal ion responses (activation by free Mg2+ and inhibition by free Mn2+), pH dependency, and substrate specificities.

Preparation and functional characterization of a catalytically active fragment of phosphorylase kinase

Limited proteolysis of rabbit muscle phosphorylase kinase catalyzed by chymotrypsin generates a 33 kD product whose kinase activity is independent of both calcium and pH over the range of 6.8 to 8.3 (Malencik, D.A. & Fischer, E.H. Calcium and Cell Function III: 161-188, 1982). This active preparation consists of three related species containing residues 1-290, 1-296, and 1-298 of the 44.7 kD gamma-subunit of phosphorylase kinase (Harris, W.R., Malencik, D.A., Johnson, C.M., Carr, S.A., Roberts, G.D., Byles, C.E., Anderson, S.R., Heilmeyer, L.M.G., Fischer, E.H. & Crabb, J.W.J. Biol. Chem. 265:11740-11745, 1991). Good recoveries of catalytic activity--with varying degrees of calcium dependence--result upon the digestion of phosphorylase kinase with assorted proteases. However, especially high yields of the chymotryptic fragment are obtainable, with purification on an Ultrogel-34 column and a DEAE Sepharose CL-6B column giving 23% of the maximum possible protein. Physical characterization shows that the 33 kD chymotryptic fragment is globular, with S20,w = 2.9S, and that it has an isoelectric point of 5.3. Our continuous catalytic assay, based on differences in the binding of the fluorescent dye 1-anilinonaphthalene-8-sulfonate by phosphorylase a and b, shows that, on a molar basis, the activity of the fragment is 2.8 fold greater than that of phosphorylase kinase (Malencik, D.A., Zhao, Z. and Anderson, S.R. Biochem. Biophys. Res. Comm. 174: 344-350, 1991). The active fragment also undergoes autophosphorylation. Incubation with Mg[gamma-P32] ATP results in the reaction of 0.7 mol 32P/mol fragment. When the catalytic subunit of the cAMP-dependent protein kinase is also present, the amount of 32P incorporated increases to 1.1 mol/mol. In the former case, phosphorylation occurs primarily at Ser30 while in the latter an additional reaction takes place at Ser81. The phosphopeptides correspond to sequences occurring in the gamma-subunit of phosphorylase kinase.

[Structure and regulation of the activity of muscle phosphorylase kinase]

Phosphorylase kinase is the key enzyme in the control of glycogen metabolism in skeletal muscles, the heart and the liver. The quaternary structure of the enzyme, the primary structure of the enzyme subunits as well as the kinetic properties and regulation of the skeletal muscle enzyme activity by covalent modification, phosphorylation and some physiological effectors (Ca2+, calmodulin, troponin C) are reviewed.

[The pre- and posttranslational regulation of the enzymes participating in glycogen phosphorolysis]

The analysis of published and own experimental data concerning function of enzymes of glycogen phosphorolysis is presented. Various types of inherited disorders in glycogen degradation are considered. The necessity of the complex approach to the investigation of enzymopathies mentioned with application of the advanced biochemical and genetical methods for more exact diagnostics and effective treatment is emphasized.

Interaction sites on phosphorylase kinase for calmodulin

Holophosphorylase kinase was digested with Glu-C specific protease; from the peptide mixture calmodulin binding peptides were isolated by affinity chromatography and identified by N-terminal sequence analysis. Two peptides originating from the alpha subunit, having a high tendency to form a positively charged amphiphilic helix and containing tryptophane, were synthesized. Additionally, a homologous region of the beta subunit and a peptide from the alpha subunit present in a region deleted in the alpha' isoform were also selected for synthesis. Binding stoichiometry and affinity were determined by following the enhancement in tryptophane fluorescence occurring upon 1:1 complex formation between these peptides and calmodulin. Finally, Ca2+ binding to calmodulin in presence of peptides was measured. By this way, the peptides alpha 542-566, alpha 547-571, alpha 660-677 and beta 597-614 have been found to bind specifically to calmodulin. Together with previously predicted and synthesized calmodulin binding peptides four calmodulin binding regions have been characterized on each the alpha and beta subunits. It can be concluded that endogenous calmodulin can bind to two calmodulin binding regions in gamma as well as to two regions in alpha and beta. Exogenous calmodulin can bind to two regions in alpha and in beta. A binding stoichiometry of 0.8 mol of calmodulin/alpha beta gamma delta promoter of phosphorylase kinase has been determined by inhibiting the ubiquitination of calmodulin with phosphorylase kinase. Phosphorylase kinase is half maximally activated by 23 nM calmodulin which is in the affinity range of calmodulin binding peptides from beta to calmodulin. Therefore, binding of exogenous calmodulin to beta activates the enzyme. A model for switching endogenous calmodulin between alpha, beta and gamma and modulation of ATP binding to alpha as well as Mg2+/ADP binding to beta by calmodulin is presented.

Phosphoprotein phosphatase activities in Alzheimer disease brain

Microtubule-associated protein tau is known to be hyperphosphorylated in Alzheimer disease brain and this abnormal hyperphosphorylation is associated with an inability of tau to promote the assembly of microtubule in the affected neurons. Our previous studies demonstrated that abnormally phosphorylated tau could be dephosphorylated after treatment with alkaline phosphatase, thereby suggesting that the abnormal phosphorylation of tau might in part be the result of a deficiency of the phosphoprotein phosphatase system in patients with Alzheimer disease. In the present study we used 32P-labeled phosphorylase kinase and poly(Glu, Tyr) 4:1 as substrates to measure phosphoprotein phosphatase activities in Alzheimer disease and control brains. The activities of phosphoseryl/phosphothreonyl-protein phosphatase types 1, 2A, 2B, and 2C and of phosphotyrosyl-protein phosphatase in frontal gray and white matters from 13 Alzheimer brains were determined and compared with those from 12 age-matched control brains. The activities of type 1 phosphatase and phosphotyrosyl phosphatase in gray matter and of type 2A phosphatase in both gray and white matters were significantly lower in Alzheimer disease brains than in controls. These findings suggest that the hyperphosphorylation of tau in Alzheimer disease brain could result from a protein dephosphorylation defect in vivo. The decrease in the phosphatase activities in Alzheimer disease might also be involved in the formation of beta-amyloid by augmenting the amyloidogenic pathway processing of beta-amyloid precursor protein.

The binding of phosphorylase kinase to immobilized calmodulin

The binding of phosphorylase kinase to calmodulin-Sepharose 4B was studied by column and batch methods. It was found that the Ca2+ dependence of the interaction strongly depended on the degree of substitution of agarose with calmodulin. Equilibrium adsorption isotherms (i.e., bulk ligand binding functions and lattice site binding functions) of phosphorylase kinase were measured on calmodulin-Sepharose. Sigmoidal bulk ligand binding functions (bulk adsorption coefficients: 1.5-5.8) were found which indicate intermolecular attraction during binding. Hyperbolic lattice site binding functions (lattice adsorption coefficients: 1.0) were obtained thus excluding the existence of a critical surface concentration of immobilized calmodulin and indicating single independent binding sites on the gel surface and on phosphorylase kinase. These findings were combined to optimize the adsorption of phosphorylase kinase on calmodulin-Sepharose, for purification procedures at low Ca2+ concentrations (5-10 microM) minimizing proteolysis by calpains. With this novel method phosphorylase kinase from rabbit and frog skeletal muscle could be purified ca 100- and 200-fold, respectively, in two steps.

Phosphorylase kinase, a metal ion-dependent dual specificity kinase

Phosphorylase kinase is shown to be a dual specificity kinase. The specificity of phosphorylation is determined by divalent cation. Mg2+ causes seryl phosphorylation of phosphorylase b, but Mn2+ activates tyrosine phosphorylation of angiotensin II. In contrast to seryl phosphorylation, the tyrosine kinase activity of holoenzyme is not regulated by Ca2+. Preincubation of the holoenzyme with Ca2+, Mg2+ and ATP that causes autophosphorylation activates tyrosine kinase activity. The tyrosyl kinase activity is a property of the gamma subunit. Addition of varying amounts of Mn2+ to a truncated form of the gamma subunit of phosphorylase kinase containing MgATP inhibits serine kinase but activates tyrosine kinase activity. This result along with an oxidative reaction caused by Cu2+ and site-directed mutagenesis of the putative catalytic base inhibiting both serine and tyrosine kinase activity suggest that one active site is involved in both activities. Kinetic studies with Mn2+ and ATP show that Km for nucleotide is not changed with a seryl or tyrosyl substrate. The Vm values are different, and the value for tyrosyl phosphorylation is similar to other tyrosyl kinases. We propose two conformations for the active site; one favors seryl phosphorylation, and the second tyrosyl phosphorylation is caused by the binding of divalent cation at a second metal ion binding site.

Isolation of an autoinhibitory region from the regulatory beta-subunit of phosphorylase kinase

An equimolar mixture of the regulatory alpha- and beta-subunits of phosphorylase kinase has been shown to inhibit its catalytic gamma-subunit (Paudel, H.K., and Carlson, G.M. (1987) J. Biol. Chem. 262, 11912-11915). The possible presence of an autoinhibitory sequence within those regulatory subunits has been evaluated by peptide isolation and characterization following chemical and proteolytic cleavage of an isolated equimolar mixture of those subunits; the peptides generated were tested for their ability to inhibit the activity of a complex of the gamma-subunit and calmodulin. An isolated inhibitory fragment, hereafter referred to as I-peptide, was sequenced and found to correspond to residues 420-436 of the beta-subunit (KRNPGSQKRFPSNCGRD). This sequence showed homology with the kinase's natural substrate, phosphorylase b. A synthetic peptide based on this sequence was constructed and used to study the mechanism of inhibition. Kinetic analysis of the inhibition of the gamma-subunit-calmodulin complex by the I-peptide revealed a competitive pattern versus the homologous substrate phosphorylase b and an uncompetitive pattern versus MgATP, suggesting an ordered binding of substrates, with the nucleotide binding first. In addition to its ability to inhibit, the I-peptide was also a substrate for the gamma-subunit-calmodulin complex, with a relatively good Km but poor Vmax. The parallel inhibition of free gamma-subunit and the gamma-subunit-calmodulin complex by progressively increasing concentrations of I-peptide provided further evidence that the peptide inhibits by interacting directly with the catalytic subunit and not with the stimulatory calmodulin molecule. The results of this study are consistent with previous findings from this laboratory showing that the conformation of the beta-subunit changes following activation of phosphorylase kinase through a variety of mechanisms.

Two exons encode the calmodulin-binding domain in the mouse phosphorylase kinase catalytic subunit gene

The catalytic subunit, gamma, of phosphorylase kinase contains two calmodulin-binding sequences that define a domain in gamma that is homologous to the troponin-C-binding domain in troponin I. The homology is based on both sequence and functional similarities. To account for this homology, it has been proposed that the calmodulin-binding sequences in gamma and the troponin-C-binding domain in troponin I have evolved from a common ancestor. We investigated this possibility by comparing the exon structure of the gamma gene with that of troponin-I gene over their homologous domains. In the quail troponin-I gene, it is known that the entire troponin-C-binding domain is encoded by a single exon. However, two exons are found to encode the calmodulin-binding domain in the gamma gene from mouse. This result indicates that convergent evolution may be responsible for the sequence and functional similarities between the homologous domains in troponin I and gamma.

Phosphorylase kinase phosphorylates the calmodulin-binding regulatory regions of neuronal tissue-specific proteins B-50 (GAP-43) and neurogranin

Neuronal tissue-specific proteins B-50 (GAP-43, neuromodulin) and neurogranin are phosphorylated by phosphorylase kinase with stoichiometries of 0.4 and 0.5 mol of phosphate/mol of protein, respectively. The apparent Km and kcat values determined at pH 8.2 for neurogranin phosphorylation are 28.4 microM and 139.3 min-1, respectively, and for B-50 phosphorylation are 22.8 microM and 33.2 min-1, respectively. As a substrate of phosphorylase kinase, phosphorylase is approximately 44 and approximately 13 times better than B-50 and neurogranin, respectively. Both proteins are better substrates of protein kinase C than of phosphorylase kinase and are phosphorylated on a single site by phosphorylase kinase. The sequence analyses of tryptic phosphopeptides isolated from neurogranin and B-50 phosphorylated by phosphorylase kinase revealed the same amino acid sequence, IQASF, indicating that phosphorylase kinase phosphorylates the calmodulin-binding regulatory regions of B-50 and neurogranin previously known to be phosphorylated by protein kinase C (Coggins, P. J., and Zwiers, H. (1989) J. Neurochem. 53, 1895-1901; Baudier, J., Deloulme, J. C., Dorsselaer, A. V., Black, D., and Matthes, W. D. (1991) J. Biol. Chem. 266, 229-237). In rat brain synaptosomes, a relatively high phosphorylase kinase specific activity is detected, and approximately 32% activity is associated with synaptic membranes where B-50 is localized. In rat brain homogenate and synaptosomal membranes, phosphorylation of a protein that co-migrates with B-50 on SDS-polyacrylamide gel electrophoresis is enhanced in the presence of exogenous phosphorylase kinase.

Features of calmodulin that are important in the activation of the catalytic subunit of phosphorylase kinase

Calmodulin (CaM) is an integral subunit, called delta, of the phosphorylase kinase hexadecamer, and the activity of the isolated catalytic gamma-subunit of the kinase is stimulated by CaM. We report here the first analysis of functionally important features of CaM for activation of the gamma-subunit. A set of genetically engineered CaMs, in which acidic residues in each of the four E-helices of the "EF-hands" were changed to basic lysine residues, was used to probe the relative importance of charge features in each domain of CaM. The maximal activation of the isolated gamma-subunit was diminished by all of the charge reversal mutations. The gamma-subunit was especially sensitive to reversals in the second and third E-helix of CaM (residues 45-47 and 82-84), the latter being present in the central helix. The results suggest the functional importance of electrostatics in the interactions between the delta-subunit (CaM) and the catalytic gamma-subunit of phosphorylase kinase, which is similar to results obtained with CaM-dependent myosin light chain kinase (MLCK) from chicken gizzard and CaM-dependent protein kinase II (CaMPK-II). However, novel features of the interaction between CaM and the gamma-subunit of phosphorylase kinase are the significant contribution of electrostatics throughout the CaM molecule, including residues in both halves and on more than one face of CaM, and the lack of a major effect of the CaM mutations on substrate kinetic parameters, unlike the effects observed with MLCK and CaMPK-II. These results are consistent with a model in which the delta-subunit (CaM) of phosphorylase kinase interacts with an extended region or multiple regions of the gamma-subunit and suggest that the mechanism of CaM activation of the gamma-subunit may have features that are distinct from those of MLCK and CaMPK-II.

Dephosphorylation of phosphorylated atrial natriuretic peptide by protein phosphatase 2A

A phosphatase which exhibits strong activity toward phosphorylated atrial natriuretic peptide (ANP) was identified in the soluble fraction of rat brain homogenate. This ANP phosphatase has a neutral pH optimum, does not require divalent cations for activity, is inhibited by low concentrations of okadaic acid (50% inhibition at 1 nM) and preferentially dephosphorylates the alpha subunit of phosphorylase kinase. These properties are characteristic of serine/threonine protein phosphatase type 2A (PP2A). The apparent molecular mass of the ANP phosphatase (160 kDa), as estimated by gel filtration, is similar to that of the native heterotrimeric form of PP2A. In addition, phosphorylated ANP is an excellent substrate for the purified catalytic subunit of PP2A (Km = 42 microM, Vmax = 10.3 mumol x min-1 x mg-1). In contrast, protein phosphatase 2B (PP2B) has only very low ANP phosphatase activity (Km = 2.5 microM, Vmax = 0.008 mumol x min-1 x mg-1), and the catalytic subunit of protein phosphatase type 1 (PP1) as well as purified protein phosphatase type 2C (PP2C) are essentially inactive on ANP. These findings are consistent with the observation that PP2A-like activity accounts for virtually all ANP dephosphorylation in brain homogenate. While the phosphorylation of ANP in vitro by cAMP-dependent protein kinase is well documented, this is a first report on a phosphatase that efficiently can reverse this modification.

Purification and properties of calmodulin from Phymatotrichum omnivorum

Mycelia of Phymatotrichum omnivorum obtained at 10 day intervals during 10 to 50 days of growth were used for isolating calmodulin, and studying its effect on glycogen synthase, phosphorylase, phosphorylase kinase, cyclic AMP phosphodiesterase and Ca++ATPase. Glycogen synthase was inhibited until the 30th day by calmodulin, whereas calmodulin obtained from the 40th day stimulated glycogen synthase activity and the 50th day sample had no effect. cAMP phosphodiesterase and Ca++ATPase of P. omnivorum were stimulated by the respective calmodulin. Molecular weight of the purified fungal calmodulin was approximately 18 kD as revealed by sodium dodecyl sulphate gel electrophoresis. Trifluoperazine, dibucaine and lidocaine inhibited calmodulin activity and calmodulin activation of PDE, respectively.

Analysis by mutagenesis of the ATP binding site of the gamma subunit of skeletal muscle phosphorylase kinase expressed using a baculovirus system

Active gamma subunit of skeletal muscle phosphorylase kinase has been obtained by expression of the rat soleus cDNA in a baculovirus system. The protein exhibited the expected pH 6.8/8.2 activity ratio of 0.6, and its activity was insensitive to Ca2+ addition, indicating that it was free gamma subunit and not a gamma subunit-calmodulin complex. It was stimulated approximately 2-fold by Ca(2+)-calmodulin addition, demonstrating that it had retained high-affinity calmodulin binding. By site-directed mutagenesis, we have examined the role of six of the amino acids that constitute the consensus ATP binding site of the protein kinase, which in the gamma subunit is represented by the sequence 26Gly.Arg.Gly.Val.Ser.Ser.Val.Val33. Changes were evaluated by the kinetic determination of the dissociation constants of gamma-ATP, gamma-ADP, gamma-AMP.PCP, and gamma-phosphorylase and the maximum catalytic activity. The mutants Ser26-gamma, Ser29-gamma, Phe30-gamma, and Gly31-gamma each exhibited an essentially identical dissociation constant for gamma subunit phosphorylase, indicating that these mutations had not caused a global alteration in the protein structure but were limited to changes in the nucleotide binding site domain. Substitution of either Val33 (by Gly) or Gly28 (by Ser), two of the most conserved residues in all protein kinases, resulted in enzyme with marginally detectable activity. In noted contrast, the Ser26 mutant, which substituted the first glycine of the consensus glycine trio motif, and which is also very highly conserved, retained at least 25% of the enzymatic activity. The Gly31 substitution, which restored a glycine to a position characteristic for most protein kinases, had little overall effect upon the maximum rate of catalysis. Restoration of Ser30 to the more typical phenylalanine, which is present in most protein kinases, had minimal effect on catalysis. These data provide the first direct evaluation of the roles that different residues play within this consensus glycine trio/valine motif of the protein kinases, which up to now have only been surmised to be of importance because of their conservation. Two unexpected findings are that for one residue that is very conserved (Gly26) there is some flexibility of substitution not apparent from the evolutionary conservation and that a second quite conserved residue in protein kinases (equivalent to Gly at position 31) does not produce a protein optimized for nucleotide binding.

Effect of starvation on glycogen and glucose metabolism in different areas of the rat brain

We have studied the changes in concentration of glycogen, glucose and the bisphosphorylated sugars, glucose 1,6-P2 and fructose 2,6-P2, in several rat brain regions during 72 h of starvation. The animals were killed by focused microwave irradiation. The activities of glycogen metabolizing enzymes in the different areas were measured. A large decrease in glycogen and glucose concentration was observed in all areas. The concentrations of bisphosphorylated sugars changed, suggesting that an increase in glycolysis could take place at the beginning of starvation, with blood glucose as a major energy source. Differences in metabolite concentration before starvation disappeared after 72 h. The activities of glycogen synthase, glycogen phosphorylase and glycogen phosphorylase kinase were similar in all areas, and they did not change during starvation.

[The characteristics of the interaction of glycogenolytic enzymes with glycogen in endothermic and ectothermic animals]

It has been shown that the dependence of glycogenphosphorylase kinase activity on the quantity of glycogen added corresponds to the glycogenphosphorylase saturation curve by glycogen obtained during investigation of complex formation with the muscle glycogenphosphorylase of the animals: rat, frog Rana temporaria and fish Raja clavata under identical conditions. We suggest that glycogenolytic enzymes organize regular structure on glycogen particle. Glycogenphosphorylases of heat-loving animals have high affinity to glycogen that is accounted for necessity to keep the structure in natural conditions.

Coordinated expression of phosphorylase kinase subunits in regenerating skeletal muscle

The developmental expression of the alpha, beta, and gamma subunits of skeletal muscle phosphorylase kinase has been examined in regenerating muscle. Rat extensor digitorum longus (EDL) muscles, treated with bupivacaine, promptly undergo a rapid degeneration of the muscle, followed by regeneration and recovery of essentially normal morphology and physiology by 3-4 weeks post-treatment (Hall-Craggs, E. C. B., and Seyan, H. S. (1975) Exp. Neurol. 46, 345-354). Phosphorylase kinase activity dropped to approximately 10% of control within 3 days of bupivacaine treatment and remained at this low level for several days but had attained at least 60% of normal levels by day 21. The pH 6.8/8.2 activity ratio was unusually high during the period of low activity, suggesting that the catalytic activity was not under normal regulation at this time. The subunit mRNAs were readily detected in control EDL but were undetectable at day 3 post-bupivacaine treatment. Very small amounts of message for all three subunits were evident by day 6 and began to approach normal levels by day 12-15. The mRNA for both the alpha and alpha' subunits of phosphorylase kinase exhibited a similar pattern of recovery, as did also the mRNA for phosphorylase. In contrast to both phosphorylase kinase and phosphorylase, actin mRNA exhibited a quite a different pattern, with a nearly full recovery of message levels by day 6 post-bupivacaine. These data indicate that synthesis of phosphorylase and the alpha, beta, and gamma subunits of phosphorylase kinase appears to be coordinately regulated at the level of message accumulation and that the expression of phosphorylase kinase activity is likely to be also regulated post-transcriptionally.

A kinetic re-interpretation of the regulation of rabbit skeletal-muscle phosphorylase kinase activity by Ca2+ and phosphorylation

The regulation of phosphorylase kinase has been proposed to occur physiologically under conditions of zero-order ultrasensitivity [Meinke & Edstrom (1991) J. Biol. Chem. 266, 2259-2266]. This is also one of the conditions that recent theoretical approaches have indicated to be essential in order for an interconvertible enzyme cascade to generate a sensitive response to an effector [Cardenas & Cornish-Bowden (1989) Biochem. J. 257, 339-345]. In contrast, all published kinetic data to date have strongly suggested that activation of phosphorylase kinase by Ca2+ or phosphorylation is attributable solely to a change in affinity for phosphorylase, with no effect on the Vmax. of the reaction. In this study an attempt is made to resolve this conflict. Findings suggest that changes in Vmax. can fully account for the activation of phosphorylase kinase by the physiological mechanisms of cyclic AMP-dependent phosphorylation and increase in Ca2+ concentration.

Cyclic inhibition-potentiation of the crosslinking of synapsin I with brain microtubules by protein kinase FA (an activator of ATP.Mg-dependent protein phosphatase)

The ATP.Mg-dependent type-1 protein phosphatase activating factor (FA) was identified as a protein kinase that could phosphorylate synapsin I, a neuronal protein that coats synaptic vesicles, binds to cytoskeleton and is believed to be involved in the modulation of neurotransmission. More importantly, more than 90% of the phosphates in 32P-synapsin I phosphorylated by FA could be removed by the activated ATP.Mg-dependent type-1 protein phosphatase and the synapsin I phosphatase activity was found to be strictly FA-dependent. Functional study further revealed that as a synapsin I kinase, factor FA could phosphorylate synapsin I and thereby inhibits crosslinking of synapsin I with tubulin, while as a synapsin I phosphatase activator, FA could promote the crosslinking copolymerization of synapsin I with tubulin. Taken together, the results provide initial evidence that a cyclic modulation of the crosslinking copolymerization of synapsin I with brain microtubules can be controlled by factor FA, representing an efficient cyclic cascade control mechanism for the regulation of axonal transport process during neurotransmission.

Glycogen storage disease diagnosed in adults

Glycogen storage diseases are usually identified in childhood. We present the clinical, biochemical and histological features of 10 patients first diagnosed in adult life. Five had glycogen storage disease type 1a, one type 1c, two type IX, and in two patients there were previously unreported abnormalities of hepatic glucose-6-phosphatase system activity. Of the latter, one patient had an inhibitor of liver glucose-6-phosphatase (pseudo-1b glycogen storage disease) the other having abnormal glucose-6-phosphatase activity and microsomal pyrophosphate transport. A glucagon test is suggested as a useful screening procedure. Glycogen storage disease should be considered in adults with symptoms suggesting hypoglycaemia.

Molecular cloning and enzymatic analysis of the rat homolog of "PhK-gamma T," an isoform of phosphorylase kinase catalytic subunit

Messenger RNA encoding a protein kinase closely related to the catalytic subunit of skeletal muscle phosphorylase kinase has previously been isolated from a human HeLa cell cDNA library, and cross-species Northern hybridization analysis has shown that the rat homolog of this transcript is abundant in the adult testis (Hanks, S.K. (1989) Mol. Endocrinol. 3, 110-116). We now propose that the protein encoded by this transcript be designated as "PhK-gamma T." In this article, the primary structure of the rat homolog of PhK-gamma T is described, as deduced from nucleotide sequences of cDNA and genomic clones. RNase protection analysis reveals that PhK-gamma T transcripts are actually present in a wide variety of adult rat tissues, but at levels 20-100-fold less than what is observed in the testis. In the testis, transcription of the PhK-gamma T gene is initiated at multiple sites as shown by RNase protection and primer extension. Enzymatic activity of PhK-gamma T was demonstrated using renatured bacterially expressed protein. In the presence of Ca2+/calmodulin, PhK-gamma T is able to efficiently phosphorylate glycogen phosphorylase and convert it from an inactive to an active form. We conclude that PhK-gamma T represents a true isoform of phosphorylase kinase catalytic subunit.

High and intermediate affinity calmodulin binding domains of the alpha and beta subunits of phosphorylase kinase and their potential role in phosphorylation-dependent activation of the holoenzyme

Phosphorylase kinase is a calcium-regulated multimeric enzyme of composition (alpha beta gamma delta)4, which contains calmodulin as the integral delta subunit and also is activated further by addition of extrinsic calmodulin. Previous studies by Dasgupta, M., Honeycutt, T., and Blumenthal, D.K. ((1989) J. Biol. Chem. 264, 17156-17163) have identified gamma 302-326 and gamma 342-366 as two calmodulin binding regions. Using peptides that were synthesized based on alpha and beta primary structure and that were predicted to contain the basic amphiphilic alpha-helix motif thought important for calmodulin binding, four additional potential calmodulin binding domains have now been identified: one of high affinity, beta 770-794; two of intermediate affinity, beta 5-28 and beta 920-946; and one with marginally low affinity, alpha 1070-1093. Peptide beta 770-794 was of higher calmodulin affinity than either gamma 302-326 or gamma 342-366; it was of higher affinity than the model synthetic peptide IV defined by O'Neil, K.T., and DeGrado, W.F. ((1990) Trends Biochem. Sci. 15, 59-64); and it is currently the most potent calmodulin-binding peptide so far described. Correlated with their affinity for calmodulin, all six phosphorylase kinase-derived peptides and several other established calmodulin-binding peptides inhibited phosphorylase kinase previously activated by cAMP-dependent phosphorylation, reducing its activity to the level of the nonactivated enzyme. However, these peptides did not inhibit (and some peptides slightly activated) the nonphosphorylated enzyme. Even in the presence of these peptides both activated and nonactivated enzyme remained fully Ca(2+)-dependent. The beta 770-794 peptide has at least a 5-fold greater calmodulin binding affinity than the holo-phosphorylase kinase. This, and its higher affinity for calmodulin than either of the sites on the gamma subunit, raises the possibility that in the native enzyme it may be involved in binding the intrinsic delta subunit. Further, inhibition of activated but not nonactivated enzyme by calmodulin-binding peptides would suggest that the phosphorylation-dependent activation of phosphorylase kinase may be mediated by changes in the binding interactions of the intrinsic calmodulin delta subunit.

The mechanism of activation of protein kinase FA (the activator of type-1 protein phosphatase) in brain synaptosomes

The ATP.Mg-dependent type-1 protein phosphatase and its activating factor (protein kinase FA) were identified to exist in brain synaptosome. The inactive protein phosphatase was found to exist in the synaptosomal cytosol whereas its activating factor (protein kinase FA) was present in the synaptosomal membrane, indicating that the inactive protein phosphatase and its activating factor FA are localized in two separate subcellular compartments. The membrane-bound FA was found to exist in two forms; approximately 75% of FA is inactive and trypsin-resistant, whereas 25% of FA is active and trypsin-labile. When membranes were incubated with exogenous phospholipase C, the inactive/trypsin-resistant FA could be activated and sequestered to become the active/trypsin-labile FA in a time- and dose-dependent manner. Taken together, the results provide initial evidence that the activation-sequestration of membrane-bound protein kinase FA may represent one mode of control modulating the activity of protein kinase FA and thereby to activate protein phosphatase in brain synaptosome, representing an efficient regulatory mechanism for regulating neurotransmission in the central nervous system.

Ca(2+)-dependent ubiquitination of calmodulin in yeast

Recently we were able to show that calmodulin from vertebrates, plants (spinach) and the mold Neurospora crassa can be covalently conjugated to ubiquitin in a Ca(2+)-dependent manner by ubiquityl-calmodulin synthetase (uCaM-synthetase) from mammalian sources [R. Ziegenhagen and H.P. Jennissen (1990) FEBS Lett. 273, 253-256]. It was therefore of high interest to investigate whether this covalent modification of calmodulin also occurs in one of the simplest eukaryotes, the unicellular Saccharomyces cerevisiae. Yeast calmodulin was therefore purified from bakers yeast. In contrast to calmodulin from spinach and N. crassa it does not activate phosphorylase kinase. Crude yeast uCaM-synthetase conjugated ubiquitin Ca(2+)-dependently to yeast and mammalian (bovine) calmodulin. Yeast calmodulin was also a substrate for mammalian (reticulocyte) uCaM-synthetase. As estimated from autoradiograms the monoubiquitination product (first-order conjugate) of yeast calmodulin has an apparent molecular mass of ca. 23-26 kDa and the second-order conjugate an apparent molecular mass of ca. 28-32 kDa. Two to three ubiquitin molecules can be incorporated per yeast calmodulin. Experiments with methylated ubiquitin in the heterologous reticulocyte system indicate that, as with vertebrate calmodulins, only one lysine residue of yeast calmodulin reacts with ubiquitin so that the incorporation of multiple ubiquitin molecules will lead to a polyubiquitin chain. These results also indicate that the ability of coupling ubiquitin to calmodulin was acquired at a very early stage in evolution.

[Isolation and properties of three forms of phosphorylase and phosphorylase kinase from human skeletal muscle]

Three forms of phosphorylase (I, II and III), two of which (I and II) were active in the presence of AMP and one (III) was active without AMP, were isolated from human skeletal muscles. The pI values for phosphorylases b(I) and b(II) were found to be identical (5.8-5.9). During chromatofocusing a low molecular weight protein (M(r) = 20-21 kDa, pI 4.8) was separated from phosphorylase b(II). This process was accompanied by an increase of the enzyme specific activity followed by its decline. During reconstitution of the complex the activity of phosphorylase b(II) returned to the initial level. Upon phosphorylation the amount of 32P incorporated into phosphorylase b(II) was 2 times as low as compared with rabbit phosphorylase b and human phosphorylase b(I). It may be supposed that in the human phosphorylase b(II) molecule one of the two subunits undergoes phosphorylation in vivo. This form of the enzyme is characterized by a greater affinity for glycogen and a lower sensitivity to allosteric effectors (AMP, glucose-6-phosphate, caffeine) compared with phosphorylase b(I). Thus, among the three phosphorylase forms obtained in this study, form b(II) is the most unusual one, since it is partly phosphorylated by phosphorylase kinase to form a complex with a low molecular weight protein which stabilizes its activity. A partially purified preparation of phosphorylase kinase was isolated from human skeletal muscles. The enzyme activity necessitates Ca2+ (c0.5 = 0.63 microM). At pH 6.8 the enzyme is activated by calmodulin (c0.5 = 15 microM). The enzyme activity ratio at pH 6.8/8.2 is equal to 0.18.