Molecular mechanism of the synergistic phosphorylation of phosphatase inhibitor-2. Cloning, expression, and site-directed mutagenesis of inhibitor-2

Inhibitor-2 (I-2) is the regulatory subunit of the ATP-Mg-dependent phosphatase, a cytosolic form of type 1 protein phosphatase. Phosphorylation of I-2 at Thr-72 by the protein kinase glycogen synthase kinase-3 (GSK-3) leads to activation of the enzyme. Casein kinase II action was shown to synergistically enhance phosphorylation and activation by GSK-3 (DePaoli-Roach, A.A. (1984) J. Biol. Chem. 259, 12144-12152). Rabbit skeletal muscle and liver I-2 cDNA clones have been isolated. Rabbit skeletal muscle cDNAs could be placed in two subtypes, differing in the length of the 3'-untranslated region. The coding sequence of 612 nucleotides was identical in the two skeletal muscle and the liver cDNAs and predicted a protein of 204 amino acids, consistent with analysis of the purified protein. Northern hybridization analysis indicated that the two mRNAs of 1.7 and 2.7 kilobase pairs were present in all rabbit tissues examined, except in liver, where only the larger transcript was detected, and in testis, where additional transcripts were present. Expression in Escherichia coli of wild-type and phosphorylation site mutants resulted in the production of I-2 polypeptides with apparent M(r) values of approximately 31,000 on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The inhibitory activity of the recombinant proteins was similar to that of native rabbit skeletal muscle I-2 and was unaffected by the substitution of alanine for the GSK-3 site (Thr-72) and for the casein kinase II sites (Ser-86 and Ser-120/121) or by substitution of glutamic acid and aspartic acid for Thr-72 and Ser-86. Recombinant wild-type I-2 and the Ala-120/121 mutant were phosphorylated synergistically by GSK-3 and casein kinase II. The Thr-72 and Ser-86 mutants, however, did not undergo this synergistic phosphorylation. Our studies indicate that Thr-72 is the only GSK-3 site and that Ser-86 is the casein kinase II site required for the potentiation of GSK-3 action. Furthermore, acidic residues cannot substitute for the phosphate group either in enhancing GSK-3 phosphorylation or in activating the phosphatase.

Isoform differences in substrate recognition by glycogen synthase kinases 3 alpha and 3 beta in the phosphorylation of phosphatase inhibitor 2

Phosphorylation of inhibitor 2, the regulatory subunit of the ATP-Mg-dependent protein phosphatase, by glycogen synthase kinase 3 (GSK-3) causes activation of the phosphatase. Prior phosphorylation by casein kinase II has been shown to enhance both phosphorylation and activation of the phosphatase by GSK-3 (DePaoli-Roach, A. A. (1984) J. Biol. Chem. 259, 12144-12152). Reported here is a comparison of the phosphorylation of inhibitor 2 by two defined isoforms of GSK-3, GSK-3 alpha and GSK-3 beta. GSK-3 beta was a significantly better inhibitor 2 kinase than was GSK-3 alpha. The Vmax/Km value for GSK-3 beta was approximately 10-fold higher than that for GSK-3 alpha. GSK-3 beta phosphorylated inhibitor 2 to a stoichiometry of approximately 1.0 mol of phosphate/mol of inhibitor 2. The phosphorylation by GSK-3 beta was determined to be exclusively at Thr-72 on the basis of the inability of the enzyme to modify a mutant inhibitor 2 in which Thr-72 was changed to alanine. Prior phosphorylation by casein kinase II promoted the action of GSK-3 alpha in keeping with earlier reports using undefined GSK-3 preparations. Phosphorylation by GSK-3 beta, in contrast, was unaffected by the previous action of casein kinase II. These results suggest that there can be important differences in substrate recognition by different isoforms of the same protein kinase and may help explain why some reported GSK-3 substrates require prior phosphorylation whereas other do not.(ABSTRACT TRUNCATED AT 250 WORDS)

Protein kinase A modulates an endogenous calcium channel, but not the calcium-activated chloride channel, in Xenopus oocytes

In Xenopus oocytes, Ca2+ influx through an endogenous voltage-gated Ca2+ channel activates a transient outward Cl- current (ICl(Ca)), which is potentiated by cAMP increase. The site of cAMP effect appears to be the Ca2+ channel instead of the Ca(2+)-activated Cl- channel, because cAMP potentiates the Ba2+ current through the Ca2+ channel in a similar way to the ICl(Ca), and cAMP does not potentiate the Ca(2+)-dependent Cl- current in cells treated with Ca2+ ionophore. Using the catalytic subunit of protein kinase A (PKA) and PKA inhibitors, it was shown that PKA is both necessary and sufficient for the cAMP effect on ICl(Ca). Furthermore, the cAMP/PKA-mediated potentiation of ICl(Ca) was inhibited by both type 1 and type 2A protein phosphatases.

Initiation of glycogen synthesis. Control of glycogenin by glycogen phosphorylase

Glycogen biosynthesis involves a specific initiation event, mediated by a specialized protein, glycogenin. Glycogenin undergoes self-glucosylation to generate an oligosaccharide primer, which, when long enough, supports the action of glycogen synthase to elongate the polysaccharide chain, leading ultimately to the formation of glycogen. We report that primed glycogenin is also a substrate for glycogen phosphorylase. Phosphorylase removed glucose from the oligosaccharide attached to glycogenin in a phosphorolysis reaction that required phosphate and produced glucose 1-phosphate. The phosphorylated form, phosphorylase a, was much more effective than the dephosphorylated phosphorylase b. However, in the presence of the allosteric effector AMP, phosphorylase b also catalyzed the phosphorolysis reaction. Glucose, an allosteric inhibitor of phosphorylase, inhibited the reaction. Glycogen, but not a short oligosaccharide (maltopentaose), also inhibited the reaction. Treatment of fully primed glycogenin with phosphorylase converted the glycogenin to a form with slightly lower apparent molecular weight, which was less effective as a substrate for glycogen synthase. These results suggest a novel role for phosphorylase in the control of glycogen biosynthesis. We propose that the glucosylation level of glycogenin would be determined by the balance between the self-glucosylation reaction and the opposing action of phosphorylase. The level of glucosylation would in turn determine whether or not glycogenin was an effective primer for glycogen synthase. In this way, several known controls of phosphorylase activity, such as epinephrine, glucagon, and insulin, could influence not only the elongation/degradation stage of glycogen metabolism but also its initiation.

Characterization of rabbit skeletal muscle glycogenin. Tyrosine 194 is essential for function

The biogenesis of glycogen involves a specific initiation event mediated by the initiator protein, glycogenin, which undergoes self-glucosylation to generate an oligosaccharide primer from which the glycogen molecule grows. Rabbit muscle glycogenin was expressed at high levels in Escherichia coli and purified close to homogeneity in a procedure that involved binding to a UDP-agarose affinity column. The resulting protein had subunit molecular weight of 38,000 as judged by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. Analysis of peptide fragments by mass spectroscopy indicated that the recombinant glycogenin was already glucosylated at Tyr-194 and contained from 1 to 8 glucose residues attached. The enzyme was active as a glucosyl transferase and could incorporate a further approximately 5 mol of glucose/mol. The apparent Km for the glucosyl donor UDP-glucose was 4.5 microM, and the pH optimum was pH 8. Of a number of nucleotides and related compounds surveyed, UDP and UTP were the most effective inhibitors. There was also a correlation between inhibition and the presence of a pyrophosphate group. Of several oligosaccharides of glucose, only maltose caused significant inhibition. The glucosylation reaction was first order with respect to glycogenin suggesting that it was intramolecular. The efficacy of the purified glycogenin as a substrate for the elongation reaction catalyzed by glycogen synthase was significantly enhanced if glycogenin was first allowed to undergo self-glucosylation. The length of the priming oligosaccharide is thus important for glycogen synthase action. A mutant of glycogenin, in which Tyr-194 was changed to Phe, behaved identically to the wild-type through purification and in particular bound to the UDP-agarose affinity matrix. Despite these indications of the protein's overall structural integrity, it was unable to self-glucosylate. This result indicates that Tyr-194 is necessary for glycogenin function and is consistent with Tyr-194 being the sole site of glucosylation.

Mechanisms of multisite phosphorylation and inactivation of rabbit muscle glycogen synthase

Glycogen synthase, a rate-determining enzyme for glycogen biosynthesis, is regulated by complex multisite phosphorylation of its subunit. Previous work has suggested that phosphorylation by some protein kinases, casein kinase II and cyclic AMP-dependent protein kinase, potentiates the ability of other protein kinases, glycogen synthase kinase 3 and casein kinase I, respectively, to modify the enzyme. In the present study, active glycogen synthase was expressed in Escherichia coli using a pET vector. The purified recombinant glycogen synthase had specific activity and subunit M(r) similar to enzyme isolated from rabbit muscle. Prior phosphorylation by casein kinase II was found to be an obligate requirement for phosphorylation by glycogen synthase kinase 3, which introduced 4 mol phosphate/mol subunit. Casein kinase II action did not affect activity, whereas the phosphorylation catalyzed by glycogen synthase kinase 3 caused a potent inactivation, reducing the +/- glucose 6-phosphate activity ratio from 0.7 to 0.10. Casein kinase I alone phosphorylated the recombinant glycogen synthase, indicating that substrate phosphorylation was not an absolute requirement. However, the prior action of cyclic AMP-dependent protein kinase significantly potentiated the ability of casein kinase I to phosphorylate and inactivate glycogen synthase. All previous analyses of glycogen synthase phosphorylation have used enzyme purified from mammalian sources and containing residual covalent phosphate. By using recombinant substrate, the present study represents a rigorous assessment of the role of prior phosphorylation in the recognition of mammalian glycogen synthase by glycogen synthase kinase 3 and casein kinase I. The conclusion is that phosphorylation of glycogen synthase can involve the concerted action of multiple protein kinases.

Molecular cloning, expression, and characterization of a 49-kilodalton casein kinase I isoform from rat testis

We report the molecular cloning and characterization of a 49-kDa form of casein kinase I from rat testis. A cDNA clone encoding the enzyme, designated casein kinase I delta, contained an open reading frame of 1284 nucleotides that predicts a polypeptide of 428 amino acids with a M(r) of 49,121. The predicted amino acid sequence shares 76% identity with casein kinase I alpha, a 37-kDa form recently cloned from bovine brain (Rowles, J., Slaughter, C., Moomaw, C., Hsu, J., and Cobb, M. (1991) Proc. Natl. Acad. Sci. U. S. A. 88, 9548-9552), and 65% identity with HRR25, a 57-kDa form of casein kinase I from yeast shown to be involved in DNA repair (Hoekstra, M. F., Liskay, R. M., Ou, A. C., DeMaggio, A. J., Burbee, D. G., and Heffron, F. (1991) Science 253, 1031-1034). Northern analysis of rat or rabbit RNA revealed three hybridizing species of 3.5-4.1, 2.2, and 1.9 kilobase pairs (kb). The largest message was detected in all tissues examined, whereas the 1.9- and 2.2-kb species were found predominantly in testis. A probe corresponding to the 3'-untranslated region of the casein kinase I delta cDNA hybridized only to the 1.9-kb transcript. Expression of the casein kinase I delta cDNA in Escherichia coli resulted in active enzyme that phosphorylated casein, phosvitin, and the peptide substrate DDDDVASLPGLRRR. Enzyme activity was associated with a predominant polypeptide of 55-kDa, although COOH-terminal degradation products of 50 and 42 kDa were also present in partially purified enzyme. Recombinant casein kinase I delta was inhibited by the specific casein kinase I inhibitor, CKI-7, half-maximally at 12 microM. Heparin inhibited recombinant casein kinase I delta when phosvitin was the substrate, with half-maximal inhibition at 11.5 micrograms/ml. However, if the peptide substrate was used, heparin activated recombinant casein kinase I delta 4-5-fold, with half-maximal activation at 9.5 micrograms/ml. A truncated form of casein kinase I delta, lacking the COOH-terminal 111 amino acids, was no longer activated by heparin. Casein kinase I delta therefore represents a separate member of the casein kinase I family distinguished by its larger size and unique kinetic behavior with respect to heparin.

Rabbit skeletal muscle glycogenin. Molecular cloning and production of fully functional protein in Escherichia coli

Glycogenin is a self-glucosylating protein involved in the initiation reactions of glycogen synthesis. Initiation occurs in two stages, requiring first the covalent attachment of a glucose residue to Tyr-194 of glycogenin and then elongation to form an oligosaccharide chain. The latter reaction is known to be catalyzed by glycogenin itself. The glycogenin sequence determined from the protein by Campbell and Cohen (Campbell, D. G., and Cohen, P. (1989) Eur. J. Biochem. 185, 119-125) was used to design oligonucleotide probes to screen a rabbit muscle lambda gt11 library. A cDNA was isolated that predicted an amino acid sequence identical to that of Campbell and Cohen, except that Cys residues replaced Ser-88 and Leu-97. Northern analysis indicated a strongly hybridizing message of 1.8 kilobases, present in most tissues including skeletal muscle, but much weaker in kidney and scarcely detectable in liver. A much weaker 3-kilobase message was also detected in muscle. Polymerase chain reaction was used to isolate DNA fragments encoding a portion of glycogenin from rat and cow. The sequence of this segment was > 90% identical at the amino acid level across the three species, indicating that glycogenin is a highly conserved protein. Using the pET-8c vector, the glycogenin protein was expressed in Escherichia coli. Incubation of the recombinant glycogenin with UDP-[14C]glucose and Mn2+ resulted in labeling of the glycogenin protein, indicating that the recombinant glycogenin was enzymatically active and capable of self-glucosylation. Furthermore, after incubation with UDP-glucose, the recombinant glycogenin could serve as a substrate for glycogen synthase, leading to the production of high M(r) polysaccharide. Therefore, production of functional glycogenin did not require the intervention of any other mammalian protein.

Recombinant rabbit muscle casein kinase I alpha is inhibited by heparin and activated by polylysine

The casein kinase I (CKI) family consists of widely distributed monomeric Ser/Thr protein kinases that have a preference for acidic substrates. Four mammalian isoforms are known. A full length cDNA encoding the CKI alpha isoform was cloned from a rabbit skeletal muscle cDNA library and was utilized to construct a bacterial expression vector. Active CKI alpha was expressed in Escherichia coli as a polypeptide of Mr 36,000. The protein kinase phosphorylated casein, phosvitin and a specific peptide substrate (D4). The enzyme was inhibited by the isoquinolinesulfonamide CKI-7, half-maximally at 70 microM. Heparin inhibited phosphorylation of the D4 peptide or phosvitin by CKI alpha. Polylysine activated when the D4 peptide was the substrate but had no effect on phosvitin phosphorylation. It is becoming clear that the individual CKI isoforms have different kinetic properties and hence could have quite distinct cellular functions.

Multiple roles for protein phosphatase 1 in regulating the Xenopus early embryonic cell cycle

Using cytostatic factor metaphase II-arrested extracts as a model system, we show that protein phosphatase 1 is regulated during early embryonic cell cycles in Xenopus. Phosphatase 1 activity peaks during interphase and decreases shortly before the onset of mitosis. A second peak of activity appears in mitosis at about the same time that cdc2 becomes active. If extracts are inhibited in S-phase with aphidicolin, then phosphatase 1 activity remains high. The activity of phosphatase 1 appears to determine the timing of exit from S-phase and entry into M-phase; inhibition of phosphatase 1 by the specific inhibitor, inhibitor 2 (Inh-2), causes premature entry into mitosis, whereas exogenously added phosphatase 1 lengthens the interphase period. Analysis of DNA synthesis in extracts treated with Inh-2, but lacking the A- and B-type cyclins, shows that phosphatase 1 is also required for the process of DNA replication. These data indicate that phosphatase 1 is a component of the signaling pathway that ensures that M-phase is not initiated until DNA synthesis is complete.

Yeast casein kinase I homologues: an essential gene pair

We report the isolation of an essential pair of Saccharomyces cerevisiae genes that encode protein kinase homologues. The two genes were independently isolated as dosage-dependent suppressors. Increased dosage of YCK1 suppressed defects caused by reduced SNF1 protein kinase activity, and increased dosage of YCK2 relieved sensitivity of wild-type cells to salt stress. The two genes function identically in the two growth assays, and loss of function of either gene alone has no discernible effect on growth. However, loss of function of both genes results in inviability. The two predicted protein products share 77% overall amino acid identity and contain sequence elements conserved among protein kinases. Partial sequence obtained for rabbit casein kinase I shares 64% identity with the two yeast gene products. Moreover, an increase in casein kinase I activity is observed in extracts from cells overexpressing YCK2. Thus YCK1 and YCK2 appear to encode casein kinase I homologues.

CDC55, a Saccharomyces cerevisiae gene involved in cellular morphogenesis: identification, characterization, and homology to the B subunit of mammalian type 2A protein phosphatase

Microscopic screening of a collection of cold-sensitive mutants of Saccharomyces cerevisiae led to the identification of a new gene, CDC55, which appears to be involved in the morphogenetic events of the cell cycle. CDC55 maps between CDC43 and CHC1 on the left arm of chromosome VII. At restrictive temperature, the original cdc55 mutant produces abnormally elongated buds and displays a delay or partial block of septation and/or cell separation. A cdc55 deletion mutant displays a cold-sensitive phenotype like that of the original isolate. Sequencing of CDC55 revealed that it encodes a protein of about 60 kDa, as confirmed by Western immunoblots using Cdc55p-specific antibodies. This protein has greater than 50% sequence identity to the B subunits of rabbit skeletal muscle type 2A protein phosphatase; the latter sequences were obtained by analysis of peptides derived from the purified protein, a polymerase chain reaction product, and cDNA clones. An extragenic suppressor of the cdc55 mutation lies in BEM2, a gene previously identified on the basis of an apparent role in bud emergence.

Molecular cloning and expression of the regulatory (RG1) subunit of the glycogen-associated protein phosphatase

DNA clones encoding the glycogen-binding (RG1) subunit of glycogen-associated protein phosphatase were isolated from rabbit skeletal muscle lambda gt11 cDNA libraries. Overlapping clones provided an open reading frame of 3327 nucleotides that predicts a polypeptide of 1109 amino acids with a molecular weight of 124,257. Northern hybridization of rabbit RNA identified a major mRNA transcript of 7.5 kilobases present in skeletal, diaphragm, and cardiac muscle, but not in brain, kidney, liver, and lung. Southern analysis of rabbit genomic DNA digested with various restriction endonucleases gave rise to a single hybridizing fragment, suggesting that a single gene is present. Expression of the complete RG1 subunit coding sequence in Escherichia coli generated a protein of apparent molecular weight on sodium dodecyl sulfate-polyacrylamide gel electrophoresis of approximately 160,000, similar to the size of the polypeptide detected by Western immunoblot in rabbit skeletal muscle extracts. The RG1 subunit shares significant homology with the Saccharomyces cerevisiae GAC1 gene product which is involved in activation of glycogen synthase and glycogen accumulation. The homology with GAC1 substantiates the role of this enzyme in control of glycogen metabolism. Hydropathy analysis of the RG1 subunit amino acid sequence revealed the presence of a hydrophobic region in the COOH terminus, suggesting a potential association with membrane. This result suggests that the same phosphatase regulatory component may be involved in targeting the enzyme both to membranes and to glycogen.

Glycogen metabolism and signal transduction in mammals and yeast

Mammalian glycogen synthase, with its complex multisite phosphorylation mechanisms, continues to provide interesting and novel examples of the regulation of protein function. The mammalian enzyme is phosphorylated in a hierarchal manner such that modification of certain sites requires the prior phosphorylation of other sites. Yeast contains two glycogen synthases that have extensive similarities to their mammalian counterpart but the greatest divergence in amino acid sequence is seen precisely in the regions likely to be involved in covalent control. We hope that examination of the control of the yeast glycogen synthase will be as informative as study of the mammalian enzymes, whether by revealing important parallels with the mammalian system or by uncovering major differences in mechanism.

Isolation of the GSY1 gene encoding yeast glycogen synthase and evidence for the existence of a second gene

Glycogen synthase preparations from Saccharomyces cerevisiae contained two polypeptides of molecular weights 85,000 and 77,000. Oligonucleotides based on protein sequence were utilized to clone a S. cerevisiae glycogen synthase gene, GSY1. The gene would encode a protein of 707 residues, molecular mass 80,501 daltons, with 50% overall identity to mammalian muscle glycogen synthases. The amino-terminal sequence obtained from the 85,000-dalton species matched the NH2 terminus predicted by the GSY1 sequence. Disruption of the GSY1 gene resulted in a viable haploid with glycogen synthase activity, and purification of glycogen synthase from this mutant strain resulted in an enzyme that contained the 77,000-dalton polypeptide. Southern hybridization of genomic DNA using the GSY1 coding sequence as a probe revealed a second weakly hybridizing fragment, present also in the strain with the GSY1 gene disrupted. However, the sequences of several tryptic peptides derived from the 77,000-dalton polypeptide were identical or similar to the sequence predicted by the GSY1 gene. The data are explained if S. cerevisiae has two glycogen synthase genes encoding proteins with significant sequence similarity The protein sequence predicted by the GSY1 gene lacks the extreme NH2-terminal phosphorylation sites of the mammalian enzymes. The COOH-terminal phosphorylated region of the mammalian enzyme over-all displayed low identity to the yeast COOH terminus, but there was homology in the region of the mammalian phosphorylation sites 3 and 4. Three potential cyclic AMP-dependent protein kinase sites are located in this region of the yeast enzyme. The region of glycogen synthase likely to be involved in covalent regulation are thus more variable than the catalytic center of the molecule.

Primary structure of rabbit skeletal muscle glycogen synthase deduced from cDNA clones

The complete amino acid sequence of rabbit skeletal muscle glycogen synthase was deduced from cDNA clones with a composite length of 3317 bp. An mRNA of 3.6 kb was identified by Northern blot analysis of rabbit skeletal muscle RNA. The mRNA coded for a protein of 734 residues with a molecular weight of 83,480. The deduced NH2-terminal and COOH-terminal sequences corresponded to those reported for the purified protein, indicating the absence of any proteolytic processing. At the nucleotide level, the 5' untranslated and coding regions were 79 and 90% identical for rabbit and human muscle glycogen synthases, whereas the 3' untranslated regions were significantly less similar. The enzymes had 97% amino acid sequence identity. Interestingly, the NH2 and COOH termini of rabbit and human muscle glycogen synthase, the regions of phosphorylation, showed the greatest sequence variation (15 of 19 mismatches and two insertion/deletion events), which may indicate different evolutionary constraints in the regulatory and catalytic regions of the molecule.

Phosphoserine as a recognition determinant for glycogen synthase kinase-3: phosphorylation of a synthetic peptide based on the G-component of protein phosphatase-1

Prior phosphorylation of its substrate has been shown to be important for substrate recognition by the protein kinase glycogen synthase kinase-3 (GSK-3). Phosphorylation of glycogen synthase by GSK-3 is known to be enhanced by the previous action of casein kinase II and the sequence -SXXXS(P)- was proposed as the minimal recognition determinant for GSK-3. The glycogen binding subunit of type 1 phosphoprotein phosphatase has been shown to be phosphorylated by cyclic AMP-dependent protein kinase at serine-13 in the sequence KPGFS(5)PQPS(9)RRGS(13)ESSEEVYV (F.B. Caudwell, A. Hiraga, and P. Cohen (1986) FEBS Lett. 194, 85-89). Inspection of the sequence revealed potential GSK-3 sites at residues 5 and 9. Using a synthetic peptide with the above sequence, we found that phosphorylation of serine-13 by cyclic AMP-dependent protein kinase permitted the recognition of serine-9 and serine-5 by GSK-3. The work provides another example of a substrate for GSK-3 and demonstrates that the action of GSK-3 is linked to the presence of phosphate in the substrate and not the action of any particular protein kinase. In the course of the analyses, a novel feature of trypsin cleavage of phosphopeptides was noted. In the sequence -SRRGS(P)- trypsin acted uniquely after the first arginine whereas in the sequence -S(P)RRGS(P)- it cleaved randomly at either arginine residue. The fact that GSK-3 could phosphorylate a peptide derived from a phosphatase subunit also raises the possibility that GSK-3 might be involved in controlling glycogen-associated type 1 phosphatase and, more generally, in mediating cyclic AMP control of protein phosphorylation in cells.

Identification of phosphorylation sites in peptides using a gas-phase sequencer

A simple procedure is described for determining the location of phosphorylation sites in phosphopeptides. The method employs measurement of 32P-labeled inorganic phosphate release during Edman degradation cycles using a gas-phase sequencer. The procedure is based on extracting peptides and inorganic phosphate from portions of the sample filter at strategic cycles in the sequence analysis followed by determination of the relative amounts of phosphate and phosphopeptide. One advantage of this technique is the very high recovery of the phosphate associated with the peptide, 80-97% in this study. In the course of this work, it was also found that phosphoserine residues themselves caused reduced efficiency of the Edman degradation as compared with unesterified serine residues. The present procedure has the merit of being simple and easy to apply.

Insulin action inhibits insulin-like growth factor-II (IGF-II) receptor phosphorylation in H-35 hepatoma cells. IGF-II receptors isolated from insulin-treated cells exhibit enhanced in vitro phosphorylation by casein kinase II

Insulin caused a rapid, dose-dependent increase in the binding of 125I-insulin-like growth factor-II (IGF-II) to the surface of cultured H-35 hepatoma cells. The [32P]phosphate content of the IGF-II receptors, immunoprecipitated from extracts of H-35 cell monolayers previously incubated with [32P]phosphate for 24 h, was decreased after brief exposure of the cells to insulin. Analysis of tryptic digests of labeled IGF-II receptors by bidimensional peptide mapping revealed that the decrease in the content of [32P]phosphate occurred to varying degrees on three tryptic phosphopeptides. Thin layer electrophoresis of an acid hydrolysate of isolated IGF-II receptors revealed the presence of [32P] phosphoserine and [32P]phosphothreonine. Insulin treatment of cells caused a decrease in the labeled phosphoserine and phosphothreonine content of IGF-II receptors. The ability of a number of highly purified protein kinases (cAMP-dependent protein kinase, protein kinase C, phosphorylase kinase, and casein kinase II) to catalyze the phosphorylation of purified IGF-II receptors was examined. Casein kinase II was the only kinase capable of catalyzing the phosphorylation of the IGF-II receptor on serine and threonine residues under the conditions of our assay. Bidimensional peptide mapping revealed that the kinase catalyzed phosphorylation of the IGF-II receptor on a tryptic phosphopeptide which comigrated with the main tryptic phosphopeptide found in receptors obtained from cells labeled in vivo with [32P]phosphate. IGF-II receptors isolated by immunoadsorption from insulin-treated H-35 cells were phosphorylated in vitro by casein kinase II to a greater extent than the receptors isolated from control cells. Similarly, IGF-II receptors from plasma membranes obtained from insulin-treated adipocytes were phosphorylated by casein kinase II to a greater extent than the receptors from control adipocyte plasma membranes. Thus, the insulin-regulated phosphorylation sites on the IGF-II receptor appear to serve as substrates in vivo for casein kinase II or an enzyme with similar substrate specificity.

Phosphorylation of phosphoprotein phosphatase inhibitor-2 (I-2) in rat fat cells

Fat cells were incubated with 32Pi for 2 h before the [32P]I-2 was immunoprecipitated, subjected to SDS/PAGE, and detected by autoradiography. [32P]I-2 (Mr = 32,000) was not recovered when excess purified I-2 was added with the antiserum or when nonimmune serum was used. Immunoprecipitated I-2 was heat-stable, inhibited phosphatase activity, and could be synergistically phosphorylated by casein kinase II and FA/GSK-3. Several times more [32P]phosphoserine than [32P]phosphothreonine was found in I-2 from 32P-labeled cells. Insulin increased the 32P-content of I-2 by as much as 40%, suggesting that phosphorylation of I-2 might be involved in the effect of insulin on stimulating protein dephosphorylation.

Multiple phosphorylation of mouse muscle glycogen synthase

Glycogen synthase was isolated from extracts of mouse diaphragm muscle by immunoprecipitation with specific antibodies raised against the rabbit muscle enzyme. A procedure was developed which permitted phosphorylation of the immunoprecipitated enzyme by several purified protein kinases. Peptide mapping techniques (including reverse-phase HPLC and thin-layer electrophoresis and chromatography) were used to compare tryptic phosphopeptides of the rabbit and mouse muscle enzymes. The results demonstrated a high degree of similarity in the chemical properties of these peptides, suggesting significant homology around the phosphorylation sites in these proteins. Thus, mouse peptides corresponding to the rabbit muscle peptides containing sites 1a, 1b, 2, 3, and 5 were identified, with protein kinase recognition specificities identical to those of the rabbit enzyme. The study indicates significant conservation in the muscle isozymes of glycogen synthase between mouse and rabbit as well as a similar distribution of phosphorylation sites throughout the enzyme subunit.

Heparin-activated protein kinase from rabbit muscle: relationship to enzymes of the glycogen synthase kinase-3 category

A heparin-activated protein kinase has been previously identified in rabbit skeletal muscle extracts (Z. Ahmad et al. (1985) FEBS Lett. 179, 96-100). Further study has indicated that this enzyme phosphorylates rabbit muscle glycogen synthase in the same tryptic peptide(s) as the protein kinase FA/GSK-3 (glycogen synthase kinase-3) and is able to activate the ATP-Mg2+-dependent protein phosphatase. These results indicate similarities in properties between the two protein kinases. Exposure of the heparin-activated enzyme to trypsin resulted in loss of heparin activation, from 3-fold to 1.3-fold. One hypothesis suggested by this result is that the enzyme FA/GSK-3 could be a derivative of the heparin-activated enzyme that has lost heparin sensitivity. The conceptual importance of this hypothesis is that it may provide a clue to the mode of regulation of this important class of protein kinases.

Multiple phosphorylation sites of rat liver glycogen synthase

Rat liver glycogen synthase was purified to homogeneity by an improved procedure that yielded enzyme almost exclusively as a polypeptide of Mr 85,000. The phosphorylation of this enzyme by eight protein kinases was analyzed by cleavage of the enzyme subunit followed by mapping of the phosphopeptides using polyacrylamide gel electrophoresis in the presence of SDS, reverse-phase high-performance liquid chromatography and thin-layer electrophoresis. Cyclic AMP-dependent protein kinase, phosphorylase kinase, protein kinase C and the calmodulin-dependent protein kinase all phosphorylated the same small peptide (approx. 20 amino acids) located in a 14 kDa CNBr-fragment (CB-1). Calmodulin-dependent protein kinase and protein kinase C also modified second sites in CB-1. A larger CNBr-fragment (CB-2) of approx. 28 kDa was the dominant site of action for casein kinases I and II, FA/GSK-3 and the heparin-activated protein kinase. The sites modified were all localized in a 14 kDa species generated by trypsin digestion. Further proteolysis with V8 proteinase indicated that FA/GSK-3 and the heparin-activated enzyme recognized the same smaller peptide within CB-2, which may also be phosphorylated by casein kinase 1. Casein kinase 1 also modified a distinct peptide, as did casein kinase II. The results lead us to suggest homology to the muscle enzyme with regard to CB-1 phosphorylation and the region recognized by FA/GSK-3, which in rabbit muscle is characterized by a high density of proline and serine residues. A striking difference with the muscle isozyme is the apparent lack of phosphorylations corresponding to the muscle sites 1a and 1b. These results provide further evidence for the presence of liver- and muscle-specific glycogen synthase isozymes in the rat. That the isozymes differ subtly as to phosphorylation sites may provide a clue to the functional differences between the isozymes.

Dephosphorylation of myosin by the catalytic subunit of a type-2 phosphatase produces relaxation of chemically skinned uterine smooth muscle

It is now well-established that phosphorylation of the 20,000-dalton light chain of smooth muscle myosin (LC20) is a prerequisite for muscle contraction. However, the relationship between myosin dephosphorylation and muscle relaxation remains controversial. In the present study, we utilized a highly purified catalytic subunit of a type-2, skeletal muscle phosphoprotein phosphatase (protein phosphatase 2A) and a glycerinated smooth muscle preparation to determine if myosin dephosphorylation, in the presence of saturating calcium and calmodulin, would cause relaxation of contracted uterine smooth muscle. Addition of the phosphatase catalytic subunit (0.28 microM) to the muscle bath produced complete relaxation of the muscle. The phosphatase-induced relaxation could be reversed by adding to the muscle bath either purified, thiophosphorylated, chicken gizzard 20,000-dalton myosin light chains or purified, chicken gizzard myosin light chain kinase. Incubation of skinned muscles with adenosine 5'-O-(thiotriphosphate) prior to the addition of phosphatase resulted in the incorporation of 0.93 mol of PO4/mol of LC20 and prevented phosphatase-induced relaxation. Under all of the above conditions, changes in steady-state isometric force were associated with parallel changes in myosin light chain phosphorylation over a range of phosphorylation extending from 0.01 to 0.97 mol of PO4/mol of LC20. We found no evidence that dephosphorylation of contracted uterine smooth muscles, in the presence of calcium and calmodulin, could produce a latch-state where isometric force was maintained in the absence of myosin light chain phosphorylation. These results show that phosphorylation or dephosphorylation of the 20,000-dalton myosin light chain is adequate for the regulation of contraction or relaxation, respectively, in glycerinated uterine smooth muscle.