Autophosphorylation and rapid dephosphorylation of the cAMP-dependent protein kinase from Blastocladiella emersonii zoospores

An unusual adenosine cyclic 3',5'-phosphate-dependent protein kinase from Dictyostelium discoideum

The CAMP-dependent protein kinase from Dictyostelium discoideum was extracted from cells at the stage of culmination. Less than 50% of the enzyme remains as a CAMP-dependent holoenzyme in the extracts, and the rest is recovered in the form of dissociated regulatory and catalytic subunits that were purified. The regulatory subunit is a monomeric protein of M, 42 000 that carries only one cAMP binding site (Kd = 3 nM). The catalytic subunit is also a monomer of M, 40000 with a sedimentation coefficient of 3.3S. The CAMP-dependent holoenzyme is a dimer consisting of one regulatory and one catalytic subunit, and the same structure is found for the holoenzyme reconstituted from the isolated subunits. Whereas cAMP binding to the regulatory subunit is independent of pH, both the catalytic activity and its ability to be inhibited by addition of regulatory subunit are increased very strongly between pH 5.5 and 7. The differences in molecular and catalytic properties of this CAMP-dependent protein kinase with those from mammalian origin are discussed in relation with the possibility that the enzyme from Dictyostelium represents an early form of the molecule in the evolutionary process.

Characterization of a type I regulatory subunit of cAMP-dependent protein kinase from the bivalve mollusk Mytilus galloprovincialis

Two isoforms of the regulatory subunit (R) of cAMP-dependent protein kinase (PKA), named R(myt1) and R(myt2), had been purified in our laboratory from two different tissues of the sea mussel Mytilus galloprovincialis. In this paper, we report the sequences of several peptides obtained from tryptic digestion of R(myt1). As a whole, these sequences showed high homology with regions of type I R subunits from invertebrate and also from mammalian sources, but homology with those of fungal and type II R subunits was much lower, which indicates that R(myt1) can be considered as a type I R isoform. This conclusion is also supported by the following biochemical properties: (1) R(myt1) was proved to have interchain disulfide bonds stabilizing its dimeric structure; (2) it failed to be phosphorylated by the catalytic (C) subunit purified from mussel; (3) it has a higher pI value than that of the R(myt2) isoform; and (4) it showed cross-reactivity with mammalian anti-RIbeta antibody.

PEST sequences in cAMP-dependent protein kinase subunits of the aquatic fungus Blastocladiella emersonii are necessary for in vitro degradation by endogenous proteases

During Blastocladiella emersonii germination, the regulatory (R) and the catalytic (C) subunits of the cAMP-dependent protein kinase (PKA) are rapidly and concurrently degraded, after PKA activation in response to a transient increase in intracellular cAMP levels. The possibility that PEST sequences could be acting as proteolytic recognition signals in this process was investigated, and high score PEST sequences were found in both B. emersonii R and C subunits. Deletions in the PEST sequences were obtained by site-directed mutagenesis and the different PKA subunits were independently expressed in Escherichia coli. Proteolysis assays of the various R and C recombinant forms, using B. emersonii cell extracts as the source of proteases, showed a strong correlation between the presence of high score PEST sequences and susceptibility to degradation. Furthermore, the amino-terminal sequence of the proteolytic fragments indicated that the cleavage sites in both subunits are located at or near the PEST regions. The PEST sequence in B. emersonii C subunit, which when deleted or disrupted leads to resistance to proteolysis, is entirely contained in the 72-amino-acid extension located in the N-terminus of the protein. C subunit mutants carrying deletions in this region displayed little difference in their kinetic properties or enzyme thermostability. These results suggest that the N-terminal extension may only play a role in C subunit degradation.

Cloning and characterization of the gene for the catalytic subunit of cAMP-dependent protein kinase in the aquatic fungus Blastocladiella emersonii

We have isolated and characterized cDNA and genomic DNA clones encoding the catalytic subunit (C) of cAMP-dependent protein kinase in the aquatic fungus Blastocladiella emersonii. The C-subunit amino acid sequence derived from the nucleotide sequence predicts a basic polypeptide of 424 residues, excluding the initiator methionine, which by amino-terminal sequence analysis has been shown to be absent from the mature protein. The Blastocladiella C presents a 70-amino-acid extension at the amino terminus, when aligned to the mouse C alpha subunit, being one of the largest C subunits already characterized. The B. emersonii C-gene-coding region is interrupted by three introns, ranging in size over 57-69 bp. The positions of the introns are quite different from those found in other species, suggesting a considerable amount of evolutionary drift in the gene structure. The 5'-flanking region lacks recognizable TATA or CCAAT sequences, is remarkably high in GC content (70%), and primer extension experiments indicate that transcription initiates from multiple sites. Several sequence motifs were identified in the promoter region which could be involved in the developmental control of this gene.

Chemotactic stimulation of aggregation-stage Dictyostelium cells induces rapid changes in energy metabolism, as measured by succinic thiokinase phosphorylation

In crude mitochondrial fractions of the cellular slime mold Dictyostelium discoideum, a 38-kDa protein can be detected in phosphorylation assays under autophosphorylation conditions in SDS polyacrylamide gels. p38 can be phosphorylated in vitro using either ATP or GTP as phosphoryl donors. After stimulation of aggregation competent cells with the chemoattractant cAMP, p38 phosphorylation pattern changes rapidly. Caffeine, a known inhibitor of cAMP relay in D. discoideum inhibits cAMP induced changes in p38 phosphorylation. The rapid changes in p38 phosphorylation after cAMP stimulation reflect changes in energy metabolism and these changes are most likely mediated by changes in internal calcium concentrations. The mitochondrial localization and other data presented on the characterization of this protein led us to the conclusion that p38 is the alpha subunit of succinic thiokinase. Data showing a correlation between in-vitro p38 phosphorylation and the metabolic state of the cells at the moment of the cell lysis are included.

Purification of a soluble casein kinase II from Dictyostelium discoideum lacking the beta subunit: regulation during proliferation and differentiation

A type II casein kinase has been purified from the soluble fraction of Dictyostelium discoideum vegetative cells. The enzyme has been purified 370 fold and behaves catalytically as casein kinase type II, in the sense that it utilizes GTP as well as ATP as phosphoryl donors, it is inhibited by low heparin concentrations and phosphorylates a specific peptide for CK II. It is a tetramer of 38 kDa-subunits with catalytic activity and ability to autophosphorylate in vitro. The comparison of this activity with the nuclear enzyme previously purified from the same organism indicates that both have the same molecular structure. Both enzymes have antigenic determinants in common with casein kinase II from bovine thymus, suggesting a high degree of conservation during evolution. Studies on the activity of this enzyme during early differentiation, and in the transition from quiescence to proliferation shows an increase in specific activity suggesting a crucial role for the enzyme in this organism.

Autophosphorylation of Mucor rouxii cAMP-dependent protein kinase and its role in holoenzyme activation

Two phosphoproteins of 53,000 and 63,000 mol. wt detected in partially purified preparations of Mucor rouxii cAMP-dependent protein kinase submitted to phosphorylation conditions with [gamma-32P]ATP are demonstrated to be the result of the autophosphorylation of its regulatory subunit, according to the following criteria: (1) linearity of phosphate incorporation with enzyme sample; (2) independence of phosphate incorporation on temperature; (3) correlation of the phosphoproteins with enzymatic activity in a DEAE-Sepharose chromatography; (4) specific elution of the phosphorylated proteins from cAMP-agarose; (5) phosphorylation of the purified regulatory subunit. Antibodies specific against Mucor regulatory subunit detected an intact subunit of 72,000 mol. wt in crude extracts. Autophosphorylation of the fungal protein kinase A promotes activation of the holoenzyme by cAMP since: (1) under conditions of partial activation, increase of activity is observed when using the phosphoform of the enzyme; (2) release of free catalytic subunit from cAMP-agarose is enhanced when the holoenzyme is previously phosphorylated.

Coordinate pretranslational control of cAMP-dependent protein kinase subunit expression during development in the water mold Blastocladiella emersonii

The aquatic fungus Blastocladiella emersonii provides a system for studying the regulation of expression of regulatory (R) and catalytic (C) subunits of cAMP-dependent protein kinase (PKA). Blastocladiella cells contain a single PKA with properties very similar to type II kinases of mammalian tissues. During development cAMP-dependent protein kinase activity and its associated cAMP-binding activity change drastically. We have previously shown that the increase in cAMP-binding activity during sporulation is due to de novo synthesis of R subunit and to an increase in the translatable mRNA coding for R (Marques et al., Eur. J. Biochem. 178, 803, 1989). In the present work we have continued these studies to investigate the mechanism by which the changes in the level of kinase activity take place. The C subunit of Blastocladiella has been purified; antiserum has been raised against it and used to determine amounts of C subunit throughout the fungus' life cycle. A sharp increase in C subunit content occurs during sporulation and peaks at the zoospore stage. Northern blot analyses, using Blastocladiella C and R cDNA probes, have shown that the levels of C and R mRNAs parallel their intracellular protein concentrations. These results indicate a coordinate pretranslational control for C and R subunit expression during differentiation in Blastocladiella.

Characterization of three casein kinases type I from Dictyostelium discoideum

Three different casein kinases type I have been characterized and partially purified from vegetative cells of Dictyostelium discoideum. The enzymes have been classified as type I because they are excluded from DEAE cellulose columns and do not utilize GTP as phosphoryl donor. We have named these activities as casein kinases IA, IB and IC respectively, according to the elution profile on phosphocellulose chromatography. The three activities differ in: the sensitivity to heparin inhibition; the salt optimum for activity and the amino acids phosphorylated, using casein as substrate. Experiments carried out in conditions that favor autophosphorylation indicate that casein kinase IB could have a 53 kDa subunit, susceptible to autophosphorylation in vitro.

Phosphorylation-dependent regulation of amidotransferase during the development of Blastocladiella emersonii

The enzyme amidotransferase [2-amino-2-deoxy-D-glucose-6-phosphate ketol isomerase (amino-transferring); EC 2.6.1.16] catalyzes the first step in the hexosamine biosynthetic pathway. In Blastocladiella emersonii the sensitivity of the enzyme to the inhibitor uridine-5'-diphospho-N-acetylglucosamine (UDP-GlcNAc) is developmentally regulated. The inhibitable form of amidotransferase activity present in the zoospore is converted to a noninhibitable form during germination. The latter form is present throughout the growth phase and sensitivity to UDP-GlcNAc gradually returns to the zoospore level during sporulation [C.P. Selitrennikoff, N.E. Dalley, and D.R. Sonneborn (1980) Proc. Natl. Acad. Sci. USA 77, 5998-6002]. The following evidence suggests that a phosphorylation/dephosphorylation mechanism underlies this interconversion: (i) Both the vegetative and zoospore enzymes have the same molecular weight of 140,000, but the vegetative enzyme elutes significantly earlier on a DEAE-cellulose column than does the zoospore enzyme. (ii) The increased sensitivity to UDP-GlcNAc occurring in vivo and in vitro correlates with increased phosphorylation of a polypeptide of apparent Mr 76,000. This component copurifies with amidotransferase activity through ion-exchange chromatography and sucrose density gradient centrifugation. (iii) Desensitization and concurrent dephosphorylation of sensitive amidotransferase can be observed in vitro after treatment with a partially purified magnesium-dependent phosphoprotein phosphatase from zoospores.

Cyclic AMP-dependent protein kinases of Paramecium. II. Catalytic and regulatory properties of type II kinase from cilia

The type II cAMP-dependent protein kinase (cAMP-PK-II) from cilia of Paramecium, purified free of type I cAMP-PK (cAMP-PK-I) and of cGMP-dependent protein kinase (cGMP-PK), phosphorylated several basic proteins and a heptapeptide containing serine (Kemptide). The enzyme was partially inhibited by the protein kinase inhibitor (Walsh inhibitor), but only at relatively high inhibitor concentrations. Half-maximal activation of cAMP-PK-II occurred at 15-25 nM cAMP. Several cAMP analogs were tested for ability to bind and activate the enzyme. 8-bromo-cGMP, a potent activator of Paramecium cGMP-PK, was a poor activator of Paramecium cAMP-PK-II. Activation of cAMP-PK-II was influenced by the phosphorylation assay buffer. Phosphate buffers provided increased activation by cAMP but decreased total activity relative to that measured in Mops-Tris buffer. The kinase was cAMP-independent when the pH of the assay buffer was high. Preincubation of cAMP-PK-II with histones also activated the enzyme in the absence of cAMP. The cAMP-PK-II bound cAMP with a Kd of 23 nM, and bound cAMP was released with a biphasic time course, suggesting two non-identical binding sites. The properties of the cAMP-PK of this ciliated protozoan appear to be closely similar to those of vertebrates.

Developmental regulation of expression of the regulatory subunit of the cAMP-dependent protein kinase of Blastocladiella emersonii

A monospecific polyclonal antiserum to the regulatory subunit (R) of the cAMP-dependent protein kinase of Blastocladiella emersonii has been developed by immunization with purified regulatory subunit. In Western blots, the antiserum displays high affinity and specificity for the intact R monomer of Mr = 58,000, as well as for its proteolytic products of Mr = 43,000 and Mr = 36,000, even though the antiserum has been raised against the Mr = 43,000 fragment. Western blots of cell extracts prepared at different times during the life cycle of the fungus indicate that the increase in cAMP-binding activity occurring during sporulation, as well as its decrease during germination, are associated with the accumulation of the regulatory subunit during sporulation and its disappearance during germination, respectively. Pulse labeling with [35S]methionine and immunoprecipitation indicate that the accumulation of R is due to its increased synthesis during sporulation. Two-dimensional gel electrophoresis of affinity purified cell extracts obtained after [35S]methionine pulse labeling during sporulation confirms de novo synthesis of R during this stage and furthermore shows that the protein is rapidly phosphorylated after its synthesis. In vitro translation studies using RNA isolated from different stages of the life cycle followed by immunoprecipitation have shown that the time course of expression of the mRNA coding for the regulatory subunit parallels the rate of its synthesis in vivo.

Yeast cAMP-dependent protein kinase can be associated to the plasma membrane

Using an anti-yeast regulatory subunit antibody and the synthetic peptide Kemptide as specific substrate we show in this work that purified preparations of yeast plasma membrane have an associated form of the regulatory subunit and cAMP-dependent protein kinase activity. Treatment of the plasma membrane "in vitro" with 1 microM cAMP releases cAMP-independent protein kinase activity while regulatory subunit remains on the membrane as revealed by immunoblotting. Incubation of the plasma membrane with [gamma-32P]ATP results in the phosphorylation of the regulatory subunit.

cAMP-dependent protein kinase from Dictyostelium discoideum

The cAMP-dependent protein kinase (cAK) from Dictyostelium discoideum is an enzyme composed of one catalytic and one regulatory subunit. Upon binding of cAMP, the holoenzyme dissociates to liberate free active catalytic subunits. The cAK is developmentally regulated, ranging from very little activity in vegetative cells to maximal expression in postaggregative cells. Although there is no immunological cross-reaction between the subunits of cAKs from Dictyostelium and from other organisms, they share several biochemical properties. A complete cDNA for the regulatory subunit has been cloned and sequenced. Only one copy of the gene for the regulatory subunit is present per haploid genome. On the basis of the comparison of the structure of the cAK from Dictyostelium with its counterparts in yeast and higher eukaryotes, we propose a model for the evolution of cyclic-nucleotide-binding proteins.

Gene isolation by direct in situ cAMP binding

The regulatory subunit of the cAMP-dependent protein kinase expressed in clones isolated by immunoscreening of a lambda gt11 cDNA library from Dictyostelium discoideum exhibits high affinity for cAMP [Mutzel et al., Proc. Natl. Acad. Sci. USA 84 (1987) 6-10]. Based on this property, we have developed a screening procedure to detect in situ cAMP-binding activity directly on phage plaques transferred to nitrocellulose filters. Highly radioactive cAMP was synthesized using [alpha-32P]ATP at 3000 Ci/mmol as the substrate of purified adenylate cyclase from Bordetella pertussis. Filter replicas of the library plated at 3 X 10(4) pfu/dish, were incubated in the presence of 2 nM [32P]cAMP and then washed thoroughly. Three clones out of 1.2 X 10(5) were detected, all of which coded for the regulatory subunit, as judged by hybridization with a specific DNA probe. The cAMP binding to the purified clones was characterized in situ by displacement with specific analogues. The ability to displace labelled cAMP was in accord with the affinities of the analogues previously reported for the regulatory subunit of the Dictyostelium cAMP-dependent protein kinase. We are able to detect fmol levels of regulatory subunit contained in phage plaques and therefore the method could be used to screen libraries from other organisms for proteins exhibiting high affinities for cyclic nucleotides.

Developmental changes in translatable RNA species and protein synthesis during sporulation in the aquatic fungus Blastocladiella emersonii

Protein synthesis during sporulation in Blastocladiella emersonii is developmentally regulated as revealed using [35S]methionine pulse labeling and two-dimensional gel electrophoresis. A large increase in the synthesis of several proteins is associated with particular stages. A large number of basic proteins are synthesized exclusively during late sporulation. Changes in translatable mRNA species were also detected by two-dimensional gel electrophoresis of the polypeptides produced in a cell-free rabbit reticulocyte lysate primed with RNA prepared at different stages of sporulation. The synthesis of several proteins during sporulation seems to be transcriptionally controlled. Most of the sporulation-specific messages are not present in the mature zoospores.

Altered autophosphorylation of adenosine 3',5'-phosphate-dependent protein kinase in the dunce memory mutant of Drosophila melanogaster

The phosphorylation-dephosphorylation, in the presence of adenosine 5'-[gamma-32P]triphosphate, of a polypeptide of apparent molecular mass 53,000 has been compared in head homogenates of wild type and memory mutant dunceM11 strains of Drosophila melanogaster. In both strains, labelling of the 53 kilodalton protein required exogenous adenosine 3',5'-phosphate (cAMP), but in dunceM11 cAMP at higher concentration (above approximately 3 microM) caused the rapid disappearance of the label. This differential dephosphorylation can be attributed to the lack of a cAMP-specific phosphodiesterase isoenzyme in the mutant. Several lines of evidence indicate that the 53 kilodalton protein is identical with the regulatory subunit of cAMP-dependent protein kinase. The findings suggest that in the mutant's nerve cells the state of phosphorylation of the regulatory subunit of cAMP-dependent protein kinase is altered, which may contribute to the biochemical disorder leading to the memory deficit.

[Mode of action of cyclic amp in prokaryotes and eukaryotes, CAP and cAMP-dependent protein kinases]

cAMP is an ubiquitous compound which is involved in the regulation of many biological processes. In bacteria such as E. coli, cAMP mediates the activation of catabolic operons via the CAP protein. The CAP-cAMP complex, whose tridimensional structure has recently been established, binds to the promoter regions of catabolic operons at a specific site, and activates their transcription by inducing RNA polymerase to bind and initiate transcription at the correct site. Various phenomenons including protein-protein interactions or CAP-induced DNA bending or kinking could be involved in the process of forming the open transcription complex. In eukaryotes, cAMP activates cAMP dependent protein kinases which covalently modify proteins by phosphorylation on serine or threonine residues. The catalytically inactive holoenzyme is generally a tetramer containing two regulatory subunits, each capable of binding two molecules of cAMP, and two catalytic subunits. In mammalian cells, two types of cAMP dependent protein kinases (I and II) can be distinguished on the basis of their regulatory subunits; their relative proportion varies from tissue to tissue. Binding of cAMP to the regulatory subunits induces the dissociation of the holoenzyme and releases the free and active catalytic subunits. Phosphorylation of proteins occurs at sequences containing two basic residues in the vicinity of the phosphorylated serine or threonine. A heat-stable protein, present in most eukaryotic cells, specifically interacts with the catalytic subunit and inhibits its activity. The amino-acid sequence of cAMP dependent protein kinases has recently been determined. It is interesting to note that the domains responsible for cAMP binding by the regulatory subunits of mammalian cAMP dependent protein kinases and CAP share important sequence homologies. The same phenomenon is observed concerning the domain responsible for ATP binding to the catalytic subunit of cAMP dependent protein kinases and that of tyrosine-specific protein kinases from oncoviruses. Other eukaryotic proteins such as S-adenosyl-L-homocysteine (SAH) hydrolase are also capable of binding cAMP. The latter is involved in the regulation of S-adenosyl-L-methionine dependent methylations, and its activity could be affected by cAMP. Besides its role as an effector of enzymatic activity via phosphorylation, such as in the regulation of glycogen metabolism, cAMP has recently been shown to activate the transcription of a number of eukaryotic genes. This process probably also involves protein phosphorylation, but its precise mechanism remains to be understood.

Phosphorylation of ribosomal protein S6 in the aquatic fungus Blastocladiella emersonii

The changes in the degree of phosphorylation of ribosomal protein S6 during the life cycle of the aquatic fungus Blastocladiella emersonii were analyzed by two-dimensional gel electrophoresis. Three phosphorylated derivatives of S6 are present throughout the entire life cycle. However, under certain germination conditions, more highly phosphorylated derivatives of S6 appear. Nonetheless, the resumption of protein synthesis that occurs during germination is not dependent on those highly phosphorylated derivatives of S6. The pattern and sites of phosphorylation of S6 labelled in vivo with [32P]orthophosphate have been compared with those of 40S ribosomal subunit labelled in vitro by partially purified protein kinases. Three major phosphopeptides were found in S6 isolated from the zoospore, while six phosphopeptides were found after zoospore germination (in germling cells). The phosphopeptide patterns of S6 phosphorylated by the cAMP-dependent protein kinase and by casein kinases I and II were completely distinct. Only the cAMP-dependent protein kinase gives rise to a phosphopeptide found in 32P-labelled cells, indicating that one of sites phosphorylated in vivo is also phosphorylated in vitro by the cAMP-dependent protein kinase.

Subcellular distribution and partial characterization of the cyclic AMP-binding proteins of Trypanosoma brucei

We assayed the cyclic AMP-binding activity of Trypanosoma brucei by two well established methods, such as the one of Gilman (Proc. Natl. Acad. Sci. U.S.A. 67, 305-312, 1970) and Ueland & Doskeland (Biochem. J., 157, 117-126, 1976). The results indicate that the former technique underestimated the total amount of cyclic AMP bound by T. brucei homogenates by up to 7.5 fold. Similar results were obtained with other Trypanosomatidae such as Leishmania tropica, and Crithidia luciliae. The bulk of the cyclic AMP-binding proteins of T. brucei appeared soluble after centrifugation of post-large-granule extracts in isopycnic sucrose-gradients. Upon fractionation by DEAE-Sephacel column chromatography, two peaks of activity eluted which were responsible for all the specific cyclic AMP-binding activity present in the cytosol of T. brucei. These two activities, which we denominated as Peak 'a' and Peak 'b' respectively, differed in a number of properties such as sensitivity to proteases, stability to storage at -20 degrees C, displacement of cyclic AMP bound by adenine analogues, and coefficients of sedimentation.