Phosphorylation of the regulatory subunit of yeast cAMP-dependent protein kinase.

In vitro phosphorylation of the regulatory subunit of yeast cAMP-dependent protein kinase was studied. The cAMP-binding regulatory subunit (R subunit) can be multiply phosphorylated. Three distinct phosphorylation sites were inferred from the different ATP concentrations required for phosphorylation and from the presence of two discrete mobility shifts in NaDodSO4/polyacrylamide gel electrophoresis of the R subunit on phosphorylation. Limited tryptic digestion of the phosphorylated R subunit showed that a Mr 37,000 cAMP-binding peptide contained one of the phosphorylation sites and that a separate Mr 12,000 peptide contained another phosphorylation site. The yeast R subunit is therefore similar to the type II R subunit of mammalian origin, although it has a larger Mr (64,000 vs. 58,000) and is multiply phosphorylated. In vivo, both phosphorylated and unphosphorylated forms of the R subunit were found in cells grown in lactate or to stationary phase in 1.5% glucose, while cells grown in 5% glucose contained the unphosphorylated form.     

Intracellular targeting of the type-I alpha regulatory subunit of cAMP-dependent protein kinase

The specificity of cyclic adenosine monophosphate (cAMP)-mediated signaling events is achieved by the composition and biochemical properties of the different cAMP-dependent protein kinase holoenzymes (PKAI and II) and by compartmentalization of PKA to discrete subcellular locations. Intracellular localization is mediated by interaction with A-kinase anchoring proteins (AKAPs) that recruit PKAII close to its substrates and to sites where it can respond optimally to local changes in intracellular cAMP concentration, thereby directing and amplifying the effects of cAMP. This review presents recent evidence that indicates that specific AKAPs mediate PKAI anchoring through interaction with its regulatory subunit RI alpha, notably at the neuromuscular junction of skeletal muscle.     

Cloning and cDNA sequence of the regulatory subunit of cAMP-dependent protein kinase from Dictyostelium discoideum.

cDNA clones encoding the regulatory subunit of the cAMP-dependent protein kinase (ATP:protein phosphotransferase, EC 2.7.1.37) from Dictyostelium discoideum were isolated by immunoscreening of a cDNA library constructed in the expression vector lambda gt11. High-affinity cAMP-binding activity was detected in extracts from bacteria lysogenized with these clones. Nucleotide sequence analysis of three overlapping clones allowed the determination of a 1195-base-pair cDNA sequence coding for the entire regulatory subunit and containing nontranslated 5' and 3' sequences. The open reading frame codes for a protein of 327 amino acids, with molecular weight 36,794. The regulatory subunit from Dictyostelium shares a high degree of homology with its mammalian counterparts, but is lacking the NH2-terminal domain required for the association of regulatory subunits into dimers in other eukaryotes. On the basis of the comparison of the regulatory subunits from Dictyostelium, yeast, and bovine tissues, a model for the evolution of these proteins is proposed.     

Genetic characterization of a brain-specific form of the type I regulatory subunit of cAMP-dependent protein kinase.

An isoform (RI beta) of the regulatory type I subunit gene of cAMP-dependent protein kinase (EC 2.7.1.37) has been characterized in mouse. The open reading frame of the RI beta cDNA is 72% identical in nucleotide sequence with the previously cloned RI gene, now referred to as RI alpha. Both genes code for a protein of 380 amino acids and their proteins are 82% identical in amino acid sequence. Sequence similarity is highest in the regions that form the pseudosubstrate-binding site of the catalytic subunit and the two cAMP binding domains. The amino-terminal portion shows the greatest dissimilarity, suggesting that the isoforms may differ in their dimerization properties or interaction with other proteins. In contrast to RI alpha, which is constitutively expressed in all tissues, RI beta is expressed in a highly tissue-specific manner. Brain and spinal cord contained significant levels of RI beta mRNA, testis RNA gave a detectable signal, and all other tissues tested were negative. Expression of a RI beta cDNA in NIH 3T3 cells resulted in the appearance of a RI subunit protein that migrated more slowly than RI alpha after NaDodSO4/PAGE. The native form of RI beta in brain could also be distinguished from RI alpha by its abnormal migration on NaDodSO4/PAGE. RI beta protein produced in 3T3 cells was shown to be functional by its ability to form a cAMP-dependent holoenzyme with the catalytic subunit.     

Isolation of a cDNA clone for the type I regulatory subunit of bovine cAMP-dependent protein kinase.

A cDNA clone for the type I regulatory subunit (RI) of cAMP-dependent protein kinase (ATP:protein phosphotransferase, EC 2.7.1.37) was isolated from bovine testis by a differential screening method. mRNA coding for RI was enriched 50- to 100-fold by polysome immunoadsorption chromatography with affinity-purified rabbit anti-RI and protein A-Sepharose. Poly(A)+ RNA from these polysomes was utilized to construct a cDNA library in pBR322, and this library was screened for hybridization to 32P-labeled cDNAs synthesized from either total or RI-enriched poly(A)+ RNA. Plasmids isolated from colonies showing preferential hybridization to the latter probe were further characterized by hybrid selection and DNA sequence analysis. One of these plasmids (designated 62C12) contains a 1,350-nucleotide insert that hybridized to RI mRNA; partial nucleotide sequence analysis confirmed its identity and indicated that it may contain the entire RI coding region. We also have identified a recombinant plasmid with a 1,550-nucleotide insert that selected through hybridization a mRNA coding for a 55,000-dalton protein that crossreacts with anti-RI antibodies. The function of this latter protein is unknown.     

Primary structure of the regulatory subunit of type II cAMP-dependent protein kinase from bovine cardiac muscle.

The complete amino acid sequence of the regulatory subunit of type II cAMP-dependent protein kinase from bovine cardiac muscle is presented. Primary fragments for the sequence determination were obtained by limited proteolysis with various proteases or by cleavage with cyanogen bromide. The sequence of the 400 amino acid residues has two homologous regions, strongly suggesting tandem gene duplication. The predicted secondary structure suggests the presence of 42% alpha-helix, 23% beta-strand, and 23 beta-turns. The molecular weight of the subunit, as derived from the sequence, is 45,084 including a phosphate group at residue 95. This is significantly less than earlier estimates based on NaDodSO4 gel electrophoresis and sedimentation experiments. The structure is discussed in terms of putative sites of interaction with cAMP and with the catalytic subunit.     

Differentiation of HL-60 leukemia by type I regulatory subunit antisense oligodeoxynucleotide of cAMP-dependent protein kinase.

A marked decrease in the type I cAMP-dependent protein kinase regulatory subunit (RI alpha) and an increase in the type II protein kinase regulatory subunit (RII beta) correlate with growth inhibition and differentiation induced in a variety of types of human cancer cells, in vitro and in vivo, by site-selective cAMP analogs. To directly determine whether RI alpha is a growth-inducing protein essential for neoplastic cell growth, human HL-60 promyelocytic leukemia cells were exposed to 21-mer RI alpha antisense oligodeoxynucleotide, and the effects on cell replication and differentiation were examined. The RI alpha antisense oligomer brought about growth inhibition and monocytic differentiation, bypassing the effects of an exogenous cAMP analog. These effects of RI alpha antisense oligodeoxynucleotide correlated with a decrease in RI alpha receptor and an increase in RII beta receptor level. The growth inhibition and differentiation were abolished, however, when these cells were exposed simultaneously to both RI alpha and RII beta antisense oligodeoxynucleotides. The RII beta antisense oligodeoxynucleotide alone has been previously shown to specifically block the differentiation inducible by cAMP analogs. These results provide direct evidence that RI alpha cAMP receptor plays a critical role in neoplastic cell growth and that cAMP receptor isoforms display specific roles in cAMP regulation of cell growth and differentiation.     

Structural similarity in the DNA-binding domains of catabolite gene activator and cro repressor proteins.

It is shown that there is a structural similarity between the presumed DNA-binding regions of the Escherichia coli catabolite gene activator protein ("CAP") and the cro repressor protein ("cro") from bacteriophage lambda. The correspondence between the two proteins is particularly striking for a structural unit consisting of two consecutive alpha-helices. The 24 alpha-carbon atoms that constitute the two-helical structural units in the two proteins can be superimposed with a root-mean-square disagreement of 1.1 A. It is shown that this agreement is very unlikely to be due to a chance correspondence. For both CAP activator and cro repressor proteins it is the second alpha-helix of the two-helical unit that has been proposed to bind within the major groove of left-handed or right-handed B DNA, respectively [McKay, D. B. & Steitz, T. A. (1981) Nature (London) 290, 744-749; Anderson, W. F., Ohlendorf, D. H., Takeda, Y. & Matthews, B. W. (1981) Nature (London) 290, 754-758]. The structural correspondence between CAP and cro seen here, together with other recent evidence of sequence homologies between cro, CAP, and other proteins that bind double-stranded DNA, suggests that the two-helical unit is likely to be a common feature of many DNA-binding proteins. The results also suggest that some principles of specific protein-double-stranded DNA interaction may be general and include recognition via alpha-helices fitting into the major groove of the DNA.     

Inhibition of the Mg(II).ATP-dependent phosphoprotein phosphatase by the regulatory subunit of cAMP-dependent protein kinase.

We report potent inhibition of the Mg(II).ATP-dependent protein phosphatase, Fc.M, by the regulatory subunit dimer of type II cAMP-dependent protein kinase, RII2. The protein kinase catalytic subunit has no effect on phosphatase activity and is unable to substitute for kinase FA in the kinase FA- and Mg(II).ATP-mediated phosphatase activation reaction. Phosphatase inhibition was investigated as a function of RII2 concentration. The results suggest that RII2 both inhibits the active phosphatase and inhibits phosphatase activation. The inhibition is shown to be noncompetitive with respect to substrate (phosphorylase a). The potential physiological significance of this inhibition is discussed in terms of phosphorylation/dephosphorylation cascade systems involving this kinase and phosphatase.     

The molecular cloning of a type II regulatory subunit of the cAMP-dependent protein kinase from rat skeletal muscle and mouse brain.

A cDNA clone for a type II regulatory (R) subunit of the cAMP-dependent protein kinase (ATP:protein phosphotransferase, EC 2.7.1.37) was isolated from a rat skeletal muscle library using a specific 47-base oligonucleotide probe. The rat cDNA was 1.2 kilobases (kb) in length and contained an open reading frame of 1.113 kb representing 92% of the coding region of the molecule. Nick-translated rat cDNA was then used to isolate a mouse RII cDNA clone from a brain library that contained an open reading frame of 1.143 kb. Because both cDNAs lacked complete coding sequences, the remainder of the RII coding region was obtained from a 15-kb mouse genomic clone. The mouse RII coding region contains 1.2 kb corresponding to a 400-amino acid protein of 51.141 kDa. The mouse cDNA hybridizes to two mRNA species, a 2.4-kb form that was only observed in testis and a 6.0-kb form found in a wide range of tissues, including testis.     

High-affinity binding of the regulatory subunit (RII) of cAMP-dependent protein kinase to microtubule-associated and other cellular proteins.

Interaction of the regulatory subunit of the type II cAMP-dependent protein kinase (RII) with tissue-specific cellular binding proteins has been demonstrated by two independent methods. Complexes of RII and its binding proteins were isolated on a cAMP analog-Sepharose affinity column, eluted from the column, and analyzed by sodium dodecyl sulfate/polyacrylamide gel electrophoresis. Alternatively, nitrocellulose blots made from polyacrylamide gels containing samples of tissue extracts or affinity column eluates were treated with sequential overlays of RII, monospecific antibody, and radioiodinated protein A. In bovine cerebrum, specific high-affinity interactions between RII and several binding proteins, including major proteins of 300, 80, and 68 kDa, were recognized by the two methods. The 300-kDa and 68-kDa proteins were identified as microtubule-associated protein 2 (300 kDa) and a protein of lower molecular weight (68 kDa) that copurifies with it. The additional major binding protein of 80 kDa requires further characterization. In addition, several binding proteins distinct from those observed in bovine cerebrum were found in bovine heart. Many of the RII binding proteins from brain and heart served to differing extents as substrates for the purified catalytic subunit of cAMP-dependent protein kinase. One hypothesis of the significance of the protein kinase regulatory subunit interaction with cellular binding proteins is that this may control the protein kinase holoenzyme localization and, thereby, define the substrate targets most accessible for phosphorylation by the activated protein kinase catalytic subunit. Alternatively, RII binding to a variety of cellular proteins might regulate their function--i.e., RII could be a regulator for multiple proteins in addition to the catalytic subunit of the cAMP-dependent protein kinase.     

Ethanol causes translocation of cAMP-dependent protein kinase catalytic subunit to the nucleus.

Short- and long-term ethanol exposures have been shown to alter cellular levels of cAMP, but little is known about the effects of ethanol on cAMP-dependent protein kinase (PKA). When cAMP levels increase, the catalytic subunit of PKA (C alpha) is released from the regulatory subunit, phosphorylates nearby proteins, and then translocates to the nucleus, where it regulates gene expression. Altered localization of C alpha would have profound effects on multiple cellular functions. Therefore, we investigated whether ethanol alters intracellular localization of C alpha. NG108-15 cells were incubated in the presence or absence of ethanol for as long as 48 h, and localization of PKA subunits was determined by immunocytochemistry. We found that ethanol exposure produced a significant translocation of C alpha from the Golgi area to the nucleus. C alpha remained in the nucleus as long as ethanol was present. There was no effect of ethanol on localization of the type I regulatory subunit of PKA. Ethanol also caused a 43% decrease in the amount of type I regulatory subunit but had no effect on the amount of C alpha as determined by Western blot. These data suggest that ethanol-induced translocation of C alpha to the nucleus may account, in part, for diverse changes in cellular function and gene expression produced by alcohol.     

Viral src gene products are related to the catalytic chain of mammalian cAMP-dependent protein kinase.

The transforming protein sequences translated from the Rous avian and Moloney murine sarcoma virus src genes are shown to be related to the catalytic chain of bovine cAMP-dependent protein kinase (ATP:protein phosphotransferase, EC 2.7.1.37). The avian transforming protein, also a protein kinase, shows greatest homology with the bovine protein kinase in the carboxyl-terminal half, where the protein kinase activity is localized. Moreover, lysine occurs in the inferred transforming protein sequences at the position homologous with the proposed ATP-binding lysine of the bovine protein kinase. This relationship is consistent with the hypothesis that the src genes originated in the host genomes, in which they are members of a superfamily of distantly related protein kinases that are normal constituents of mammalian cells. In the host, these sequences are much more highly conserved than in the viruses.     

Regulation of Cl- transport in T84 cell clones expressing a mutant regulatory subunit of cAMP-dependent protein kinase.

Cl- channels in the apical membranes of salt-secreting epithelia are activated by both cAMP and Ca2+ second-messenger systems, and dysfunctions in their hormonal regulation have been demonstrated in patients with cystic fibrosis. We have transfected the epithelial cell line T84 with an expression vector containing a mutant form of the regulatory subunit of the cAMP-dependent protein kinase. Stable transformants that express this construct have reduced basal cAMP-dependent protein kinase activity and do not increase kinase activity beyond the basal level of control cells in response to cAMP. Forskolin, vasoactive intestinal peptide, and prostaglandin E2 each stimulate intracellular cAMP accumulation in both mutant and control clones; however, the activation of Cl- channels in response to elevated cAMP is blocked in mutant clones, indicating direct involvement of the cAMP-dependent protein kinase. In contrast, Ca2+ ionophores retain their ability to activate the Cl- channel in T84 cells expressing the mutant regulatory subunit, suggesting that activation of the channel by means of Ca2+ does not require the participation of cAMP-dependent protein kinase activity. These clones will be useful for further studies of the interactions between the cAMP- and Ca2(+)-dependent regulatory pathways in salt-secreting epithelial cells. They can also be used to identify the mediators of Ca2(+)-dependent Cl- channel activation in isolation from interactions with the cAMP second-messenger pathway.     

Hippocampal long-term depression and depotentiation are defective in mice carrying a targeted disruption of the gene encoding the RI beta subunit of cAMP-dependent protein kinase.

The cAMP-dependent protein kinase (PKA) has been shown to play an important role in long-term potentiation (LTP) in the hippocampus, but little is known about the function of PKA in long-term depression (LTD). We have combined pharmacologic and genetic approaches to demonstrate that PKA activity is required for both homosynaptic LTD and depotentiation and that a specific neuronal isoform of type I regulatory subunit (RI beta) is essential. Mice carrying a null mutation in the gene encoding RI beta were established by use of gene targeting in embryonic stem cells. Hippocampal slices from mutant mice show a severe deficit in LTD and depotentiation at the Schaffer collateral-CA1 synapse. This defect is also evident at the lateral perforant path-dentate granule cell synapse in RI beta mutant mice. Despite a compensatory increase in the related RI alpha protein and a lack of detectable changes in total PKA activity, the hippocampal function in these mice is not rescued, suggesting a unique role for RI beta. Since the late phase of CA1 LTP also requires PKA but is normal in RI beta mutant mice, our data further suggest that different forms of synaptic plasticity are likely to employ different combinations of regulatory and catalytic subunits.     

Post-translational regulation of steroidogenic acute regulatory protein by cAMP-dependent protein kinase A

Adrenal steroid production is stimulated by adrenocorticotropin hormone activation of the cAMP-dependent protein kinase A (PKA) signaling pathway and subsequent induction of Steroidogenic Acute Regulatory (StAR) protein expression. Herein we have compared StAR mRNA and protein levels in 8-Br-cAMP-treated mouse adrenocortical Y1 and the derived PKA mutant Kin-8 cell lines to evaluate the PKA requirement in StAR expression. StAR mRNA was induced by 8-Br-cAMP-treatment of both Y1 and Kin-8 cells with maximal expression levels in Kin-8 cells approximately 50% of that observed in Y1 cells. StAR protein levels, as detected by Western analysis, were concomitantly increased in Y1 cells but were not detected in the Kin-8 cells. StAR mRNA colocalized with the active polysome fractions in both 8-Br-cAMP-treated Y1 and Kin-8 cells, indicating translation was not blocked in Kin-8 cells. Consistent with this data, a 2-fold increase in incorporation of [35S]methionine into StAR was also observed after 8-Br-cAMP treatment of both cell lines. Since StAR protein levels were not sufficient to detect by Western analysis, these data indicate that PKA functions at the post-translational level to regulate StAR expression and we propose that phosphorylation of StAR by PKA contributes to protein stability.     

Protein kinase C and cAMP-dependent protein kinase phosphorylate the beta subunit of the purified gamma-aminobutyric acid A receptor.

A number of recent studies have suggested that phosphorylation of the gamma-aminobutyric acid A (GABAA) receptor could modulate receptor function. Activators of protein kinase C and cAMP-dependent protein kinase have been shown to influence GABAA receptor function. In addition, Sweetnam et al. [Sweetnam, P. M., Lloyd, J., Gallombardo, P., Malison, R. T., Gallager, D. W., Tallman, J. F. & Nestler, E. J. (1988) J. Neurochem. 51, 1274-1284] have reported that a kinase associated with a partially purified preparation of the receptor could phosphorylate the alpha subunit of the receptor. Moreover, Kirkness et al. [Kirkness, E. F., Bovenkerk, C. F., Ueda, T. & Turner, A. J. (1989) Biochem. J. 259, 613-616] have recently shown that cAMP-dependent protein kinase could phosphorylate a muscimol binding polypeptide of the GABAA receptor. To explore the issue further, we have examined the ability of specific kinases to catalyze significant phosphorylation of the GABAA receptor that has been purified to near homogeneity. The GABAA receptor was purified as previously described using benzodiazepine affinity chromatography. The purified receptor possessed no detectable kinase activity. Protein kinase C and cAMP-dependent protein kinase catalyzed the phosphorylation of the beta and alpha subunits of the receptor. However, most of the phosphate incorporation was associated with the beta subunit. Two muscimol binding polypeptides designated beta 58 (Mr 58,000) and beta 56 (Mr 56,000) were present in the preparation. The higher molecular weight polypeptide, beta 58, was phosphorylated specifically by cAMP-dependent protein kinase. beta 56 was phosphorylated specifically by protein kinase C. beta 58 and beta 56 gave distinct patterns in a one-dimensional phosphopeptide analysis. The stoichiometry of phosphorylation (mol of phosphate/mol of muscimol binding) catalyzed by cAMP-dependent protein kinase was 0.52 and that catalyzed by protein kinase C was 0.38. Taken together these data confirm that there are two forms of the beta subunit of the GABAA receptor and suggest that these two forms of the beta subunit are phosphorylated by distinct kinases.     

Isolation of cDNA clones coding for the catalytic subunit of mouse cAMP-dependent protein kinase.

mRNA coding for the catalytic (C) subunit of cAMP-dependent protein kinase (ATP: protein phosphotransferase, EC 2.7.1.37) was partially purified from bovine testis by polysome immunoadsorption and oligo(dT)-chromatography. This enriched mRNA preparation was used to prepare and differentially screen a cDNA library. One of the selected cDNA clones was shown to hybrid-select mRNA coding for a 40-kDa protein that was specifically precipitated with antibodies to the C subunit. This bovine cDNA clone was then used to isolate a series of mouse cDNA clones that are complementary to the entire mouse C subunit mRNA. The mouse clones code for a protein of 351 amino acids that shows 98% homology to the bovine C subunit and hybridize to a single mRNA of 2.4 kilobases in mouse heart and brain. Southern blot analysis of total genomic DNA suggests that there is a single mouse gene coding for the C subunit. mRNA levels for both the C subunit and the type I regulatory subunit in various mouse tissues and cell lines were quantitated and compared by using single-stranded RNA probes prepared with SP6 polymerase.     

Mutations in the catalytic subunit of cAMP-dependent protein kinase result in unregulated biological activity.

Mutations were identified in the catalytic subunit (C) of the cAMP-dependent protein kinase (EC 2.7.1.37) that block inactivation by regulatory subunit (R) without compromising catalytic activity. Randomly mutagenized mouse C expression vectors were screened functionally for clones that stimulated gene induction in the presence of excess R. Point mutations in the C coding sequence were identified that result in a His----Gln substitution at amino acid 87 (His87Gln) and a Trp----Arg change at amino acid 196 (Trp196Arg). In contrast to wild-type C, both mutants retained partial activity in the presence of excess R isoform RI alpha, although only Trp196Arg retained partial activity in the presence of excess R isoform RII alpha. A C expression vector that included both mutations was fully active in promoting gene induction and was virtually unaffected by an 80-fold excess of either RI alpha or RII alpha. These results demonstrate that mutations at His-87 and Trp-196 alter R interactions with C at a site that is not involved in substrate recognition or enzymatic activity. In contrast to these randomly generated mutations, a site-specific alteration of the autophosphorylated Thr-197 to an Ala resulted in an 80% loss of biological activity and partial resistance to R inhibition. The location and proximity of His-87 and Trp-196 in the crystal structure of C suggest a surface domain that may interact with a region of R that is outside of the substrate/pseudosubstrate site.