Association of elevated protein kinase CK2 activity with aggressive behavior of squamous cell carcinoma of the head and neck.

BACKGROUND: Protein kinase CK2 (also known as casein kinase 2) is a messenger-independent protein serine/threonine kinase ubiquitously distributed in eukaryotes. CK2 has been found to phosphorylate a wide variety of cytosolic and nuclear substrates which are intimately involved in regulation of DNA, RNA, and protein synthesis, and differentiation. We therefore addressed the hypothesis that malignant transformation of upper aerodigestive tract mucosa to squamous cell carcinoma of the head and neck (SCCHN) might be associated with altered CK2 activity. MATERIALS AND METHODS: To this end, we subjected surgical specimens of SCCHN tumors and of normal oropharyngeal mucosa to subcellular fractionation. We then quantitated CK2 activity in cytosol and nuclei of these specimens using a CK2-specific peptide substrate (Arg-Arg-Arg-Glu-Glu-Glu-Thr-Glu-Glu-Glu). RESULTS: We found that CK2 activity was significantly elevated in both nuclear (p < 0.0005) and cytosolic (p < 0.0034) compartments of SCCHN tumors, relative to normal oropharyngeal mucosa. Moreover, CK2 activity in the cellular cytosolic fraction of SCCHN tumors was associated with less differentiated histologic grade (p < 0.037), positive nodal metastatic status (p < 0.056), and a poor clinical outcome (p < 0.028). Kaplan-Meier cumulative survival analysis revealed greatly reduced survival in the high-CK2 activity patient group, with high statistical significance (p < 0.023). CONCLUSIONS: These preliminary data reveal that malignant transformation of the upper aerodigestive tract mucosa is associated with altered CK2 activity. The results further suggest that dysregulation of this protein kinase may play a significant role in the pathobiology of SCCHN, and that CK2 activity may be a prognostic indicator in this malignancy.     

Altered protein kinase activities of lymphoid cells transformed by Abelson and Moloney leukemia viruses

Five different types of protein kinase activities have been evaluated in cell lines from murine lymphomas induced by Abelson leukemia virus (A-MuLV), whose oncogene codes for a tyrosine protein kinase. Such activities were compared with those of normal cells and of cells transformed by Moloney leukemia virus (M-MuLV), lacking oncogene sequences in its genome. While cAMP-dependent protein kinase and casein kinase-1 do not undergo significant changes, casein kinase-2 rises in both A-MuLV and M-MuLV infected lymphocytes, becoming largely associated with the particulate fraction of transformed cells. Protein kinase-C on the other hand is unchanged in M-MuLV transformed cells but it undergoes a 2-3-fold increment in both soluble and particulate fractions of A-MuLV transformed lymphocytes, which also display high tyrosine protein kinase activity.     

Multiple nuclear protein kinase activities in rat liver and hepatoma 3924A.

Multiple protein kinase activities were isolated from nuclei of rat and hepatoma 3924A, and purified 40- to 140-fold, respectively. Hepatic protein kinase-I exhibited high activity with casein as substrate, but was relatively inactive with either liver and hepatoma chromatin or mixed histone. In contrast, hepatoma protein kinase-I showed equivalent activity with casein and liver chromatin. Protein kinase-IIA, -IIB and-IIC from both tissues were more active with liver chromatin in comparison to casein and hepatoma chromatin, and exhibited similar electrophoretic profiles of 32P-chromatin.     

Non-histone chromatin proteins in beef thyroid: distinct phosphorylation patterns of several protein kinases

Summary Non-histone chromatin protein (NHCP) fractions were extracted from purified beef thyroid nuclear preparations and tested for the presence of protein kinase activities using several known mediators of thyroid regulation, as well as potential phosphotransferase substrates using purified or partially purified protein kinase activities. The addition of cAMP/3-isobutyl-l-methylxanthine had no effect on NHCP historic kinase activity; the addition of 10 g of the heat-stable cAMP-dependent protein kinase A inhibitor, however, resulted in a 47% reduction in histone H₂ kinase activity. Nuclear casein kinase II activity was present in the NHCP fractions as evidenced by the capacity of spermine to stimulate (ED₅₀ = 0.19 mM) and heparin to inhibit (ID₅₀ = 0.09 g/ml) the phosphorylation of casein; further, the phosphotransferase activity could be purified by sequential casein-agarose and spermine-agarose affinity chromatography. Neither calcium-calmodulin nor calcium/phosphatidylserine/diolein had an effect on NHCP casein kinase or histone kinase activities, respectively. The addition of cAMP-dependent protein kinase A catalytic subunit, nuclear casein kinase II, calcium-activated calmodulin-dependent protein kinase and diacylglycerol-activated calcium/phospholipid-dependent protein kinase C activities exhibited distinct phosphorylation patterns when NHCP were used as substrates and analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis and autoradiography. We conclude that NHCP fraction from beef thyroid: 1) contains both cAMP-dependent protein kinase A catalytic subunit and nuclear casein kinase II and 2) substrates for cAMP-dependent protein kinase A, calcium-activited calmodulin-dependent protein kinase, protein kinase C, and nuclear casein kinase II.Abbreviations NHCP Non-Histone Chromatin Proteins - PK-A cAMP-Dependent Protein Kinase - CAMPK Calcium-Activated Calmodulin-Dependent Protein Kinase - PK-C Diacylglycerol-Activated Calcium/phospholipid-dependent Protein Kinase - NK-11 Nuclear Casein Kinase 11 - CK-G Cytosolic Casein Kinase G or 11 - PMSF Phenylmethyl Sulfonyl Fluoride - PKI the Heat Stable PK-A Inhibitor (Walsh inhibitor) - SDS-PAGE Sodium Dodecylsulfate Polyacrylamide Gel Electrophoresis - EDTA Ethylenediamine Tetraacetic Acid - EGTA Ethyleneglycol bis- (B-aminoethyl ether) N,N,N,N,-Tetraacetic Acid - PS Phosphatidylserine - DO 1,2-Diolein     

Effect of a sunflower oil-supplemented diet on protein kinase activities of rat liver plasma membranes.

1. The effect of a sunflower oil-enriched diet on plasma membrane-bound protein kinase C, protein kinase A, casein and tyrosine kinase activities was studied. 2. The diet induced an increase in the content of linoleic acid and a decrease in the content of palmitic acid. The anisotropy parameter (rs) of the fluorescence probe DPH and SDPH decreased strongly in the experimental group. 3. Protein kinase C was stimulated more than two times. Tyrosine kinase, protein kinase A and casein kinase activities were increased by 65, 57 and 40%, respectively. 4. We suggest that a more fluid lipid environment favours higher plasma membrane-bound protein kinase activities.     

Protein kinase activities in neoplastic squamous epithelia and normal epithelia from the upper aero-digestive tract

In the present study the activities of three different protein kinase were determined in squamous cell carcinoma from the upper aero-digestive tract, and compared with the activities in normal oral mucosa. The protein kinases investigated are: a) cAMP-dependent protein kinase; b) cGMP-dependent protein kinase, and c) casein kinase II. The basal protein kinase activity, when histone IIa was used as substrate, was about 3-fold higher in tumors, as compared to normal mucosa, in the soluble fraction (32.0 +/- 4.2 and 10.9 +/- 2.4 pmol 32P/mg prot. X min, respectively). In the particulate fraction the basal protein kinase activity was about 9 times higher in tumors as compared to normal mucosa (19.4 +/- 5.2 and 2.1 +/- 0.3 pmol 32P/mg prot X min, respectively). The protein kinase activity in the presence of cyclic nucleotide (cAMP/cGMP) minus the basal protein kinase activity was taken as the cAMP- and the cGMP-dependent protein kinase activity, respectively. Maximal protein kinase activity was obtained in the presence of 0.5 microM of cyclic nucleotide both in squamous cell carcinoma and normal mucosa. In the cytosolic fraction the cAMP-dependent protein kinase activity was 33.9 +/- 13.0 pmol 32P/mg prot. X min in tumors, and 28.2 +/- 5.8 pmol 32P/mg prot. X min in normal tissue, after stimulation with 0.5 microM cAMP. The cGMP-dependent protein kinase activity was 5-10% of the cAMP-dependent protein kinase activity, and no concentration-dependent stimulation with cGMP was seen. The cGMP-dependent protein kinase activity in the presence of 0.5 microM cGMP was 2.4 +/- 1.3 and 1.8 +/- 0.6 pmol 32P/mg prot. X min in tumors and normal mucosa, respectively. Casein kinase II activity was determined only in the cytosolic fraction and was found to be 3-fold higher in tumors as compared to normal mucosa (31.8 +/- 5.2 and 8.6 +/- 3.5 pmol 32P/mg prot X min, respectively). This study shows a general increase in histone phosphorylation and casein kinase activity in neoplastic squamous epithelia compared to normal epithelia. No evidence for an increase in cyclic nucleotide dependent protein kinase activities in neoplastic squamous epithelia was found. This study thus supports the idea that phosphorylation/dephosphorylation reactions may play an important role in the control of cell growth, differentiation and proliferation.     

Identification and isolation of casein kinase type II from RNA-binding proteins of amphibian oocytes

Protein kinase previously detected in RNA-binding proteins of amphibian oocytes phosphorylates casein far more efficiently than histones to form phosphoserine and phosphothreonine and utilizes both ATP and GTP. Heparin in concentrations below 1 microgram/ml inhibits protein kinase. This allows to relate the enzyme to casein kinases II. Protein kinase was extensively purified (more than 15000-fold) with respect to proteins of ribosome-free extract. The homogeneous enzyme consists of three polypeptide chains (Mr 43,000, 41,000, and 29,000). The 125I-labelled enzyme possessing casein kinase and RNA-binding activities when injected into amphibian oocytes was detected in the particles identical to free cytoplasmic informosomes in terms of their sedimentation properties.     

Separation of different protein kinase activities in the human thyroid gland. Evidence for an abnormal form in hyperfunctioning adenomas.

Protein kinases from normal and from hyperfunctioning “toxic” adenoma human thyroid tissue were analyzed by DE 52 cellulose chromatography. In normal, as well as in toxic adenoma, three peaks of histone kinase activity were eluted. The first two corresponded to types I and II cAMP dependent histone kinases. The third, eluting at 350 mM, was cAMP independent. Two peaks of phosvitin kinase (125 mM and 450 mM) and one of casein kinase (125 mM) activities were also observed. The toxic adenoma elution pattern differed from normal by the presence of an additional peak of histone kinase activity which coeluted with the second peak of phosvitin kinase (450 mM). This additional histone kinase activity was cAMP independent.     

Casein kinase II is elevated in solid human tumours and rapidly proliferating non-neoplastic tissue

Protein kinase CKII (i.e. casein kinase II, CKII, NII) is expressed at a higher level in rapidly proliferating tissues and in solid human tumours (e.g. colorectal carcinomas) when compared to the corresponding non-neoplastic colorectal mucosa. This could be shown by (a) Western blotting of cellular extracts from solid tumours followed by immunostaining with an anti-CKII polyclonal antibody, (b) immunohistochemical staining of cells from tissue sections and (c) by activity measurements using the CKII-specific synthetic peptide (RRRDDDSDDD). The maximum observed activity in the colorectal carcinomas investigated was up to eightfold higher in the tumour specimens than in the non-neoplastic tissue (i.e. colorectal mucosa). The activity range was between 33-350 U/mg protein and in the case of colorectal mucosa 13-106 U/mg protein. The amount of CKII determined in the individual tumours was in the range 0.4-1.6 nmol/g tissue.     

Purification and characterization of two cyclic AMP-independent casein/glycogen synthase kinases from rat liver cytosol

Two cyclic AMP-independent protein kinases (ATP: protein phosphotransferase, EC 2.7.1.37) (casein kinase 1 and 2) have been purified from rat liver cytosol by a method involving chromatography on phosphocellulose and casein-Sepharose 4B. Both kinases were essentially free of endogeneous protein substrates and capable of phosphorylating casein, phosvitin and I-form glycogen synthase, but were inactive on histone IIA, protamine and phosphorylase b. They were neither stimulated by cyclic AMP, Ca2+ and calmodulin, nor inhibited by the cyclic AMP-dependent protein kinase inhibitor protein. The casein and glycogen synthase kinase activities of each enzyme decreased at the same rate when incubated at 50 degrees C. Casein kinase 1 and casein kinase 2 showed differences in molecular weight, sensitivity to KCl, Km for casein and phosvitin and Ka for Mg2+, whereas their Km values for ATP and I-form glycogen synthase were similar. The phosphorylation of glycogen synthase by these kinases correlated with a decrease in the +/- glucose 6-phosphate activity ratio (independence ratio). However, casein kinase 1 catalyzed the incorporation of about 3.6 mol of 32P/85000 dalton subunit, decreasing the independence ratio from 83 to about 15, whereas the phosphorylation achieved by casein kinase 2 was only about 1.9 mol of 32P/850000 dalton subunit, decreasing the independence ratio to about 23. The independence ratio decrease was prevented by the presence of casein but was unaffected by phosphorylase b. These data indicate that casein/glycogen synthase kinases 1 and 2 are different from cyclic AMP-dependent protein kinase and phosphorylase kinase.     

Characteristics of protein tyrosine kinase activities of Y-79 retinoblastoma cells and retina

Protein tyrosine kinase activities were determined in retina and Y-79 retinoblastoma cells. Kinase activity was associated with particulate subcellular fractions. Specific activities were similar in both retina and Y-79 cells; apparent Km values for ATP and casein were also similar. Retinoblastoma-derived growth factor stimulated endogenous protein tyrosine phosphorylation in Y-79 cells significantly more than in retina. Differences in protein tyrosine phosphorylation between retina and Y-79 retinoblastoma appear to be due to differences in regulation of the activity by the growth factor or differences in the endogenous protein substrates present in the Y-79 cells.     

In vivo progesterone regulation of protein phosphatase activity in Xenopus oocytes

Exogenous beta casein, previously phosphorylated in vitro by protein kinase A and casein kinase II, was microinjected into Xenopus oocytes to monitor in vivo protein phosphatase activities. Phosphatase activities were 1.6 and 3.4 fmol/min/oocyte, respectively, for beta casein phosphorylated by casein kinase II and beta casein phosphorylated by protein kinase A. Progesterone induced an early decrease (35% after 10 min) in phosphatase activity restricted to the protein kinase A sites of beta casein.     

MODIFICATIONS IN SOLUBLE PROTEIN KINASE AND CYCLIC-AMP BINDING CAPACITY OF DEVELOPING RAT BRAIN

Histone and casein phosphoprotein-kinase activities were determined in rat brain soluble fraction at various stages of development. Cyclic AMP -independent or basal histone kinase activity increased, whereas cyclic AMP -dependent activity decreased in whole soluble fraction with the age. On the contrary, whole soluble cyclic AMP -dependent and -independent casein kinases activities did not show any difference during development. The percentage of activation by cyclic AMP of histone kinase activity and [³H] cyclic- AMP binding activity in the soluble fraction decreased markedly during development. By DEAE-cellulose chromatography the histone kinase was separated mainly into 4 peaks; the fourth peak was strongly stimulated by cyclic AMP . Stimulation by cyclic AMP was higher in the 4-day-old rat brains than in the 9- and 30-day-old. In the 9-day-old rats the ratio of cyclic AMP -dependent histone kinase in respect to the cyclic AMP -independent enzyme was higher than in 4- and 30-day-old rats. Casein kinase activities in the brains of 9- and 30-day-old rats were separated by DEAE-cellulose chromatography into three peaks of which the third one was stimulated by cyclic AMP . Little, if any, difference was observed for casein kinase during the development. These results suggest that brain histone and casein kinase are different enzymes:     

Cyclic AMP-independent casein/glycogen synthase kinases from pig polymorphonuclear leucocytes

1. Two cyclic AMP-independent casein/glycogen synthase kinases were purified from pig polymorphonuclear leucocytes by chromatography on phosphocellulose followed by affinity chromatography on casein-Sepharose 4B or gel filtration on Bio-Gel A-1.5m. When the affinity step was used, the specific activities were 86 and 43units/mg of protein for casein kinase 1 and 2, respectively, whereas these values were 94 and 90units/mg of protein when the gel-filtration step was used. 2. These kinases differ as follows: (a) the molecular weight of casein kinase 1 (38000) is very much lower than that of casein kinase 2 (185000); (b) the K(m) for casein (0.46mg/ml) and K(a) for Mg(2+) (0.3mm) of casein kinase 1 are lower than those of casein kinase 2 (0.90mg/ml and 1.7mm respectively); (c) KCl stimulates the phosphorylation of casein by casein kinase 1, whereas it inhibits phosvitin phosphorylation by this enzyme; on the contrary, the effect of KCl on casein kinase 2 is very similar with either casein or phosvitin as substrate; (d) although both kinases phosphorylate rabbit muscle glycogen synthase I, the ratio of glycogen synthase to casein phosphorylation by casein kinase 1 is about 4-fold greater than that by casein kinase 2. Furthermore, (32)P incorporation into glycogen synthase promoted by casein kinase 1 (3.6mol of (32)P/mol of 85000-dalton subunit) is twice that observed with casein kinase 2 (1.8mol of (32)P/mol of 85000-dalton subunit). Such a phosphorylation results in a decrease in the glucose 6-phosphate-independence ratio of glycogen synthase to 10-15 with casein kinase 1 and to 35-45 with casein kinase 2. 3. The activity of both kinases is neither stimulated by cyclic AMP, Ca(2+) and calmodulin nor inhibited by cyclic AMP-dependent protein kinase inhibitor protein. 4. No phosphorylation kinase activity was observed with casein kinase 1 and 2 at either pH6.8 or 8.2 in the presence of Ca(2+). 5. Activities of both kinases on casein and glycogen synthase decreased in parallel when incubated at 50 degrees C.     

Principal neurofilament-associated protein kinase in squid axoplasm is related to casein kinase I

A cytoskeletal extract of pure axoplasm, highly enriched with neurofilaments (ANF), was prepared from the giant axon of the squid. This ANF preparation also contained potent kinase activities which phosphorylated the Mr greater than 400,000 (high molecular weight) and Mr 220,000 squid neurofilament protein subunits. High salt (1 M) extraction of this ANF preparation solubilized most of the neurofilament proteins and kinase activities and gel filtration on an AcA 44 column separated these two components. The neurofilaments eluted in the void volume of the column while the kinase activities eluted in the 17-44-kDa range of the column. Two major kinase activities were measured in this peak of activity. One of these strongly phosphorylated the phosphate acceptor peptide Leu-Arg-Arg-Ala-Ser-Leu-Gly (Kemptide) and was completely inhibited by the selective inhibitor of cAMP-dependent kinase Thr-Thr-Tyr-Ala-Asp-Phe-Ile-Ala-Ser-Gly-Arg-Thr-Gly-Arg-Arg-Asn-Ala-Ile- NH2 (Wiptide). Since addition of cAMP did not stimulate activity, this suggested that this kinase was a free catalytic subunit of cAMP-dependent kinase associated with the neurofilaments. The second kinase activity most effectively phosphorylated alpha-casein, and this activity was not affected by Wiptide. The alpha-casein phosphorylating activity (ANF kinase) was the principal activity responsible for neurofilament protein phosphorylation, and was not inhibited by various inhibitors against second messenger regulated kinases, suggesting it was related to the casein kinase family. Four lines of evidence indicate ANF kinase was similar to casein kinase I. These were: 1) the apparent molecular weight determined by gel filtration and the chromatographic elution profile on phosphocellulose column corresponded to casein kinase I; 2) heparin, an inhibitor of casein kinase II at 2-5 micrograms/ml, stimulated both ANF kinase and purified casein kinase I at these concentrations, while CKI-7, a relatively selective inhibitor of casein kinase I, inhibited ANF kinase in a comparable dose-response fashion; 3) purified casein kinase I strongly phosphorylated both ANF protein subunits (like ANF kinase) whereas casein kinase II was relatively ineffective; and 4) tryptic peptide maps of the HMW and Mr 220,000 neurofilament proteins after phosphorylation by ANF kinase or purified casein kinase I showed similar 32P-peptide patterns.     

Cyclic AMP-dependent and -independent protein kinases of the water mold, Blastocladiella emersonii.

Protein kinase (ATP:protein phosphotransferase, EC 2.7.1.37) and cyclic adenosine 3',5'-monophosphate binding activities have been identified in zoospore extracts of the water mold Blastocladiella emersonii. More than 75% of these activities is found in the soluble fraction. Soluble protein kinase activity is resolved in three peaks(I, II and III) by DEAE-cellulose chromatography. Peak I is casein dependent and insensitive to cyclic AMP. Peak II is histone dependent and cyclic AMP independent; this enzyme is inhibited by the heat-stable inhibitor from bovine muscle. Peak III utilizes histone as substrate and is activated by cyclic AMP.     

Developmental changes in rat hepatic casein kinases 1 and 2

Cytosolic histone kinase and casein kinase activities varied considerably in the late fetal and postnatal periods of liver development. Both activities showed a maximum at day 21 of gestation and decreased at birth to values close to those of adult rats. The changes in total casein kinase activity were due to variations of casein kinase 1 and casein kinase 2. Similarly the activities of both the cyclic-AMP-dependent protein (histone) kinase and the cyclic-AMP-independent histone kinase varied during development. Besides the changes in total activity, the affinity of casein kinases 1 and 2 for casein also varied in fetal and postnatal development. The Km values of casein kinase 2 increased from day 18, reached a maximum at day 20 of gestation and then started to decrease until one day after birth. In contrast the Km values of casein kinase 1 decreased from day 18, reached its lowest value at day 21 of gestation and attained values similar to those in the adult at the day of birth. Changes in this parameter were also observed when insulin (3 IU/kg) was administered by intraperitoneal injection to one-day-old rats. The Km values of casein kinase 1 decreased while those of casein kinase 2 increased after administration of this hormone. On the other hand, the Km values for ATP of casein kinases 1 and 2 as well as their apparent molecular masses and sensitivity to heparin and GTP did not significantly change during ontogeny of rat liver.     

Protein Kinase C-Mediated Phosphorylation of Kvβ2 in Adult Rat Brain

The phosphorylation of Kvβ2 was investigated by different protein kinases. Protein kinase A catalytic subunit (PKA-CS) yielded the greatest phosphorylation of recombinant Kvβ2 (rKvβ2), with limited phosphorylation by protein kinase C catalytic subunit (PKC-CS) and no detectable phosphorylation by casein kinase II (CKII). Protein kinase(s) from adult rat brain lysate phosphorylated both rKvβ2 and endogenous Kvβ. The PKA inhibitor, PKI 6-22, fully inhibited PKA-mediated phophorylation of rKvβ2 yet showed minimal inhibition of kinase activity present in rat brain. The inhibitor Gö 6983, that blocks PKCα, PKCβ, PKCγ, PKCδ and PKCζ activities, inhibited rKvβ2 phosphorylation by rat brain kinases, with no inhibition by Gö 6976 which blocks PKCα and PKCβΙ activities. Dose-response analysis of Gö 6983 inhibitory activity indicates that at least two PKC isozymes account for the kinase activity present in rat brain. Τhus, while PKA was the most active protein kinase to phosphorylate rKvβ2 in vitro, Kvβ2 phosphorylation in the rat brain is mainly mediated by PKC isozymes.     

The effect of polyamines on protein kinase activities of wheat (Triticum aestivum) L. anthers

Protein kinase activities were extracted from anther tissue of wheat (Triticum aestivum L. cv. Zaragoza) and characterized with regard to the effects of polyamines, second messengers and plant growth hormones. The protein kinases were inhibited by polyamines and cyclic nucleotides, but were stimulated by the addition of auxins, gibberellic acid and kinetin. The dominant polyamine-sensitive kinase activity was partially purified and characterized. The optimal pH of the reaction was 7.5 to 8.0 and casein was the preferred exogenous substrate. Polyamines were inhibiting in the decreasing order of putrecine > spermidine > spermine. The results are discussed against the context of the anther culture technique.     

Multiple protein kinase activities in the dimorphic fungus Mucor rouxii. Comparison with a cyclic adenosine 3',5'-monophosphate binding protein.

Protein kinase and cyclic adenosine 3′,5′-monophosphate (cAMP) binding activities have been detected in cell extracts of the dimorphic fungus Mucor rouxii. The subcellular distribution of both activities indicates that most of the binding protein is in the high-speed supernatant (S₁₀₀), while about 70% of the total protein kinase activity remains in particulate fractions. S₁₀₀ preparations have been analyzed by diethylaminoethyl cellulose column chromatography. Binding activity can be resolved in two peaks (A and B) and protein kinase in three peaks (I, II, and III). Peaks I and II are casein dependent and insensitive to cAMP. Peak III utilizes histone as substrate and is activated (two- to fourfold) by cAMP. Theophylline strongly inhibits cAMP binding activity and mimics the effect of cAMP on cAMP-dependent protein kinase. The possible relationship between cAMP binding activity and cAMP-dependent protein kinase is suggested.