Journal of cyclic nucleotide and protein phosphorylation research最新文献

A novel protein kinase which specifically binds single strand DNA was identified in rat liver by chromatography on double strand- and single strand- DNA cellulose. This protein kinase activity was stimulated by cAMP and was inhibited by the heat stable inhibitor, suggesting it was a cAMP-dependent protein kinase. Isoelectric focusing studies confirmed that the single strand DNA-binding protein kinase was indeed a cAMP-dependent protein kinase and had the same pI as cAMP-dependent protein kinase, Type II. The DNA binding capacity of this kinase was primarily localized in the regulatory subunit. These results support the recent hypothesis that in addition to regulating enzymatic activity by phosphorylating proteins, cAMP-dependent protein kinase, Type II, may regulate mammalian gene expression through a mechanism similar to that in prokaryotes.

Strips of bovine mesenteric artery mounted in disposable organ baths made of polyethylene showed a biphasic relaxation pattern when exposed to glyceryl trinitrate (GTN). The concentration response curve could be resolved into a high affinity component (pD2 11.9) and a low affinity component (pD2 7.5) by means of non-linear regression analysis. The relaxation induced by both low (0.01 nM - 0.1 nM) and high (1 microM) concentrations of GTN seemed to be mediated by cyclic GMP. We found a 2-3-fold increase in cGMP at 0.01 - 0.1 nM GTN and a 5-fold increase at 1 microM GTN. Cyclic AMP levels were unchanged. We also found that GTN-induced relaxation was increased, for a given GTN concentration, when the endothelium was removed, especially in the low concentration range.

A simple method for the simultaneous assay of both substrate utilization and product formation by Bordetella pertussis adenylate cyclase has been developed. This method involves measurement of ATP remaining in the reaction mixture and cyclic 3',5'-AMP (cAMP) formation by 31p-NMR spectroscopy. No separation of the nucleotides is required. The measurement of the rate of cAMP formation compared very well with other methods that require separation of product from the substrate. With this method it has been possible to show calmodulin activation of B. pertussis adenylate cyclase and to demonstrate an inhibition of calmodulin activation by melittin. The inhibition of calmodulin-activated adenylate cyclase by melittin is not permanent and can be overcome by long-term incubation.

The polyamines, spermine and spermidine, activate a high molecular weight form of phosphorylase a phosphatase isolated from rat liver. This broad specificity protein phosphatase (type 2A) was partially purified, using both protein and non-protein phosphoester substrates. Spermine and spermidine activated isolated protein phosphatase-2A1 (apparent Mr 210,000) approximately 2-fold, when p-nitrophenyl phosphate (PNPP) was used as substrate. Freeze-thawing, which activated the phosphatase activity against a variety of phosphoprotein substrates, also increased the extent of stimulation of PNPP phosphatase activity by spermine (8 to 9-fold with Ka of 93 microM) and spermidine (6 to 7-fold with Ka 280 microM). Kinetic analysis indicated that the activation of phosphatase by polyamines was accomplished by an increase in Vmax of the enzyme, by a mechanism independent of that achieved by other cations. The data indicate that polyamines, at physiological concentrations, can activate a form of protein phosphatase widely distributed in mammalian tissues, and thereby influence cellular protein phosphorylation.

The distribution of beta-adrenergic receptors in lysates from several mammalian cell lines was analyzed on nonlinear sucrose density gradients before and after desensitization by isoproterenol. On the nonlinear gradients, the receptors in lysates from untreated HeLa, A431, S49 cyc- and C6 cells were well resolved into light and heavy density membrane fractions. In contrast, with the former three cell lines, there was very poor or no separation of the two peaks of receptors on linear sucrose gradients. With C6 cells, resolution was better on the nonlinear than on the linear gradient. In all cases, successful separation of the two density fractions of the receptor was achieved only when cells had been treated with concanavalin A prior to lysis. Adenylate cyclase activity cosedimented with the heavy membrane fraction of the receptor, and no activity was detected with the light fraction. After desensitization of adenylate cyclase by isoproterenol, there was a redistribution of the receptors to the light density fraction. This shift of receptors, but not desensitization, was prevented when cells were pretreated at 37 degrees C with concanavalin A prior to exposure to isoproterenol. Thus, sequestration of beta-adrenergic receptors away from the plasma membrane and adenylate cyclase to a lighter density membrane fraction appears to accompany, but may not be a prerequisite for desensitization in mammalian cells. This receptor redistribution, however, can be readily detected on nonlinear sucrose gradients.

Chronic treatment of neuroblastoma X glioma hybrid cells (NG 108-15) with the muscarinic cholinergic agonist carbachol, which acutely inhibits adenylate cyclase, resulted in a 104 +/- 10% increase in PGE1-stimulated cAMP accumulation. Pretreatment of intact cells with pertussis toxin can structurally modify the inhibitory regulatory protein, Gi, by ADP-ribosylation and thus abolish the acute inhibition by carbachol. Pretreatment of the cells with pertussis toxin also resulted in a 27 +/- 8% increase in PGE1-stimulated cAMP accumulation. In the pertussis toxin-treated cells, chronic treatment with carbachol did not further enhance the PGE1 stimulation. These results suggest that functional Gi is required for the development of muscarinic cholinergic-induced enhancement of PGE1-stimulated cAMP accumulation.

Rat antral mucosae maintained for 6 h in organ culture responded to carbamylcholine with a significant increase in endogenous cyclic GMP production and gastrin secretion. The acetylcholine analogue exerted a stimulatory action within a defined concentration range: exposure of antral explants to carbachol concentrations greater than the optimal stimulatory dose was accompanied by a marked decrease in both cyclic GMP production and gastrin release. Exogenous 8-Br-cyclic GMP (1 mM) significantly augmented gastrin secretion into the culture media during 6-12 h culture periods. Cycloheximide (0.1 mM) and the Ca2+ channel-blocker verapamil (5 microM) prevented 8-Br-cyclic GMP from acting as a gastrin secretagogue. Addition of cyclic somatostatin-14 (0.1 mM) to culture media was attended by complete inhibition of 8-Br-cyclic GMP-stimulable gastrin secretion. These results provide evidence that cyclic GMP may play a mediatory role in the coupling of gastrin secretory processes to agonist stimulation. It would seem that the secretagogue action of 8-Br-cyclic GMP requires unabated Ca2+ transmembrane fluxes and protein biosynthesis. Since somatostatin-14 abrogates the stimulatory effect of 8-Br-cyclic GMP on antral gastrin secretion, it is surmised that the inhibitory tetradecapeptide acts at a locus (or loci) distal to domains involved in the actual generation of the cyclic nucleotide.

Cyclic AMP-dependent protein kinase (cAMP-PrK) regulatory subunits, RI and RII, and cyclic GMP-dependent protein kinase (cGMP-PrK) have been simultaneously purified from skeletal muscle, utilizing sequential affinity chromatography on cyclic AMP-Sepharose. Rat skeletal muscle extract was chromatographed over DEAE-cellulose. Appropriate fractions, enriched in RI, RII or cGMP-PrK were further purified by affinity chromatography on cAMP-Sepharose. The protein kinase units were specifically eluted with cAMP or cGMP. A novel procedure, using two affinity columns, differing in their linkage of cAMP via either N6 or C-8 bonds, was developed to obtain RII free of other cyclic nucleotide binding proteins. In all cases, affinity chromatography was followed by HPLC gel exclusion chromatography to remove residual contaminating proteins. Proteins were purified to essential homogeneity as judged by silver stained SDS polyacrylamide gels. This procedure yields protein kinase subunits of high purity, and may be applicable to the isolation of these proteins from other sources.

Highly purified 4-aminobutyrate aminotransferase from pig brain is susceptible to phosphorylation by the purified cAMP-dependent protein kinase catalytic subunit. Up to 0.7 moles of phosphate from ATP-(gamma)-32P can be incorporated per mole of dimeric holoenzyme. Maximum phosphorylation was observed within about 90 minutes at 30 degrees C. Despite the extensive degree of phosphorylation observed, no kinetic property of the enzyme was perceptibly altered. Removal of cofactor had no detectable impact on the extent of phosphorylation but thermal inactivation of the enzyme increased and mild reduction with sodium borohydride decreased the phosphorylatability of the aminotransferase. It was possible to separate the enzyme into phospho and dephospho forms by the use of DEAE chromatography. Validation that the two fractions represented genuine aminotransferase was obtained by proteolytic peptide mapping. The phospho form of the enzyme was found to possess little or no aminotransferase activity while that of the dephospho form exhibited higher specific activity than the purified enzyme prior to phosphorylation. Furthermore, the dephospho form of the enzyme could not be detectably phosphorylated by reincubation with the kinase following DEAE chromatography unless it was subjected to thermal inactivation. The stoichiometry of phosphorylation of the fraction containing 32P from DEAE chromatography was approximately 1 mole/mole of dimer. These results suggest that the substrate for phosphorylation by the kinase is a form of the aminotransferase which is somehow inactivated during routine purification even when extensive precautions are taken to maximally preserve catalytic activity.