Synaptic membrane proteins as substrates for cyclic AMP-stimulated protein phosphorylation in various regions of rat brain

Synaptosomal plasma membranes from mammalian brain contain protein kinase activity which phosphorylates endogenous membrane proteins and is stimulated by cyclic AMP. Using polyacrylamide gel electrophoresis it was shown that at least ten proteins in the synaptosomal plasma membrane fraction could be phosphorylated by endogenous cyclic AMP-stimulated protein kinase activity. The number of proteins whose phosphorylation was stimulated by cyclic AMP was strongly influenced by the pH and Mg²⁺ concentration used in the phosphorylation reaction. A complex pattern of cyclic AMP-stimulated protein phosphorylation was obtained only with synaptosomal plasma membranes and a crude microsomal fraction. Mitochondrial and myelin fractions exhibited no cyclic AMP-stimulated protein kinase activity. Investigation of the distribution of substrates for cyclic AMP-stimulated phosphorylation among various brain regions failed to reveal any regional differences.     

Endogenous cyclic AMP-stimulated phosphorylation of a Wolfgram protein component in rabbit central-nervous-system myelin.

Cyclic AMP-stimulated phosphorylation of membrane proteins in central-nervous-system myelin was investigated, with rabbit brain myelin. Subfractionation of a myelin membrane preparation by sucrose-density-gradient centrifugation produced a rapidly sedimenting population of membrane vesicles containing 5'-nucleotidase and acetylcholinesterase, a light membrane fraction containing myelin basic protein and 2',3'-cyclic nucleotide 3'-phosphodiesterase, and an intermediate membrane fraction containing the highest specific activity of 2',3'-cyclic nucleotide 3'-phosphodiesterase and a small proportion of myelin basic protein. Cyclic AMP stimulation of protein phosphorylation was confined to a protein of Mr 49 700, which co-electrophoresed with the upper component of the Wolfgram protein doublet. Cyclic AMP did not affect the phosphorylation of myelin basic protein. Cyclic AMP-stimulated phosphorylation of this protein followed 2',3'-cyclic nucleotide 3'-phosphodiesterase activity on subcellular fractionation and was correspondingly high in the intermediate or 'myelin-like' fraction on sucrose-density-gradient centrifugation.     

Increased phosphorylation of nuclear substrates for rat brain protein kinase C in regenerating rat liver nuclei.

Protein phosphorylation catalysed by rat brain protein kinase C (PKC) has been studied in nuclei isolated from normal and regenerating rat liver. Histone H1 and a 40,000 molecular weight protein were hyperphosphorylated at all the explored regeneration times, ranging from 3 to 22 h after partial hepatectomy. Phosphorylation of the two substrates was totally dependent on calcium and lipids and was abolished by low concentration of staurosporine. The observed early change of phosphate content of histone H1 and of the 40,000 molecular weight protein on the time scale of liver regeneration suggests that PKC might be involved in the initial nuclear events leading to cell proliferation.     

Calcitonin inhibits the phosphorylation of various proteins in rat brain synaptic membranes

The present report examines the effect of different calcitonins and analogs on the in vitro phosphorylation of brain synaptic membrane proteins. The findings demonstrate that calcitonin is a potent inhibitor of several brain synaptic proteins and that salmon and eel calcitonins are considerably more potent than thyrocalcitonin in eliciting this effect. Deletion of leucine from position 16 in salmon calcitonin sequence resulted in a drastic loss of inhibitory activity, indicating the importance for a hydrophobic residue at position 16 in the intact calcitonin molecule. The mechanism of calcitonin inhibition of protein phosphorylation was likely due to the blockade of stimulation of protein kinases by calmodulin.     

Calcium accumulation and cyclic AMP-stimulated phosphorylation in plasma membrane-enriched preparations of myocardium.

Plasma membranes were prepared from guinea pig ventricle by a procedure which involved differential centrifugation at low gravitational forces, extraction with KCl, and centrifugation in a discontinuous sucrose gradient. Adenylate cyclase was purified 10–15-fold over the starting homogenate with a yield of 75%. The membranes contained an active Ca²⁺ binding and uptake system as well as Ca²⁺-activated adenosine triphosphatase; protein kinase and phosphoprotein phosphatase activities were also present. The membranes could be phosphorylated by either intrinsic or exogenous protein kinase, and phosphorylation was stimulated by cyclic AMP and was reversible. Phosphorylated membranes accumulated twice as much Ca²⁺ as control preparations.     

Stimulation of tyrosine phosphorylation of rat brain membrane proteins by calmodulin

Calmodulin stimulates the alkali-resistant phosphorylation of peptides of 50 and 58-60 kDa in rat brain membrane. Phosphoamino acid analysis indicated a calmodulin stimulated increase of phosphotyrosine in these peptides. Calmodulin also stimulated the phosphorylation of these peptides at serine and threonine residues. This suggests the involvement of the calmodulin regulatory system in the effects of tyrosine protein kinases.     

Evidence for phosphorylation of rat brain guanylate cyclase by cyclic AMP-dependent protein kinase

Direct phosphorylation of purified rat brain guanylate cyclase by cyclic AMP-dependent protein kinase is demonstrated. In the presence of [γ-³²P]ATP, ³²P was incorporated into the protein to the extent of 0.8 to 0.9 mol/mol of guanylate cyclase. The presence of ³²P in the guanylate cyclase molecule was demonstrated by gel-filtration and by autoradiography after gel electrophoresis. The phosphorylation was accompanied by an increase in enzyme activity, characterized by an increase of V_M. These results suggest that the activity of guanylate cyclase may be regulated in vivo by phosphorylation.     

Effect of L-DOPA pretreatment on invivo protein synthesis in various rat brain regions

A single dose of L-dihydroxyphenylalanine (L-DOPA, 500 mg/kg, i.p.) that caused massive disaggregation of brain polysomes also suppressed the incorporation of [³H] lysine into trichloracetic acid (TCA)-precipitable proteins of cortex, cerebellum, hypothalamus, brainstem and striatum. The magnitude of this inhibition of [³H] protein synthesis was similar in all brain regions studied, and was not related to changes in the specific activity of the precursor amino acid.     

Cyclic AMP-stimulated phosphorylation of bovine tracheal smooth muscle contractile and non-contractile proteins.

1. Various proteins isolated from bovine tracheal smooth muscle were examined as phosphate acceptor substrates for a cyclic AMP-dependent protein kinase isolated from the same tissue. A fraction prepared in a manner similar to that of skeletal muscle troponin was the best substrate of the presumptive contractile proteins isolate. Actomyosin and tropomyosin were relatively poor substrates. 2. An assay was developed for the rapid detection in a large number of samples of the muscle specific substrate for the protein kinase on which we reported previously. 3. Using this assay, the muscle specific substrate found in bovine tracheal smooth muscle was partially purified resulting in a preparation which when resolved by polyacrylamide gel electrophoresis showed a single peak of 32P incorporated, and which could be further characterized. 4. Our findings suggest that the substrate contains a protein subunit of molecular weight 19 000, which can be phosphorylated at serine and threonine residues, in the presence of cyclic AMP and protein kinase. The phosphate is in a covalent ester linkage with these residues. 5. A phosphoprotein phosphatase was isolated from the bovine tracheal smooth muscle. 6. Bovine tracheal smooth muscle contains cyclic AMP dependent protein kinase and phosphoprotein phospahatase activity as well as the muscle specific substrate, suggesting that these elements may be part of a mechanism which regulates smooth muscle tone.     

Endogenous substrates for cyclic AMP-dependent and calcium-dependent protein phosphorylation in rabbit peritoneal neutrophils

As in other cells, cAMP-dependent (protein kinase A) and calcium-dependent protein kinases are present in the rabbit peritoneal neutrophil. The major substrates for protein kinase A in the cytosol of rabbit peritoneal neutrophil is a 43 kDa protein which appears to be actin (pI 5.7). The other substrates for protein kinase A in the cytosol are very acidic proteins with molecular weights of 135 000 (pI 4.6) and 130 000 (pI 4.8). Two classes of calcium-dependent protein kinases are present in the rabbit peritoneal neutrophil: one is calcium, calmodulin-dependent, the other is calcium, phosphatidylserine-dependent. Phosphatidylserine appears to be much more effective than calmodulin in stimulting calcium-dependent protein kinase activity. The phospolipid-sensitive, calcium-dependent protein kinase (protein kinase C), present only in the cytosol fraction, exhibits much higher activity than the cAMP-dependent protein kinase from the same source. At least four substrates (M_r 130 000 (pI 4.6) 43 000 (pI 4.8), 41 000 (pI 6.3) and 34 000) of the protein kinase C in the cytosol were identified. Trifluoperazine, a compound which inhibits the degranulation, aggregation and stimulated oxygen consumption of rabbit peritoneal neutrophils. (Alobaidi, T., Naccache, P.H. and Sha'afi, R.I. (1981) Biochim. Biophys. Acta 675, 316–321), also inhibits the activity of protein kinase C. The possible role of cAMP-dependent and calcium-dependent phosphorylation system in neutrophil function is discussed.     

Plasma membrane cyclic AMP-dependent protein phosphorylation system in L6 myoblasts.

Plasma membranes can be isolated without disruption of cells by the plasma membrane vesiculation technique (Scott, R.E. (1976) Science 194, 743-745). A major advantage of this technique is that it avoids contamination of plasma membranes with intracellular membrane components. Using this method, we prepared plasma membranes from L6 myoblasts grown in tissue culture and studied the characteristics of the protein phosphorylation system. We found that these plasma membrane preparations contain protein kinase which is tightly bound to the membrane and cannot be removed by washing in EDTA or in high ionic strength salt solutions. This protein kinase activity can catalyze the phosphorylation of several exogenous substrates with decreasing efficiency as acceptors of phosphate: calf thymus histones f2b, protamine and caseine. Cyclic AMP causes a dose-dependent stimulation of protein kinase activity; the highest stimulation (4-fold) is achieved at concentration 10(-5) M cyclic AMP. Cyclic AMP-dependent stimulation can be completely inhibited by heat-stable protein kinase inhibitor isolated from rabbit skeletal muscle. On the other hand, cyclic GMP does not affect the activity of protein kinase. Plasma membrane-bound protein kinase also catalyzes the phosphorylation of endogenous membrane protein substrates and this is also stimulated by addition of cyclic AMP. Analysis of plasma membrane proteins by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis showed that specific polypeptides are phosphorylated by cyclic AMP-independent and by cyclic AMP-dependent protein kinase systems. The results of these studies demonstrate the presence of endogenous cyclic AMP-dependent and -independent protein phosphorylating systems (enzyme activity and substrates) in purified plasma membrane preparations. These data provide a basis for further investigations on the role of plasma membrane phosphorylation as a regulator of membrane functions including those that may control cellular differentiation.     

The cyclic nucleotide-dependent phosphorylation of aortic smooth muscle membrane proteins

Membrane proteins of Mr 240,000, 130,000, and 85,000 (GS-proteins) were rapidly and selectively phosphorylated in particulate fractions of rabbit aortic smooth muscle in the presence of [Mg-32P]ATP and low concentrations of cGMP (Ka = 0.01 microM) or cAMP (Ka = 0.2 microM). The effects of both cyclic nucleotides in this preparation were mediated entirely by an endogenous, membrane-bound form of cGMP-dependent protein kinase (G-kinase). The GS-proteins were also phosphorylated by the soluble form of G-kinase purified from bovine lung; this effect was most evident following removal of endogenous G-kinase from the membranes using Na2CO3 and high salt washes. The membrane-bound and cytosolic forms of G-kinase phosphorylated the Mr 130,000 GS-protein with the same specificity as determined by two-dimensional peptide mapping. Despite this functional homology between the two forms of G-kinase, only the particulate enzyme appears to play a role in phosphorylating the GS-proteins. Although little endogenous cAMP-dependent protein kinase (A-kinase) activity was detected in washed aortic smooth muscle membranes, the GS-proteins could be phosphorylated when purified A-kinase catalytic subunit was added to this preparation. Peptide mapping of the Mr 130,000 GS-protein indicated that A-kinase phosphorylated a subset of the same peptides labeled by the two forms of G-kinase. The endogenous A-kinase of rabbit aortic smooth muscle homogenates was also found to phosphorylate the GS-proteins. Since the intracellular concentrations of cGMP or cAMP can be selectively elevated by different stimuli, these results suggest several possible mechanisms by which the phosphorylation state of the GS-proteins may be regulated by cyclic nucleotides: activation of the membrane-bound G-kinase by cGMP or cAMP; and activation of cytosolic A-kinase by cAMP.     

Selective phosphorylation of erythrocyte membrane proteins by the solubilized membrane protein kinases.

This report describes the substrate and phosphoryl donor specificities of solubilized erythrocyte membrane cyclic adenosine 3',5'-monophosphate (cAMP)-independent protein kinases toward human and rabbit erythrocyte membrane proteins. Three types of substrate preparations have been utilized: heat-inactivated ghosts, isolated spectrin, and 2,3-dimethylmaleic anhydride (DMMA)-extracted membranes. A 30 000-dalton protein kinase, extracted from either human or rabbit erythrocyte membranes, catalyzes the phosphorylation of heat-inactivated membranes in the presence of ATP. The resulting phosphorylation profile is analogous to that of the autophosphorylation of membranes with ATP (in the absence of cAMP). These kinases also phosphorylate band 2 of isolated spectrin and band 3, but not glycophorin, in the DMMA-extracted ghosts. The ability of the 30 000-dalton kinases to use GTP as a phosphoryl donor appears to be related to the substrate or some other membrane factor. A second kinase, which is 100 000 daltons and derived from rabbit erythrocyte membranes, uses ATP or GTP to phosphorylate membrane proteins 2, 2.1, 2.9-3 in heat-inactivated ghosts, band 2 in isolated spectrin, glycophorin, and to a lesser extent, band 3 in the DMMA-extracted ghosts.     

Tubulin and high molecular weight microtubule-associated proteins as endogenous substrates for protein carboxymethyltransferase in brain

The endogenous substrate for protein carboxymethyltransferase in brain was examined. Several polypeptides were methylated when brain slices were incubated with L-methionine or when subcellular fractions of brain, such as the cytosolic fraction, were incubated with S-adenosyl L-methionine. Two methyl-accepting proteins in the cytoplasm were identified as tubulin and high molecular weight microtubule-associated proteins (300 kDa), which are components of microtubules. Tubulin behaved as a 43 kDa protein in acidic polyacrylamide gel electrophoresis, but as a 55 kDa protein in SDS-polyacrylamide gel electrophoresis. The methyl moiety transferred to these proteins from L-methionine was labile at alkaline pH. The high molecular weight microtubule-associated proteins showed higher methyl-accepting activity than tubulin or ovalbumin, which was used as a standard substrate: about 20 mmol of high molecular weight microtubule-associated proteins, 2 mmol of tubulin and 10 mmol of ovalbumin were methylated per mol of each protein in 30 min under the experimental conditions used.     

Synaptic scaffolding proteins in rat brain. Ankyrin repeats of the multidomain Shank protein family interact with the cytoskeletal protein alpha-fodrin

The postsynaptic density is the ultrastructural entity containing the neurotransmitter reception apparatus of excitatory synapses in the brain. A recently identified family of multidomain proteins termed Src homology 3 domain and ankyrin repeat-containing (Shank), also known as proline-rich synapse-associated protein/somatostatin receptor-interacting protein, plays a central role in organizing the subsynaptic scaffold by interacting with several synaptic proteins including the glutamate receptors. We used the N-terminal ankyrin repeats of Shank1 and -3 to search for interacting proteins by yeast two-hybrid screening and by affinity chromatography. By cDNA sequencing and mass spectrometry the cytoskeletal protein alpha-fodrin was identified as an interacting molecule. The interaction was verified by pull-down assays and by coimmunoprecipitation experiments from transfected cells and brain extracts. Mapping of the interacting domains of alpha-fodrin revealed that the highly conserved spectrin repeat 21 is sufficient to bind to the ankyrin repeats. Both interacting partners are coexpressed widely in the rat brain and are colocalized in synapses of hippocampal cultures. Our data indicate that the Shank1 and -3 family members provide multiple independent connections between synaptic glutamate receptor complexes and the cytoskeleton.     

Endogenous substrates for in vitro phosphorylation by cyclic AMP-dependent protein kinase from Dictyostelium discoideum

Endogenous proteins which could serve as substrates for cyclic AMP-dependent protein kinase in vitro were measured in cytosolic fractions at four stages of development. A peak of cyclic AMP-dependent phosphorylation occurred at the slug stage, coincident with the appearance of cyclic AMP-dependent protein kinase. After partial purification of the slug-stage extracts by DE-52 cellulose and Sephacryl S-300 chromatography, cyclic AMP dependency of six proteins was observed. The apparent subunit molecular weights of the proteins were greater than 200,000, 110,000, 107,000, 91,000, 75,000 and 69,000. Upon further purification of the cyclic AMP-dependent protein kinase by chromatofocusing, the endogenous substrates were separated from the enzyme. In addition, the enzyme separated into catalytic and regulatory subunits. If the purified catalytic subunit was added to heated S300 fractions, proteins with apparent molecular weights of 91,000 and 107,000 were specificity phosphorylated. The results show the stage-dependent appearance of a cyclic AMP-dependent protein kinase and point out several in vitro substrates for the enzyme.     

Study of the endogenous cyclic nucleotide-dependent phosphorylation of synaptic membrane proteins

The highest activity of cyclic nucleotide-dependent (cAMP--2 X 10(-5) M, GMP--2 X 10(-4) M) phosphorylation of synaptic membrane proteins in vitro is revealed at equimolar concentrations of ATP and Mg2+ (10(-3)M) and depends on the ratio of the ATP concentration, protein amount in the assay and the period of exposure. At concentrations exceeding 10(-3) M ATP inhibits cyclic nucleotide-dependent phosphorylation. Optimal concentrations of ATR and Mg2+ to provide basal phosphorylation are found to be equal to 10(-2) M. Possible role of cyclic nucleotide-dependent phosphorylation in synaptic transmission is discussed.     

Adenosine 3':5'-monophosphate-regulated phosphorylation of erythrocyte membrane proteins. Separation of membrane-associated cyclic adenosine 3':5'-monophosphate-dependent protein kinase from its endogenous substrates.

An adenosine 3':5'-monophosphate (cyclic AMP)-binding protein in the human erythrocyte plasma membrane was isotopically labeled using a photoaffinity analog of cyclic AMP, N6-(ethyl 2-diazomalonyl) cyclic [3H]AMP. The cyclic AMP-binding site is located in a polypeptide chain having a molecular weight of 48,000. Cyclic AMP-binding protein and cyclic AMP-dependent protein kinase were solubilized with 0.5% Triton X-100 in 56 mM sodium borate, pH 8, but 32P-labeled membrane phosphoproteins were retained in the Triton-insoluble fraction, suggesting that the membrane-associated binding protein is not a primary substrate for protein kinase. Triton-solubilized and membrane-associated protein kinase activities were stimulated 15- and 17-fold by cyclic AMP, suggesting that the degree of association between the catalytic anc cyclic AMP-binding components was very similar in both preparations. Fractionation and characterization of membrane phosphoproteins have shown that protein III and a co-migrating minor protein are substrates for protein kinase but membrane sialoglycoproteins are not phosphorylated.