Identification of endogenous substrate proteins for cAMP-dependent protein kinase in bovine brain

Targeting the purinome

Purines are critical cofactors in the enzymatic reactions that create and maintain living organisms. In humans, there are approximately 3,266 proteins that utilize purine cofactors and these proteins constitute the so-called purinome. The human purinome encompasses a wide-ranging functional repertoire and many of these proteins are attractive drug targets. For example, it is estimated that 30% of modern drug discovery projects target protein kinases and that modulators of small G-proteins comprise more than 50% of currently marketed drugs. Given the importance of purine-binding proteins to drug discovery, the following review will discuss the forces that mediate protein:purine recognition, the factors that determine druggability of a protein target, and the process of structure-based drug design. A review of purine recognition in representatives of the various purine-binding protein families, as well as the challenges faced in targeting members of the purinome in drug discovery campaigns will also be given.

Calcium-Stimulated Protein Phosphorylation in Synaptic Membranes

Synaptic membranes from rat brain contain several calcium-requiring protein kinase (PK) activities with different substrate specificities: (a) an activity (CaH-PK) effective at high concentrations of Ca2+ ion in the absence of Mg2+ (active on class F substrates); (b) a (Ca + Mg)-PK activity that is mediated by Ca2+ ion in the presence of Mg2+ (active on class B substrates); (c) (Ca-CaM)-PK activities that exhibit simultaneous requirements for both Ca2+ ion and CaM (for class C and D substrates). Also described are three activities (d-f) that do not require Ca2+ ion: (d) a Mg-PK activity in which the presence of Ca2+ causes the inhibition of phosphorylation (active on class A substrates); (e) an activity affecting a diverse group of substrates (class E substrates), the phosphorylation of which occurs in the presence of Mg2+ ion alone (Mg-PK activity) and is unaffected by the addition of Ca2+ ion and CaM, the substrates of which show different responses to several types of inhibitors; and, finally, (f) the previously well characterized cAMP-dependent PK activities. Several of the substrates of these kinases have been identified in a fairly unambiguous manner: among them are P43 (class A), as the alpha subunit of pyruvate dehydrogenase; P54 (class B), as the presynaptic protein B50; and the doublet P75-P80, as proteins IA and IB of Ueda and Greengard. The most interesting activity is that requiring both Ca2+ and CaM. The half-maximal stimulation (K0.5) for Ca2+ in the presence of CaM was found to be 1.0 microM Ca2+F in untreated membranes. There is little change in this value on prior EGTA extraction of the membranes, which removes the bulk of its Ca2+ and reduces its residual CaM by greater than or equal to 50%. The apparent K0.5 for CaM in the presence of excess Ca2+ ion was found to equal 0.4 microgram per reaction mixture (8 micrograms/ml) or 1.35 micrograms per reaction mixture (27 micrograms/ml), for the untreated and EGTA-treated membranes, respectively.

Cyclic nucleotide research -- still expanding after half a century

Since the discovery in 1957 that cyclic AMP acts as a second messenger for the hormone adrenaline, interest in this molecule and its companion, cyclic GMP, has grown. Over a period of nearly 50 years, research into second messengers has provided a framework for understanding transmembrane signal transduction, receptor-effector coupling, protein-kinase cascades and downregulation of drug responsiveness. The breadth and impact of this work is reflected by five different Nobel prizes.

Expression and modification of PKA and AKAPs during meiosis in rat oocytes

Meiosis in oocytes is initiated during fetal life, arrested around birth and resumed after puberty. Meiotic arrest is controlled by a cAMP-dependent protein kinase (PKA)-mediated cAMP action. We examined oocytes for the presence and modulation of the regulatory (R) subunits of PKA and the A-kinase anchoring proteins (AKAPs) that target PKA to specific subcellular locations. We found that rat oocytes express the two regulatory subunit isoforms, RI and RII of PKA. Immunocytochemistry revealed that the regulatory subunits underwent cellular translocation upon resumption of meiosis. We also demonstrated the presence of a novel 140 kDa AKAP, AKAP140 that exhibited a retarded electrophoretic motility at reinitiation of meiosis. The mobility shift of AKAP140 was susceptible to alkaline phosphatase and prevented by inhibition of p34cdc2 kinase. We conclude that rat oocytes express AKAP140 that is phosphorylated during meiosis. AKAP140 phosphorylation is sensitive to p34cdc2 kinase inhibitors. We hypothesize that AKAP140 and its phosphorylation state may influence the translocation of the R subunits of PKA throughout resumption of meiosis.

Visualization of detergent insoluble cyclic AMP-dependent protein kinase RIalpha aggregates in the rat brain

Regulatory subunits of the cAMP dependent protein kinases are the most abundant receptor for cAMP in eukaryotic cells. Four isoforms of regulatory subunits (RIalpha and -beta, RIIalpha and -beta) have been distinguished. Distribution of the most abundant RII isoforms has been extensively studied in the brain, by immunohistochemistry and biochemical fractionation, while the least abundant RI isoforms have been neglected. In neurons most regulatory subunits are bound to the cytoskeleton. A protocol is presented that allows immunohistochemical and biochemical characterization of detergent-insoluble RI isoforms in the brain.

Mitosis-specific phosphorylation and subcellular redistribution of the RIIalpha regulatory subunit of cAMP-dependent protein kinase

Phosphorylation of the RII regulatory subunits of cyclic AMP-dependent protein kinases (PKAs) was examined during the HeLa cell cycle. Three RIIalpha isoforms of 51, 54, and 57 kDa were identified by RIIalpha immunodetection and labeling with 8-azido[32P]cAMP in different cell cycle phases. These isoforms were characterized as different phosphorylation states by the use of selective PKA and cyclin-directed kinase inhibitors. Whereas RIIalpha autophosphorylation by PKA caused RIIalpha to shift from 51 to 54 kDa, phosphorylation of RIIalpha by one other or a combination of several kinases activated during mitosis caused RIIalpha to shift from 51 to 57 kDa. In vivo incorporation of [32P]orthophosphate into mitotic cells and RIIalpha immunoprecipitation demonstrated that RIIalpha was hyperphosphorylated on a different site than the one phosphorylated by PKA. Deletion and mutation analysis demonstrated that the cyclin B-p34(cdc2) kinase (CDK1) phosphorylated human recombinant RIIalpha in vitro on Thr54. Whereas RIIalpha was associated with the Golgi-centrosomal region during interphase, it was dissociated from its centrosomal localization at metaphase-anaphase transition. Furthermore, particulate RIIalpha from HeLa cell extracts was solubilized following incubation with CDK1 in vitro. Our results suggest that at the onset of mitosis, CDK1 phosphorylates RIIalpha, and this may alter its subcellular localization.

Modification of hippocampal synaptic proteins by nitric oxide-stimulated ADP ribosylation

Nitric oxide has been shown to be an important neuronal signaling molecule that participates in both behavioral and synaptic plasticity. To better understand the potential mechanisms by which NO regulates synaptic function, the ability of NO to stimulate the modification of synaptic proteins by ADP ribosylation was examined. Two NO donors, sodium nitroprusside and 3-morpholinosydnonimine, stimulated the ADP ribosylation of proteins at apparent molecular masses of 42, 48, 51, 54, and 74 kD in hippocampal synaptosomes. This stimulation was likely owing to the production of NO by the donors; ADP ribosylation was not stimulated by non-NO decomposition products of sodium nitroprusside, and quenching of superoxide anion did not inhibit Sin-1-induced ADP ribosylation. Experiments using NAD+ that was radiolabeled on the nicotinamide moiety demonstrated that the modification of proteins of molecular masses of 30, 33, and 38 kD are not true ADP ribosylation, whereas labeling of the 42-, 48-, 51-, 54-, and 74-kD proteins likely represent ADP ribosylation. Some of the substrates were brain specific (54 and 74 kD), whereas others (42 and 51 kD) were present in multiple nonbrain tissues.

Evidence for the existence of cAMP-dependent protein kinase phosphorylation system associated with specific phosphoproteins in stable microtubules from rat cerebral cortex

Cyclic AMP is a second messenger by which different extracellular signals are transduced into biological responses. Within the cell, most of the effects of cAMP are mediated through the cAMP protein kinase which appears to be localized in specific compartments of the cell near to their substrate proteins. In the present study, we have investigated the possible association of cAMP-dependent protein kinase, its substrate proteins and RII binding proteins in stable microtubules from rat cerebral cortex. The results show that in this fraction there is a cAMP binding protein of 52-54 kDa. This cAMP receptor is in the inactive holoenzyme form, since the addition of cAMP (5 microM) induces an increase in the endogenous phosphorylation of different stable microtubules polypeptides, which is completely inhibited in the presence of a specific protein kinase inhibitor (PKI 5-24 1 microM). Interestingly, overlay binding assay reveals that beside MAP2, 32P/R II is able to bind stable microtubule proteins of M(r) 150 and 75 kDa which, according to their electrophoretic mobility, can also be endogenous substrates for the enzyme. We conclude that cAMP-dependent phosphorylation system is indeed associated with stable microtubules from rat cerebral cortex.

Two lipid-anchored cAMP-binding proteins in the yeast Saccharomyces cerevisiae are unrelated to the R subunit of cytoplasmic protein kinase A

We show that the yeast, Saccharomyces cerevisiae, contains two cAMP-binding proteins in addition to the well-characterized regulatory (R) subunit of cytoplasmic cAMP-dependent protein kinase (PKA). We provide evidence that they comprise a new type of cAMP receptor, membrane-anchored by covalently attached lipid structures. They are genetically not related to the cytoplasmic R subunit. The respective proteins can be detected in sral mutants, in which the gene for the R subunit of PKA has been disrupted and a monoclonal antibody raised against the cytoplasmic R subunit does not cross-react with the two membrane-bound cAMP-binding proteins. In addition, they differ from the cytoplasmic species also with respect to their location and the peptide maps of the photoaffinity-labeled proteins. Although they differ from one another in molecular mass and subcellular location, peptide maps of the cAMP-binding domains resemble each other and both proteins are membrane-anchored by lipid structures, one to the outer surface of the plasma membrane, the other to the outer surface of the inner mitochondrial membrane. Both anchors can be metabolically labeled by Etn, myo-Ins and fatty acids. In addition, the anchor structure of the cAMP receptor from plasma membranes can be radiolabeled by GlcN and Man. After cleavage of the anchor with glycosylphosphatidylinositol-specific phospholipase C from trypanosomes, the solubilized cAMP-binding protein from plasma membranes reacts with antibodies which specifically recognize the cross-reacting determinant from soluble trypanosomal coat protein, suggesting similarity of the anchors. Degradation studies also point to the glycosylphosphatidylinositol nature of the anchor from the plasma membrane, whereas the mitochondrial counterpart is less complex in that it lacks carbohydrates. The plasma membrane cAMP receptor is, in addition, modified by an N-glycosidically linked carbohydrate side chain, responsible mainly for its higher molecular mass.

Localization and characterization of the binding site for the regulatory subunit of type II cAMP-dependent protein kinase on MAP2

Microtubule-associated protein 2 (MAP2) binds, and is a substrate for, type II cAMP-dependent protein kinase. The structural domain in MAP2 that binds the regulatory subunit (RII) of protein kinase II was identified by expressing fragments of a human MAP2 cDNA in E. coli using the pATH11 vector. Fusion proteins were resolved by SDS-PAGE and transferred to nitrocellulose. The filters were probed with purified bovine heart or brain RII, anti-RII monoclonal antibodies, and 125I-labeled protein A. Binding of RII was localized to a 31 amino acid sequence near the N-terminus of the MAP2 molecule. Fusion proteins containing this fragment bound both heart and brain RIIs in a concentration-dependent manner, but bound heart RII with a higher apparent affinity than brain RII. The amino acid sequence of this fragment (DRETAEEVSARIVQVVTAEAVAVLKGEQEKE) is totally conserved between human and mouse MAP2, suggesting an important role for the RII binding site of MAP2 in neuronal function.

cAMP-dependent phosphorylation of soluble and crude microtubule fractions of rat cerebral cortex after prolonged desmethylimipramine treatment

We have analyzed the cAMP-dependent phosphorylation system in the cerebral cortex and hippocampus of rats after acute and chronic administration of desmethylimipramine. Prolonged desmethylimipramine administration modified the cAMP-dependent endogenous phosphorylation of a protein band with apparent molecular weight 280 kDa from the cerebrocortical-soluble fraction. The effect appeared to be specific and associated with the chronic but not the acute administration of desmethylimipramine since we did not obtain any modification in other endogenous cAMP phosphoproteins of either the particulate or soluble fraction of the cerebral cortex. 280 kDa was identified as the soluble microtubule associated protein 2 on the basis of molecular weight, endogenous phosphorylation and immunological recognition. Prolonged desmethylimipramine administration did not induce any modification in the soluble cAMP-dependent endogenous phosphorylation of 280 kDa in other brain areas such as hippocampus, striatum or cerebellum, suggesting a region-specific effect of chronic desmethylimipramine treatment. Microtubule-associated protein 2 is a neuronal protein highly enriched in the dendritic portion of neurons and represents one of the major substrates in the cell for the type II cAMP protein kinase. Since the type II cAMP protein kinase that catalyzes the phosphorylation of microtubule-associated protein 2 copurifies with microtubules, we performed endogenous phosphorylation using increasing concentrations of cAMP in a crude microtubule preparation where microtubule-associated protein 2 appeared to be more concentrated. Under our conditions the maximal effect occurred at 1 microM cAMP, revealing increased 32P incorporation in microtubule-associated protein 2 from a crude microtubule preparation obtained from the cerebral cortex of rats treated with desmethylimipramine. Photoaffinity labelling with 8-azido-[32P]cAMP of the various fractions obtained during the preparation of crude microtubules (S1, S2 and crude microtubules) revealed an increase in the labelling of a protein band with apparent molecular weight of 52 kDa after desmethylimipramine treatment. The labelling of a 47 kDa protein band, which is also present in S1 and S2 fractions was, however, not altered by drug treatment. In conclusion, our studies demonstrated that prolonged desmethylimipramine treatment elicited specific changes in the phosphorylation system associated with a crude microtubule fraction.

Protein phosphorylation systems in postmortem human brain

Protein phosphorylation systems regulated by cyclic adenosine 3',5'-monophosphate (cyclic AMP), or calcium in conjunction with calmodulin or phospholipid/diacylglycerol, have been studied by phosphorylation in vitro of particulate and soluble fractions from human postmortem brain samples. One-dimensional or two-dimensional gel electrophoretic protein separations were used for analysis. Protein phosphorylation catalyzed by cyclic AMP-dependent protein kinase was found to be highly active in both particulate and soluble preparations throughout the human CNS, with groups of both widely distributed and region-specific substrates being observed in different brain nuclei. Dopamine-innervated parts of the basal ganglia and cerebral cortex contained the phosphoproteins previously observed in rodent basal ganglia. In contrast, calcium/phospholipid-dependent and calcium/calmodulin-dependent protein phosphorylation systems were less prominent in human postmortem brain than in rodent brain, and only a few widely distributed substrates for these protein kinases were found. Protein staining indicated that postmortem proteolysis, particularly of high-molecular-mass proteins, was prominent in deeply located, subcortical regions in the human brain. Our results indicate that it is feasible to use human postmortem brain samples, when obtained under carefully controlled conditions, for qualitative studies on brain protein phosphorylation. Such studies should be of value in studies on human neurological and/or psychiatric disorders.

Synapsin Ia, synapsin Ib, protein IIIa, and protein IIIb, four related synaptic vesicle-associated phosphoproteins, share regional and cellular localization in rat brain

The regional and cellular distribution of four synaptic vesicle-associated proteins, synapsins Ia and Ib (Mr 86,000 and 80,000, collectively referred to as synapsin I) and proteins IIIa and IIIb (Mr 74,000 and 55,000, collectively referred to as protein III), has been compared in selected rat brain regions, using both radioimmunoassays and back-phosphorylation assays. Lesions of several neuronal populations in the basal ganglia (corticostriatal fibers, intrinsic striatal neurons, striatonigral fibers, nigrostriatal fibers) induced decreases in the levels of these various proteins that were highly correlated (r = 0.96-0.97). Moreover, the synaptic vesicle-associated phosphoproteins displayed a similar and widespread distribution throughout the CNS. This evidence for colocalization indicates that the majority of, and possibly all, CNS neurons and nerve terminals may contain both forms of synapsin I and both forms of protein III.

Distribution of calmodulin- and cyclic AMP-stimulated protein kinases in synaptosomes

The subcellular location of calmodulin- and cyclic AMP stimulated protein kinases was assessed in synaptosomes which were prepared on Percoll density gradients. The distribution of the protein kinases between the outside and the inside and between the soluble and membrane fractions was determined by incubating intact and lysed synaptosomes, as well as supernatant and pellet fractions obtained from lysed synaptosomes, in the presence of [gamma-32P]ATP. Protein kinase activity was assessed by the labelling of endogenous proteins, or exogenous peptide substrates, under conditions optimized for either calmodulin- or cyclic AMP-stimulated protein phosphorylation. When assessed by calmodulin-stimulated autophosphorylation of the alpha subunit of calmodulin kinase II, 44% of this enzyme was on the outside of synaptosomes, and 41% was in the 100,000 g supernatant. Using an exogenous peptide substrate, the distribution of total calmodulin-stimulated kinase activity was 27% on the outside and 34% in the supernatant. The high proportion of calmodulin kinase II on the outside of synaptosomes is consistent with its known localization at postsynaptic densities. The proportion of calmodulin kinase II which was soluble depended on the ionic strength conditions used to prepare the supernatant, but the results suggest that a major proportion of this enzyme which is inside synaptosomes is soluble. When assessed by cyclic AMP-stimulated phosphorylation of endogenous substrates, no cyclic AMP-stimulated kinase activity was observed on the outside of synaptosomes, whereas 21% was found with an exogenous peptide substrate. This suggests that if endogenous substrates are present on the outside of synaptosomes, then the enzyme does not have access to them. The cyclic AMP-stimulated protein kinase present inside synaptosomes was largely bound to membranes and/or the cytoskeleton, with only 10% found in the supernatant when assessed by endogenous protein phosphorylation and 25% with an exogenous substrate. The markedly different distribution of the calmodulin- and cyclic AMP-stimulated protein kinases presumably reflects differences in the functions of these enzymes at synapses.

Characterization of two mutants of the LLC-PK1 porcine kidney cell line affected in the catalytic subunit of the cAMP-dependent protein kinase

The catalytic (C) subunit activity of the cAMP-dependent protein kinase (cAMP-PK) from the mutant cell lines, FIB4 and FIB6, is only 10% compared with the parent cell line, LLC-PK1 [Jans and Hemmings (1986) FEBS Lett. 205, 127-131]. In order to understand the nature of the mutant phenotypes the cAMP-PK from parent and mutant cell lines was studied in more detail. Analysis of mutant cAMP-PK activity by ion-exchange chromatography revealed that kinase activity associated with type I holoenzyme of both FIB4 and FIB6 was only 5% parental, and the activity of the type II holoenzyme was about 20% parental. The type I regulatory (RI) subunits associated with the type I were also found to be reduced by 70-80% in both mutants, whereas the type II R subunit levels were similar to that of the parent. The residual kinase activity associated with the type I holoenzyme from FIB4 and FIB6 could not be activated by cAMP whereas the type II holoenzyme was activated by cAMP (Ka of 5.5 X 10(-8) M), and showed normal affinities for Kemptamide and ATP. A polyclonal antibody to the catalytic subunit was used to quantify the level of this protein in wild-type and mutant cells. This analysis showed that FIB4 and FIB6 had nearly normal levels of C subunit, suggesting that the C subunit synthesized by the mutants was mostly inactive. As both type I and type II cAMP-PK holoenzymes were abnormal, the most likely explanation of the mutant phenotype is a defect either in the structural gene for the C subunit or in an enzyme involved in its posttranslational processing. However, a second lesion affecting the RI subunit cannot be ruled out at this moment.

Demonstration by phase-partitioning in Triton X-114 solutions that phosphoprotein B-50 (F-1) from rat brain is an integral membrane protein

The Triton X-114 phase separation technique was employed to fractionate phosphoproteins present in membrane fragments from rat brain. Membranes were labelled with [gamma-32P]ATP in media containing Ca2+, Ca2+ plus calmodulin or cyclic AMP, and then treated with Triton X-114. Phosphoproteins recovered in the detergent-insoluble fraction, aqueous and detergent phases were detected by SDS-polyacrylamide gel electrophoresis and autoradiography. Of the proteins solubilised by the detergent, a known substrate of protein kinase C, the B-50 phosphoprotein (45 kD; also known as F-1), partitioned quantitatively into the detergent-rich phase, making it very probable that this phosphoprotein is an integral membrane protein. The detergent-rich phase also contained an 80 kD phosphoprotein, which probably corresponds to the widespread acidic 87 kD substrate of protein kinase C.

The cAMP cascade in the nervous system: molecular sites of action and possible relevance to neuronal plasticity

Many intercellular messages regulate the activity of their target cells by altering the intracellular level of cAMP and, as a consequence, the phosphorylation state of proteins which serve as substrates for cAMP-dependent protein kinase. Such regulation plays a crucial role in neuronal development, neuronal function, and neuronal plasticity (e.g., elementary learning mechanisms). Ample information has been accumulated in recent years on the enzymes that regulate the level of cAMP or respond to it, on the regulation of cAMP synthesis by neurohormones, neurotransmitters, ions, and toxins, on neuronal-specific substrate proteins that are phosphorylated by the cAMP-dependent kinase, and on the interaction of the cAMP-cascade with other second-messenger systems within neurons. Such data, obtained by a combination of molecular-biological, biochemical, and cellular approaches, shed light on the detailed mechanisms by which modulation of a ubiquitous molecular cascade leads to a great variety of short-term as well as long-term specific neuronal responses and alterations.

Expression cloning of a cDNA encoding the type II regulatory subunit of the cAMP-dependent protein kinase

We report here the isolation and sequence of a cDNA for the type II regulatory subunit of the cAMP-dependent protein kinase (cAMP-PK) from a lambda gt-11 cDNA library derived from a porcine epithelial cell line (LLC-PK1). The cDNA was detected by immunological screening using an affinity purified polyclonal antibody for bovine RII. DNA sequence analysis of the 467 bp EcoRI insert confirmed the identity of the clone, because the deduced amino acid sequence corresponded to the published sequence for the bovine RII protein. Northern analysis of total RNA from the LLC-PK1 cells indicated a single mRNA species of about 6.0 kb, probably derived from a single copy gene.

Acceptors for cyclic AMP-dependent and calcium ion-dependent protein kinases in rat brain cytosol fractions: a comparison of occluded (synaptosomal) cytosol with non-occluded cytosol

Endogenous protein phosphorylation patterns were compared in occluded and non-occluded cytosol fractions prepared from rat forebrain. The occluded fraction was taken as representative of synaptosomal cytosol. One- and two-dimensional autoradiographs revealed the presence in non-occluded cytosol of a substrate for cAMP- and Ca2+/calmodulin-dependent protein kinase activities of Mr 300kD, corresponding to phosphorylated microtubule-associated protein-2 (MAP-2); this protein was absent in occluded cytosol. In contrast, a major substrate for protein kinase C was observed exclusively in occluded cytosol after phosphorylation under basal conditions. However, after phosphorylation in the presence of exogenous lipids, approximately equal amounts of the 82kD substrate were detected in both fractions, suggesting that protein kinase C in the occluded fraction was present in a partially activated state. Other minor differences in phosphorylation patterns between the two fractions were observed.

Comparison of the regulatory and catalytic subunits of cAMP dependent protein kinase from Dictyostelium discoideum and bovine heart using polyclonal antibodies

The purified regulatory (R) and catalytic (C) subunits of cAMP dependent protein kinase (cAK) from the primitive eukaryote Dictyostelium discoideum have been compared with the homologous proteins from bovine heart by SDS-PAGE followed by Western blotting using polyclonal antibodies. No cross-reaction could be demonstrated by this technique although the slime mold subunits share several functional properties with their mammalian counterparts and are able to form functional hybrid holoenzymes.

Immunocytochemical characterization of neuron-rich primary cultures of embryonic rat brain cells by established neuronal and glial markers and by monospecific antisera against cyclic nucleotide-dependent protein kinases and the synaptic vesicle protein synapsin I

Primary cell cultures derived from embryonic rat brain were characterized by immunocytochemical methods using established cell markers and monospecific antisera against cyclic nucleotide-dependent protein kinases and the synaptic vesicle protein, synapsin I. The cultures contained predominantly neurons, few astroglial cells and no oligodendroglial cells, based on immunocytochemical studies of the distribution of neuron-specific enolase, glial fibrillary acidic protein, myelin basic protein and galactocerebroside. Subsequently, the immunocytochemical localization of synapsin I, the cyclic GMP-dependent protein kinase and the various subunits of cyclic AMP-dependent protein kinase was determined. Synapsin I, a substrate for both the cyclic AMP- and Ca2+/calmodulin-dependent protein kinases, appeared particularly useful as a specific neuronal marker in primary cultures. Both immunocytochemical and immunoblotting techniques readily detected synapsin I in neuron-rich embryonic brain cultures, but indicated that synapsin I was absent from glia-rich primary cultures of newborn rat brain cells which lacked neurons. The intracellular localization of synapsin I in neurons changed markedly during the time of cell culture. In the first 10 days of cell culture, synapsin I appeared to be confined to neuronal cell bodies, whereas later it shifted to a patchy distribution in neuronal processes, perhaps indicating the transport of synapsin I in synaptic vesicles from the compartment of protein synthesis to its final synaptic location. Within neuron-rich embryonic cultures, the regulatory subunit (R-II) and the catalytic subunit (C) of cyclic AMP-dependent protein kinase appeared to be highly concentrated in neurons examined immunocytochemically. However, biochemical experiments demonstrated that R-II and C were also present in non-neuronal cell types of brain cell primary cultures. Cyclic GMP-dependent protein kinase, a marker protein for cerebellar Purkinje cells and for smooth muscle cells, was not detected immunocytochemically in neuron-rich cultures of embryonic brain cells, suggesting that Purkinje cells and smooth muscle cells were either absent from or not sufficiently developed in these cultures.

Changes in in vitro brain and spinal cord protein phosphorylation after a single oral administration of tri-o-cresyl phosphate to hens

The effect of a single oral 750 mg/kg dose of tri-o-cresyl phosphate (TOCP) on the endogenous phosphorylation of brain and spinal cord proteins was assessed in hens during the development of and recovery from delayed neurotoxicity. Crude membrane and cytosolic fractions were prepared from the brains and spinal cords of control and TOCP-treated hens at 1, 7, 14, 21, 35, and 55 days after treatment. Brain and spinal cord protein phosphorylation with [gamma-32P]ATP was analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), autoradiography, and microdensitometry. TOCP administration conferred calcium and calmodulin dependence on the phosphorylation of a few brain cytosolic proteins and caused an increase in the phosphorylation of a number of other cytosolic and membrane proteins. This effect of TOCP was large in magnitude, and its time course reflected the onset of and recovery from the signs of ataxia and paralysis associated with delayed neurotoxicity in the hen. The molecular weights (Mr) and maximal phosphorylation (percent of control) for the most prominently affected bands were as follows: brain cytosol--50K (183%), 55K (575%), 60K (529%), 65K (273%), and 70K (548%); brain membranes--50K (622%) and 60K (697%); and spinal cord cytosol--20K (182%). The role of endogenous phosphorylation reactions in and their potential usefulness as biochemical indicators of delayed neurotoxicity are being explored further.

[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.

Comparison of proteins involved with cyclic AMP metabolism between synaptic membrane and postsynaptic density preparations isolated from canine cerebral cortex and cerebellum

Synaptic membrane and postsynaptic density (PSD) fractions isolated from canine cerebral cortex and cerebellum were assayed for the following proteins: adenylate cyclase and phosphodiesterase (PDE) activities against cyclic AMP and cyclic GMP, the regulatory subunit of the cyclic AMP-dependent protein kinase, and the substrate proteins for this kinase. The results were expressed on the basis of both the protein content of the fractions and the number of synapses in the synaptic membrane fractions. The number of synapses on a constant protein content basis was about three times higher in the cerebral cortex synaptic membrane fraction than in the comparable cerebellar fraction. Adenylate cyclase activity was from 3.4 to 5.6 times higher in the cerebral cortex membrane fraction than in the cerebellar membrane fraction based on protein content but only slightly higher based on synapse counts. PSD fractions had no adenylate cyclase activity. The cyclic AMP-PDE activity was from 17 to 27 times higher in the cerebral cortex membrane fraction than in the cerebellar membrane fraction based on protein content, and about five times higher based on synapse counts. By doing PDE histochemistry at the electron microscopy level it was found that all the cerebral cortex PSDs in the isolated fraction contained PDE activity, none being found associated with the broken-up material in the fraction. The amount of the regulatory subunit of the cyclic AMP-dependent protein kinase was about equal in the two fractions based on protein, but about one-third lower in cerebral cortex fraction than in cerebellar fractions. In the cerebral cortex membrane fraction the primary substrate for the cyclic AMP-dependent protein kinase is synapsin I, with much lower amounts in the cerebellar membrane fraction. The PSD fraction from the two sources also showed these differences in synapsin I content. In the cerebellar membrane fraction, the primary substrate for the enzyme is a approximately 245,000 Mr protein not found in the cerebral cortex membrane fraction. The findings that the turnover of cyclic AMP is much higher in cerebral cortex synapses than in cerebellar synapses, and that differences are found between the cerebral cortex and cerebellum with regard to the substrate proteins for the cyclic AMP-dependent protein kinase indicate a divergence in the effect of cyclic AMP between cerebral cortex and cerebellar synapses.

Cross-reaction of anti-rat B-50: characterization and isolation of a "B-50 phosphoprotein" from bovine brain

Antibodies to the phosphoprotein B-50 of rat brain were used to trace cross-reacting brain proteins of vertebrates. With the SDS-gel-immunoperoxidase method, a cross-reacting protein (CP) of apparent Mr 53,000 was demonstrated in the homogenate and the synaptic plasma membrane fraction of bovine brain. Sequence 1-24 of adrenocorticotropin (ACTH1-24) (10(-5) M and 10(-4) M) inhibited endogenous phosphorylation of CP in synaptic plasma membranes. The protein was partially characterized and purified to homogeneity from bovine brain by procedures previously described for rat B-50. CP was enriched in ammonium sulfate precipitated protein (ASP) fractions and phosphorylated by an endogenous protein kinase. Two-dimensional gel analysis of bovine and rat ASP showed that the cross-reacting protein had an isoelectric point less acidic than B-50. Limited proteolysis by Staphylococcus aureus protease yielded a "peptide map" analogous to B-50. Two major fragments of Mr 30,000 and 17,000 were produced. In addition, CP exhibited other similarities to rat B-50: phosphorylation by rat brain protein kinase C, microheterogeneity observed after isoelectric focusing, and possibly degradation by endogenous proteolysis. Cross-reaction of proteins in brain homogenates of other mammalian species and of chicken was demonstrated: the Mr of the proteins ranged from 47,000 to 53,000. We conclude that (1) the cross-reacting bovine protein is a "B-50 protein," and (2) the Mr of the "B-50 protein" varies from species to species.

The major calmodulin-stimulated phosphoprotein of synaptic junctions and the major post-synaptic density protein are distinct

The major post-synaptic density protein (mPSDp) present in isolated synaptic junction fractions is distinct from the major phosphoprotein (50Kpp) that is labelled by an endogenous calmodulin-stimulated protein kinase. mPSDp and the 50Kpp have different apparent molecular weights on sodium dodecyl sulphate polyacrylamide gels and the presence of 50Kpp in brain soluble fractions indicates that the two proteins have different subcellular distributions.

Endogenous protein phosphorylation in chick and rat brain synaptic membranes

The protein kinase activities endogenous to synaptic membranes prepared by an identical procedure from avian (chick) and mammalian (rat) brains were compared. Both species showed similar responses towards both protein kinase effector molecules cyclic adenosine monophosphate and Ca2+. Kapp for cyclic adenosine monophosphate-dependent protein kinase activity occurred at 0.4-0.8 microM cAMP and Kapp for Ca2+-dependent, calmodulin-requiring protein kinase activity occurred at 1-2 microM Ca2+ (free ion concentration) both in the absence or presence of calmodulin added to the reaction mixture. This suggests that endogenous calmodulin in these membranes was able to modulate the Ca2+-dependent, calmodulin requiring protein kinase activity. After EGTA-treatment of the membranes to remove endogenous Ca2+ and calmodulin, no significant response towards Ca2+ on the phosphorylation of the membrane polypeptides was measured unless exogenous calmodulin was added after which the Kapp for Ca2+ was increased to 15 microM Ca2+ (free ion concentration). There was a difference in the maximal levels of kinase activity in these membranes with chick membranes containing 57% less cyclic adenosine monophosphate-dependent protein kinase activity, but 65% more Ca2+-dependent, calmodulin-requiring protein kinase activity than the rat membranes. Similar results were determined when either low (5 microM) or high (5.8 microM) concentrations of adenosine 5'-triphosphate were added to the reaction mixtures. Besides certain species differences in the molecular weights of the resulting phosphoproteins, we observed several major differences with respect to the absence or presence of some of the phosphoproteins. Chick synaptic membranes may lack the cyclic adenosine monophosphate-requiring, microtubule-associated phosphoprotein, MAP2, one of the 2 neurospecific, cyclic adenosine monophosphate-requiring and Ca2+, calmodulin-requiring phosphoproteins (Protein Ib, although Protein Ia apparently is present), and the Ca2+-requiring, calmodulin-independent, ACTH-sensitive phosphoprotein, B50. The phenothiazines, trifluoperazine, fluphenazine and chlorpromazine were found to inhibit the Ca2+-dependent, calmodulin-requiring protein kinase activities of both the chick and rat synaptic membranes. This inhibition appeared to be specific for calmodulin because at the same concentrations the phenothiazine analogue, chlorpromazine-sulfoxide, had no effect on this activity. Also found to inhibit Ca2+-dependent calmodulin-requiring protein kinase activity were dibucaine and adrenocorticotropin. These data suggest that rat forebrain synaptic plasma membranes are activated by cyclic adenosine monophosphate

Localization of an endogenous substrate for cyclic AMP-stimulated protein phosphorylation in retina

Incubation of mouse or rabbit whole retina homogenates in the presence of [gamma 32P]-ATP and Mg2+ leads to the phosphorylation of various proteins, as demonstrated using SDS-polyacrylamide gel electrophoresis and autoradiography. The phosphorylation of one protein (ca. approximately equal to 31000 mol. wt) was increased by cyclic AMP in both species, with half-maximal stimulation at 5 x 10(-7)M. Cyclic GMP was also active, but much less potent. Protein phosphorylation patterns were compared in retina homogenates from normal mice (C57BL/6J), from adult C57BL/6J mice homozygous for the retinal degeneration gene (rd/rd) in which rod photoreceptor cells are absent, and from 21-day-old 020/Cpb mice homozygous for the retinal degeneration slow gene (rds/rds) in which only the outer segments of the rod photoreceptors are missing. The 31 K protein was only present in normal and in 21-day-old rds/rds mice. When rabbit retina was microdissected into outer segment, inner segment plus outer nuclear, and inner retina layers, cyclic AMP-stimulated phosphorylation of the 31 K protein was evident only in the inner segment plus outer nuclear layer. These data indicated the presence of a specific, endogenous substrate for a cAMP-dependent protein kinase which is found in the inner portions of rod photoreceptor cells.