Rat Brain Membrane Fractions Research Articles

Comparison of the endogenous heptapeptide Met-enkephalin-Arg6-Phe7 binding in amphibian and mammalian brain

In previous communications [4, 38] we published that [3H]Met-enkephalin-Arg6-Phe7 (MERF) binds to opioid (kappa2 and delta) and sigma2 sites in frog and rat brain membrane preparations, however no binding to kappa1 sites could be established. In the present paper we compare the frog, rat and guinea pig brain membrane fractions with respect to their MERF binding data. No qualitative differences were found between the three species but specific binding of labelled MERF was maximal in frog brain and lowest in guinea pig brain, which corresponds to their kappa2 opioid receptor distribution. The naloxone resistant binding was also present in all investigated species and varied from 25% in frog and guinea pig cerebrum, to 50% in rat cerebrum and cerebellum, but no naloxone inhibition was found in guinea pig cerebellum where no kappa2 opioid receptors have been found. The presence of sigma2-like receptor was demonstrated in each investigated membrane fraction with displacement experiments using (-)N-allyl-normetazocine as competitor of tritiated MERF. It was shown that this site was responsible for 60-80% of [3H]MERF binding. The remaining part of the naloxone resistant labelled MERF binding could be displaced only with endogenous opioid peptides as met-enkephalin, dynorphin and beta-endorphin. The eventual physiological role of multiple MERF receptors is discussed.

Identification of rat brain p42 IP4, a high-affinity inositol(1,3,4,5)tetrakisphosphate/phosphatidylinositol(3,4,5)trisphosphate binding protein

Inositol(1,3,4,5)tetrakisphosphate (InsP 4) and phosphatidylinositol(3,4,5)trisphosphate (PtdInsP 3) are two potential second messengers with a still largely unknown mode of action. We recently cloned the 42 kDa protein p42 IP4 previously purified from pig cerebellum, which binds InsP 4 ( K d ∼2 nM) and PtdInsP 3 with comparable affinities (Stricker et al., FEBS Lett. 405 (1997) 229). The protein p42 IP4 (pig) is highly homologous to centaurin-α, a larger protein of 46 kDa, derived from a rat brain cDNA library clone (Hammonds-Odie et al., J. Biol. Chem. 271 (1996) 18859). Here we investigated whether also p42 IP4 is expressed in rat brain and how it might be related to centaurin-α. When we carried out RT-PCR using mRNA from brain of rats of different ages we obtained several clones corresponding to p42 IP4, but not to centaurin-α. The existence of p42 IP4 in rat brain is supported by the following findings: (1) biochemical analysis of the purified rat brain protein shows inositol phosphate ligand affinities identical to those of the protein from other species; (2) Western blot analysis of rat brain membrane fractions using a peptide-specific antiserum revealed only the 42 kDa protein (p42 IP4), but did not give evidence for the occurrence of a larger 46 kDa centaurin-α-like protein in rat brain; and (3) the amino acid sequences deduced from p42 IP4 cDNA are highly homologous in several species and are confirmed by protein fragment microsequences. Thus, p42 IP4 from rat brain which has two pleckstrin homology domains is a protein largely conserved between different species and most likely has an important function in inositol phosphate or inositol lipid signal transduction.

Expression and Initial Characterization of a Soluble Glycine Binding Domain of the N-Methyl-d-aspartate Receptor NR1 Subunit

Glycine is an essential co-agonist of the excitatory N-methyl-D-aspartate (NMDA) receptor, a subtype of the ionotropic glutamate receptor family. The glycine binding site of this hetero-oligomeric ion channel protein is formed by two distinct extracellular regions, S1 and S2, of the NR1 subunit, whereas the homologous domains of the NR2 subunit mediate glutamate binding. Here, segments S1 and S2 of the NR1 polypeptide were fused via a linker peptide followed by N- and C-terminally tagging with Flag and His6 epitopes, respectively. Infection of High Five insect cells with a recombinant baculovirus containing this glycine binding site construct resulted in efficient secretion of a soluble fusion protein of about 53 kDa. After affinity purification to near-homogeneity, the fusion protein bound the competitive glycine site antagonist [3H]MDL105,519 with high affinity (Kd = 5.22 +/- 0. 13 nM) similar to that determined with rat brain membrane fractions. This high affinity binding could be competed by the glycine site antagonist 7-chlorokynurenic acid as well as the agonists glycine and D-serine but not by L-glutamate. This indicates that the S1 and S2 domains of the NR1 subunit are sufficient for the formation of a glycine binding site that displays pharmacological properties similar to those of the NMDA receptor in vivo.

SAP97 Is Associated with the α-Amino-3-hydroxy-5-methylisoxazole-4-propionic Acid Receptor GluR1 Subunit

Rapid glutamatergic synaptic transmission is mediated by ionotropic glutamate receptors and depends on their precise localization at postsynaptic membranes opposing the presynaptic neurotransmitter release sites. Postsynaptic localization of N-methyl-D-aspartate-type glutamate receptors may be mediated by the synapse-associated proteins (SAPs) SAP90, SAP102, and chapsyn-110. SAPs contain three PDZ domains that can interact with the C termini of proteins such as N-methyl-D-aspartate receptor subunits that carry a serine or threonine at the -2 position and a valine, isoleucine, or leucine at the very C terminus (position 0). We now show that SAP97, a SAP whose function at the synapse has been unclear, is associated with alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)-type glutamate receptors. AMPA receptors are probably tetramers and are formed by two or more of the four AMPA receptor subunits GluR1-4. GluR1 possesses a C-terminal consensus sequence for interactions with PDZ domains of SAPs. SAP97 was present in AMPA receptor complexes immunoprecipitated from detergent extracts of rat brain. After treatment of rat brain membrane fractions with the cross-linker dithiobis(succinimidylpropionate) and solubilization with sodium dodecylsulfate, SAP97 was associated with GluR1 but not GluR2 or GluR3. In vitro experiments with recombinant proteins indicate that SAP97 specifically associates with the C terminus of GluR1 but not other AMPA receptor subunits. Our findings suggest that SAP97 may be involved in localizing AMPA receptors at postsynaptic sites through its interaction with the GluR1 subunit.

Purification, cDNA Cloning, and Expression of UDP-Gal: Glucosylceramide β-1,4-Galactosyltransferase from Rat Brain

Lactosylceramide synthase is an enzyme that catalyzes the transfer of galactose from UDP-Gal to glucosylceramide, and thus participates in the biosynthesis of most glycosphingolipids in mammals. We purified this enzyme over 61,000-fold to near homogeneity with a 29. 7% yield from rat brain membrane fractions. The isolation procedure included solubilization with Triton X-100, affinity chromatography on wheat germ agglutinin-agarose and UDP-hexanolamine-agarose, and hydroxylapatite column chromatography, followed by ion exchange chromatography. The final preparation migrated as a broad band with an apparent molecular mass of 61 kDa on SDS-polyacrylamide gel electrophoresis. This apparent molecular mass was reduced to 51 kDa by N-glycanase digestion, suggesting that the enzyme has a glycoprotein nature. The enzyme required Mn2+ for its activity, and glucosylceramide was its preferred substrate. The cDNA for the enzyme was cloned from a rat brain cDNA library. The cDNA insert encoded a polypeptide of 382 amino acid residues, with a molecular weight of 44,776. The polypeptide contained eight putative glycosylation sites and a 20-amino acid residue transmembrane domain at its N terminus. Amino acid sequence homology analysis revealed that this enzyme shared 39% homology with mouse beta-1, 4-galactosyltransferase (EC 2.4.1.38), which catalyzes the transfer of Gal to beta-1,4-GlcNAc in glycoproteins.

A monoclonal antibody recognizing nestin and the phosphorylated form of neurofilament-M.

In the course of an immunohistochemical screening of antibodies raised against a postnatal day 7 rat brain membrane fraction, we obtained a monoclonal antibody that recognizes both neurofilament-M, a type VI intermediate filament protein, and nestin, a type IV intermediate filament protein. In newborn rat cerebellum, the 5A7 antibody stained Bergmann glial fibers and subsets of axons simultaneously, but in adult cerebellum it only stained neuronal elements. Upon immunoblot analysis of neonatal rat cerebellum, 5A7 gave two prominent bands corresponding to 140 and > 200 kDa. The former was identified as neurofilament-M and the latter as nestin. Dephosphorylation of neurofilament-M resulted in a dramatic reduction in the immunoreactivity, indicating that 5A7 recognized the phosphorylated form of neurofilament-M. Immunoblot analysis of V8 protease digests of neurofilament-M revealed that the epitope on neurofilament-M is located on the carboxy-half of the tail domain of neurofilament-M. These results indicate that 5A7 is the first monoclonal antibody which recognizes a common or similar epitope on different classes of intermediate filament proteins other than the conserved rod domain. The presence of a common or similar epitope on both nestin and neurofilament-M recognized by 5A7 may suggest the possibility that nestin and neurofilament-M have a common functional motif.

Specific Modulation of two Neuronal Digitalis Receptors by Anaesthesia

Three isoenzymes of digitalis receptors (α1, α2, α3) in the brain and only one in the kidney (α1) can be distinguished by their ouabain affinities and their responsiveness to sodium. Since we have reported modulations for these digitalis receptors by their fatty acid membrane environment, anaesthesics could bind on and modulate either directly these receptors or indirectly by disturbing membrane lipids. The aim of this study was to evaluate this anaesthetic action on apparent ouabain affinities and sodium dependence of cerebral and renal Na+,K+-ATPase isoenzymes activities. Rat brain and kidney membrane fractions with pentobarbital-induced anaesthetized state were compared to an unanaesthetized state for their (1) fatty acid composition of total membrane phospholipids, (2) responsiveness to ouabain and (3) Na+ dependence of digitalis receptors. An anaesthesia period of 10 minutes induced (1) a fatty acid modification of brain membranes and (2) a significant sensibilization to ouabain for the α2 and α3 isoforms of digitalis receptors (α2, IC50; 8.2 ± 0.5 × 107 mol/l vs 4.5±0.2 × 107 mol/l; α3, IC50; 6.0±0.3 × 10-8 mol/l vs 2.5±0.1 × 10-8 mol/l). In contrast, the ouabain affinity of the α1 subunit expressed in kidney and brain membranes was unaltered. No anaesthetic effect was observed on the Na+ dependence of the α1 isoenzyme in the brain (4 mmol/l) and the kidney (8 mmol/l). Pentobarbital induced a desensibilization for α2-receptors (8.3±0.5 vs 16.0±1.4 mmol/l Na+) and a sensibilization for α3-receptors (14.4±0.8 vs 10±1.3 mmol/l Na+). These altered properties could be related to a selective modification of the fatty acid composition and/or to the presence of a specific binding site for pentobarbital on these two neuronal digitalis receptors.

CMP-NeuAc:(NeuAc alpha 2-->8)n (colominic acid) sialyltransferase activity in rat brain and in tumour cells that express polysialic acid on neural cell adhesion molecules.

A method for the assay of CMP-NeuAc:(NeuAc alpha 2-->8)n (colominic acid) sialyltransferase activity was developed. Using a 1-day-old rat brain membrane fraction as an enzyme preparation optimal activity was obtained at pH 6.5, 0.3% Triton X-100, and 5 mM MnCl2. However, no absolute cation requirement was found as EDTA only partially inhibited the activity. Within a concentration range of 0.3-3 mg colominic acid (which consists of a mixture of oligomers of alpha 2-->8-linked sialic acid) per 50 microliters a V of 0.61 nmol per mg protein h-1 was estimated while a half-maximal reaction velocity was obtained at a concentration of 1.75 mg per 50 microliters. High performance anion-exchange chromatography of the radioactive products formed in the reaction showed that sialic acid oligomers ranging in size from a degree of polymerization (DP) of 2 up to at least DP 9 could serve as acceptor substrates. Comparison of the acceptor properties of DP 3 and DP 6 showed that the larger oligomer was acted upon with a 10-fold higher efficiency. Periodate oxidation of the products followed by reduction and hydrolysis yielded the C7 analogue of NeuAc as the only radioactive product, indicating that under the conditions of the assay only a single sialic acid residue was introduced into the acceptor molecules. Using the assay it appeared that in rat brain the activity of this sialyltransferase decreased six-fold during postnatal development to the adult stage. The assay method was also applied to lysates of several neuroblastoma and small cell lung tumour cell lines, which differ in the expression of polysialic acid as well as of the neural cell adhesion molecule NCAM, a major carrier of this polymer. Activity of the sialyltransferase appeared to be correlated with the expression of polysialic acid present on NCAM. These results indicate that this sialyltransferase might function in the process of poly-sialylation.

Immunochemical Identification and Differential Phosphorylation of Alternatively Spliced Forms of the α1A Subunit of Brain Calcium Channels

Biochemical properties of the alpha 1 subunits of class A brain calcium channels (alpha 1A) were examined in adult rat brain membrane fractions using a site-directed anti-peptide antibody (anti-CNA3) specific for alpha 1A. Anti-CNA3 specifically immunoprecipitated high affinity receptor sites for omega-conotoxin MVIIC (Kd approximately 100 pM), but not receptor sites for the dihydropyridine isradipine or for omega-conotoxin GVIA. In immunoblotting and immunoprecipitation experiments, anti-CNA3 recognized at least two distinct immunoreactive alpha 1A polypeptides, a major form with an apparent molecular mass of 190 kDa and a minor, full-length form with an apparent molecular mass of 220 kDa. The 220- and 190-kDa alpha 1A polypeptides were also specifically recognized by both anti-BI-Nt and anti-BI-1-Ct antibodies, which are directed against the NH2- and COOH-terminal ends of alpha 1A predicted from cDNA sequence, respectively. These data indicate that the predicted NH2 and COOH termini are present in both size forms and therefore that these isoforms of alpha 1A are created by alternative RNA splicing rather than post-translational proteolytic processing of the NH2 or COOH termini. The 220-kDa form was phosphorylated preferentially by cAMP-dependent protein kinase, whereas protein kinase C and cGMP-dependent protein kinase preferentially phosphorylated the 190-kDa form. Our results identify at least two distinct alpha 1A subunits with different molecular mass, demonstrate that they may result from alternative mRNA splicing, and suggest that they may be differentially regulated by protein phosphorylation.

D-myo-Inositol 1,4,5-Trisphosphate-Binding Proteins in Rat Brain Membranes1

Rat brain membrane fractions obtained using Triton X-100 were applied to a D-myo-inositol 1,4,5-trisphosphate [D-Ins(1,4,5)P3] immobilized column, followed by gel filtration and anion-exchange chromatography. Two proteins with molecular masses of 130 and 85 kDa, as assessed by SDS-polyacrylamide gel electrophoresis, were purified to apparent homogeneity as D-[3H]Ins(1,4,5)P3-binding proteins with no D-Ins(1,4,5)P3-metabolizing activity. Partial amino acid sequence determinations of these proteins revealed that the 130 kDa protein appears to be a new D-Ins(1,4,5)P3-binding protein and the 85 kDa protein is a delta 1-isozyme of phospholipase C. We have previously purified 130 and 85 kDa proteins as D-[3H]Ins(1,4,5)P3-binding proteins, from rat brain cytosol fraction. Antibodies against the 130 kDa protein from the cytosol cross-reacted with the membrane 130 kDa protein purified in this study, suggesting that the membrane 130 kDa protein is likely to be the same as the protein from the cytosol fraction. The inhibition of D-[3H]Ins(1,4,5)P3 binding by D-isomers of inositol phosphates available clarified that the 130 kDa protein has a similar affinity for D-Ins(1,4,5,6)P4 to that for D-Ins(1,4,5)P3, while the 85 kDa protein is specific to D-Ins(1,4,5)P3.

Potent and stereospecific anticonvulsant activity of 3-isobutyl GABA relates to in vitro binding at a novel site labeled by tritiated gabapentin

3-Isobutyl GABA is a derivative of the inhibitory neurotransmitter γ-aminobutyric acid (GABA) and is also structurally related to the novel anticonvulsant gabapentin. The S(+) enantiomer of 3-isobutyl GABA blocks maximal electroshock seizures in mice and also potently displaces tritiated gabapentin from a novel high-affinity binding site in rat brain membrane fractions. The R(−) enantiomer is much less active in both assays, suggesting that the gabapentin binding site is involved in the anticonvulsant activity of 3-isobutyl GABA.

Lack of potent antinociceptive activity by substance P antagonist CP-96,345 in the rat spinal cord

Substance P (SP) binds to the NK-1 receptor and has been implicated in the transmission of pain as well as in physiological responses such as salivary gland secretion and neurogenic inflammation. Studies in this field have been limited due to the lack of specific antagonists that are not degraded rapidly and are not neurotoxic. However, a recently developed non-peptide SP antagonist, CP-96,345, is specific for the NK-1 receptor. The purpose of this study was to assess the effects of this antagonist on nociception. The tail flick, paw pinch and hot plate tests were used to assess nociception in rats. Following baseline determination of tail skin temperature and analgesiometric tests, the rats received intrathecal injections of various doses of CP-96,345, and pain sensitivity was assessed at several time intervals up to two hours after injection. The ability of CP-96,345 to inhibit SP induced biting and scratching was also assessed. Results from the analgesiometric tests indicated that there were no significant elevations in the latency of tail flick test or the paw pinch thresholds even at 240 μg of CP-96,345. The hot plate latency was elevated at the highest dose of antagonist. In addition, there was a significant dose-related elevation in latency on the hot plate test. CP-96,345 also produced a dose-related decrease in tail skin temperature. CP-96,345 did not block SP induced biting and scratching. CP-96,345 and SP were evaluated for their ability to displace 125I-Tyr 8-SP from rat spinal cord, brain and submandibular gland membrane fractions. It was found that although the affinity of CP-96,345 was 56 fold lower than that of SP in the brain, the antagonist was nearly as potent as SP in the spinal cord and submandibular gland. The results of this study suggest that, while CP-96,345 binds to the NK-1 receptor in the spinal cord, this receptor is most likely not involved in mediating some types of nociception at the spinal cord level.

A rapid and simple assay method for UDP-glucose: ceramide glucosyltransferase

We have developed a simple rapid method for measuring UDP-glucose: ceramide glucsoyltransferase; the method utilizes ceramide immobilized on the surface of silica gell and [ 14C]UDP-glucose as substrate. The reaction product, [ 14C]glucosylceramide formed on the surface of the silica gel was easily separated from free [ 14C]UDP-glucose, either by centrifugation or by filtration. The reliability of this solid phase method was evaluated by using rat brain membrane fraction as an enzyme source. This enzyme had an optimal pH of 6.4–6.5 and required Mn 2+, Mg 2+ in the presence of 3-[(3-cholamidopropyl)dimethylammonio]-1- propanesulfonate (CHAPS). Apparent K m values of 8.7 μM for UDP-glucose and 292 μM for ceramide were determined using the new method. Under the optimal conditions, the solid phase method yielded 2–5 times more product than did the method using micellar system. Moreover, the reaction was highly quantitative in its enzyme dose-activity relationship.

Bimodal regulation of protein phosphorylation by a ganglioside in rat brain membrane.

Whether or not a ganglioside influences the protein phosphorylation in the rat brain membrane fraction was investigated. Phosphorylation of the 72 kDa protein was significantly affected by the addition of 80 nM GQ1b in vitro, which is far below the reported concentration of gangliosides that affects protein phosphorylation in the neuronal membrane fraction. This action of GQ1b was bimodal: it being not only stimulatory as to the incorporation of phosphate into the 72 kDa protein on incubation of 20 sec, but also as to the release of phosphate from or breakdown of the 72 kDa protein on incubation for more than 5 min. Eighty nM GQ1b did not noticeably affect ATPase in the same fraction. These results suggest that the transphosphorylation of the 72 kDa protein is affected by the interaction of GQ1b with either the responsible enzymes or the 72 kDa protein as a substrate.

Proteolytic fragment of protein kinase C (kinase M) phosphorylates in vitro phosphatidylinositol-4-phosphate

Limited tryptic proteolysis of homogeneous protein kinase C induces the formation of a catalytically active fragment of 50 kDa (kinase M) which, unlike native PK C acquires the ability to phosphorylate PIP. Both ATP and GTP were found to be capable of serving as phosphate donors in this process. Incubation of purified kinase M with a preparation of rat brain membrane fraction enhanced the level of phosphorylation of PIP in the presence and in the absence of exogenous PIP. A scheme of the interrelationship of phosphoinositide metabolism and the proteolytic processing of protein kinase C is proposed.

Syntheses and biological activities of endothelin-1 analogs.

Endothelin-1 analogs replaced by various amino acids at position 21, namely [X21]-ET-1, were synthesized, and their agonistic vasoconstrictor activity on rat thoracic aortic strips and receptor binding activity on rat brain membrane fraction were examined to elucidate their structure-activity relationship. The vasoconstrictor activities of [Tyr21]- and [Phe21]-ET-1 were one order of magnitude smaller than that of ET-1, and those of [His21]-, [Gly21]-, [Ser21]-, [Ala21]- and [Lys21]-ET-1 were more than two orders of magnitude smaller than that of ET-1. On the other hand, the replacements by Ile, Glu, Gln and Pro resulted in distinguished losses of the vasoconstrictor activities. In addition, preincubation with these analogs did not blunt ET-1-induced vasoconstriction and showed no antagonistic activity. The binding inhibitory activities of these analogs against 125I-ET-1 were approximately conformable to the vasoconstrictor activities with only a slight exception. These findings demonstrate that the phenyl group at position 21 is important for both the vasoconstrictor activity and the receptor binding activity.

Interactions of N-cyclopropylmethyl(−)-6β-acetylthionormorphine with μ-, κ-, δ- and σ-opioid receptors

1. 1. Affinities of N-cyclopropylmethyl(−)-6β-acetylthionormorphine (KT-90) to μ-, κ-, δ- and σ-receptors were tested in rat brain membrane fractions using radioligand-receptor assays. 2. 2. Though KT-90 had nonselectively high affinities to μ-, κ- and δ-receptors, affinity of KT-90 to σ-receptors was lower than those to the other three receptors. 3. 3. Affinity of KT-90 to σ-receptors was 1000 times lower than that of buprenorphine.

Solid phase synthesis and opioid receptor binding properties of presumable dermorphin precursor derivatives.

Presumable dermorphin precursor peptide derivatives comprised of 35 amino acids and their fragments, which are based on the amino acid sequence determined by recombinant deoxyribonucleic acid (DNA) techniques, were synthesized by the solid phase method. A 35-residue peptide amide containing L-Ala2-dermorphin sequence at the N-terminus (1) as well as its D-Ala2 isomer (2) and the C-terminal 20-residue peptide amide were found to be unexpectedly stable against aminopeptidase M digestion and in rat brain membrane fractions mixture, suggesting that the C-terminal Glu-rich moiety of 1 and 2 serves to protect from enzymatic breakdown. In the opioid receptor binding assay, 2 showed 40 and 25-fold higher affinities than 1 for mu and delta-receptors, respectively. The N-terminal 15-residue peptide fragment of 2 showed greatly increased affinities for both receptors, being one half of those of dermorphin, whereas that of 1 showed low affinities. Opioid receptor binding properties of these synthetic peptides may be useful in investigation of the processing to dermorphin.

Purification and characterization of membrane-bound and cytosolic forms of diacylglycerol kinase from rat brain.

Two different types of diacylglycerol kinase (DGK) have been purified 10,455-fold (DGK I) and 7,410-fold (DGK IV) from the cytosol and membrane fractions of rat brain, respectively. The cytosolic DGK was purified by successive chromatographies on Affi-Gel Blue, Q-Sepharose F.F., Mono Q, hydroxylapatite, and ATP-agarose. The membrane-bound DGK was purified from the 2 M NaCl extract of membranes by chromatography on Affi-Gel Blue, phenyl-Superose, hydroxylapatite, and ATP-agarose. The resultant preparations contained homogeneous enzymes with a Mr of 110,000 (DGK I) and 150,000 (DGK IV) as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. These enzymes both phosphorylate 1,2-dioleoyl glycerol at rates of 11.5 mumol/min/mg protein for DGK I and 5.2 mumol/min/mg protein for DGK IV. Both enzymes require divalent cations and ionic detergents for activity. Magnesium is the most potent cation for both enzymes, but Ca2+ was also found to be fairly effective. Manganese is less effective than Mg2+ or Ca2+. Anionic detergents such as sodium deoxycholate or sodium cholate stimulate the activities of both enzymes, although DGK IV is stimulated more markedly than DGK I at lower concentrations. The optimal pH for the two enzymes was found to be the same, pH 7.4. Some phospholipids such as phosphatidylserine and phosphatidylinositol elevate the kinase activities of these kinases even in the absence of detergents. DGK IV is activated more significantly than DGK I by low amounts of phospholipids. The two enzymes also show structural differences. DGK I and DGK IV give different peptide maps after digestion with Staphylococcus aureus V8 protease or alpha-chymotrypsin. The results suggest that these enzymes are different forms of DGK and may be involved in different biological processes.

A Protein Tyrosine Kinase Involved in Regulation of pp60c-src function

We recently identified a novel protein tyrosine kinase that specifically phosphorylates truncated pp60c-src (Mr = 53,000) at a tyrosine residue(s) distinct from its autophosphorylation site. In this study, we examined whether this enzyme phosphorylates intact pp60c-src (Mr = 60,000) and determined its phosphorylation site. Non-neuronal and neuronal forms of intact pp60c-src were separately purified from the membrane fraction of neonatal rat brain by sequential column chromatographies. The novel kinase phosphorylated tyrosine residues of both forms of intact pp60c-src. The phosphorylation occurred in parallel with autophosphorylation of pp60c-src, and in both forms the final stoichiometry estimated was quite similar to that of autophosphorylation (about 5%). The enzyme also phosphorylated pp60c-src in which the kinase activity had been destroyed by an ATP analogue, p-fluorosulfonylbenzoyl 5'-adenosine. The phosphorylation site of the non-neuronal form was analyzed by sequential peptide mapping with tosylphenylalanyl chloromethyl ketone-treated trypsin and alpha-chymotrypsin. Tryptic digestion of the phosphorylated pp60c-src yielded a unique phosphopeptide that cross-reacted with an antibody specific for the carboxyl-terminal sequence of chicken pp60c-src. Digestion of the phosphopeptide with chymotrypsin yielded a product that comigrated with a synthetic phosphopeptide corresponding to the carboxyl-terminal 15 residues of chicken pp60c-src. These results clearly indicate that the carboxyl-terminal sequence of rat pp60c-src is identical to that of chicken pp60c-src, and a tyrosine residue corresponding to chicken Tyr527 is the phosphorylation site. This phosphorylation resulted in a decrease in the enolase phosphorylating activity of pp60c-src. Kinetic experiments indicated that this decrease in activity was due to a decrease in the Vmax value of pp60c-src. These findings support our previous proposal that the novel tyrosine kinase acts as a specific regulator of pp60c-src in cells.