GABAA receptors in mouse cortical homogenates are phosphorylated by endogenous protein kinase A

Biochemical, molecular, and electrophysiological studies suggest that phosphorylation of beta subunits of the GABAA receptor (GaR) by exogenous protein kinase A inactivates the receptor channels. We have developed a method which for the first time allows the study of GaR phosphorylation in brain tissues by endogenous PKA. Desalted homogenates or crude synaptic membranes from mouse cerebral cortex were incubated with [gamma-32P]ATP and 8-Br-cAMP or chlorophenylthio-cAMP. Extracts from these incubations were immunoprecipitated by polyclonal antibodies against native GaR and analyzed by SDS-gel electrophoresis and autoradiography. In both homogenates and membranes, cAMP-dependent incorporation of 32P was observed for a 57-kDa peptide, and to a lesser extent 51- to 53-kDa peptides. Phosphorylation of affinity-purified GaR by the catalytic subunit of PKA also produced a major 57-kDa phosphopeptide and a minor 51-kDa phosphopeptide. Limited digestion by S. aureus V-8 protease of the 57-kDa phosphopeptide from the desalted homogenates or from purified receptors produced a major 32P-labeled fragment of 11 kDa, suggesting that the phosphorylation site is similar to that shown previously to reduce GaR function. The phosphorylation of GaRs in homogenates was time dependent and blocked by H-89 or protein kinase inhibitor 5-24, specific inhibitors of protein kinase A. Prolonged incubations resulted in dephosphorylation of the 57-kDa phosphoprotein by a microcystin-LR sensitive phosphatase. In cortical homogenates the level of cAMP-dependent phosphorylation of the 57-kDa GaR peptide was more than 5 times that obtained with washed synaptic membranes. However, assays of PKA using the heptamer kemptide as substrate showed that the specific activity in the particulate fraction was 57% that of the homogenate. This suggests that GaRs on synaptic membranes are preferentially phosphorylated by a cytoplasmic form of protein kinase A. By comparing the [3H]flunitrazepam-photolabeled 53-kDa GaR subunit with the 51-57 kDa [32P]peptides from cortical homogenates, the molar ratio of [32P]/[3H] was estimated at 0.43, suggesting that a substantial fraction of the GaR pool is phosphorylated under these conditions.

Phosphorylation reverses the membrane association of peptides that correspond to the basic domains of MARCKS and neuromodulin

Several groups have observed that phosphorylation causes the MARCKS (Myristoylated Alanine-Rich C Kinase Substrate) protein to move off cell membranes and phospholipid vesicles. Our working hypothesis is that significant membrane binding of MARCKS requires both hydrophobic insertion of the N-terminal myristate into the bilayer and electrostatic association of the single cluster of basic residues in the protein with acidic lipids and that phosphorylation reverses this electrostatic association. Membrane binding measurements with myristoylated peptides and phospholipid vesicles show this hydrophobic moiety could, at best, barely attach proteins to plasma membranes. We report here membrane binding measurements with basic peptides that correspond to the phosphorylation domains of MARCKS and neuromodulin. Binding of these peptides increases sigmoidally with the percent acidic lipid in the phospholipid vesicle and can be described by a Gouy-Chapman/mass action theory that explains how electrostatics and reduction of dimensionality produce apparent cooperativity. The electrostatic affinity of the MARCKS peptide for membranes containing 10% acidic phospholipids (10(4) M-1 = chi/[P], where chi is the mole ratio of peptide bound to the outer monolayer of the vesicles and [P] is the concentration of peptide in the aqueous phase) is the same as the hydrophobic affinity of the myristate moiety for bilayer membranes. Phosphorylation decreases the affinity of the MARCKS peptide for membranes containing 15% acidic lipid about 1000-fold and produces a rapid (t1/2 < 30 s) dissociation of the peptide from phospholipid vesicles.

In vitro phosphorylation of the 46-kDa mannose 6-phosphate receptor by casein kinase II. Structural requirements for efficient phosphorylation

Some steps in the receptor-mediated transport of newly synthesized mannose 6-phosphate-containing lysosomal enzymes are assumed to be accompanied by changes in the phosphorylation state of receptors. In vitro, the metabolically phosphorylated 46-kDa mannose 6-phosphate receptor (MPR 46) was dephosphorylated by protein phosphatase 2A. The synthetic cytoplasmic domain of MPR 46 was phosphorylated in vitro by casein kinase II. Tryptic phosphopeptide mapping showed that casein kinase II phosphorylates MPR 46 in vitro at the same site that is phosphorylated in vivo. Inhibition studies using synthetic peptides corresponding to different amino acid sequences of the cytoplasmic tail of MPR 46 revealed that the sequence 26-32 (ADGCDFV) contribute to efficient phosphorylation of serine 56. Baby hamster kidney cells were transfected with wild type human MPR 46 cDNA or cDNAs containing mutations in the cytoplasmic tail and assayed for their phosphorylation state in vivo. The phosphorylation of mutant receptors with deleted residues 23-28 (NLVADG) was strongly reduced. These data indicate that residues on the N-terminal side of the phosphorylatable serine 56 may influence the efficiency with which a casein-kinase II-like kinase phosphorylates MPR 46.

Protein kinase C in yeast. Characteristics of the Saccharomyces cerevisiae PKC1 gene product

The Saccharomyces cerevisiae PKC1 gene encodes a homolog of mammalian protein kinase C (Levin, D. E., Fields, F.O., Kunisawa, R., Bishop, J.M., and Thorner, J. (1990) Cell 62, 213-224). A protein of 150 kDa is recognized by a polyclonal antiserum raised against a trpE-Pkc1 fusion protein. In subcellular fractionations, Pkc1p associates with the 100,000 x g particulate fraction. This association is resistant to extraction with high salt concentrations, alkali buffer, or nonionic detergents, suggesting that Pkc1p may be associated with a large protein complex. Pkc1p modified at its COOH terminus with two repeats of the Staphylococcus aureus protein A IgG-binding fragment (ZZ sequence tag) was able to fully restore the growth defects of a pkc1ts strain at restrictive temperature. ZZ-tagged Pkc1p was partially purified by chromatography on DEAE-Sepharose, followed by IgG-Sepharose. In vitro, Pkc1p phosphorylates the pseudosubstrate peptide and myelin basic protein, but not histones. Replacing an isoleucine with an arginine 2 amino acids COOH-terminal of the acceptor serine in the substrate peptide resulted in a 10-fold decrease of Km. Pkc1p activity was independent of cofactors such as phospholipids, diacylglycerol, and Ca2+, known to activate several mammalian protein kinase C isoenzymes, making it a rather distantly related member of the protein kinase C superfamily.

Phosphorylation of the erythroid transcription factor GATA-1

GATA-1 is a zinc finger DNA-binding protein thought to be involved in the expression of the vast majority of erythroid specific genes. We have examined the phosphorylation of GATA-1 in murine erythroleukemia (MEL) cells and have mapped the sites of phosphorylation by overexpression of GATA-1 in monkey kidney COS cells. We show that GATA-1 is phosphorylated on 6 serines within its amino terminus in uninduced MEL cells and that a 7th site, serine 310, becomes phosphorylated after MEL cells are induced to differentiate by exposure to dimethyl sulfoxide. This site lies near the carboxyl boundary of the DNA-binding domain in a conserved region of the protein believed to be involved in DNA bending. Detailed analyses indicate, however, that phosphorylation at this site, or the other sites identified, does not significantly influence DNA-binding affinity or specificity, DNA bending, or transcriptional transactivation by GATA-1.

Regulation of c-Fgr protein kinase by c-Src kinase (CSK) and by polycationic effectors

The protein tyrosine kinase expressed by the protooncogene c-fgr is phosphorylated and down-regulated in vitro by the c-Src kinase (CSK). CSK catalyzed phosphorylation affects Tyr-511 of c-Fgr, homologous to Tyr-527 of c-Src and it prevents the autophosphorylation normally occurring at c-Fgr Tyr-400, homologous to c-Src Tyr-416. Polylysine, histones H1 and H2A and other polycationic proteins on the other hand stimulate c-Fgr activity while promoting enhanced autophosphorylation of both Tyr-400 and Tyr-511. Once phosphorylated at Tyr-511 and down-regulated by CSK, c-Fgr is no more susceptible to polylysine stimulation. Previous autophosphorylation (at Tyr-400) reduces c-Fgr susceptibility to down-regulation by CSK, although Tyr-511 can be still phosphorylated by it. If a more exhaustive autophosphorylation (of both Tyr-400 and Tyr-511) is performed in the presence of polylysine, c-Fgr becomes totally insensitive to CSK down-regulation. These data support the concept that down-regulation of c-Fgr by Tyr-511 phosphorylation is prevented if Tyr-400 is also phosphorylated and they are consistent with an outcompetition of phospho-Tyr-511 from the Src homology 2 domain by phospho-Tyr-400, which, in c-Fgr, is surrounded by an amino acid sequence divergent from that of the other Src-related protein tyrosine kinases.

Protein kinase C-dependent desensitization of the atrial natriuretic peptide receptor is mediated by dephosphorylation

Stimulation of guanylyl cyclase A (GC-A) by atrial natriuretic peptide (ANP) is antagonized by activators of protein kinase C (PKC). Thus, it has been suggested that PKC phosphorylates and desensitizes GC-A. Here, we have developed stable GC-A transfectants of NIH3T3 cells, which display marked reductions in hormone-dependent cGMP elevations and guanylyl cyclase activity after incubation with ANP or phorbol 12-myristate 13-acetate (PMA). ANP binding and immunoblot analysis indicated that the decreases were not due to receptor internalization or degradation. GC-A isolated from 32PO4-labeled cells contained phosphoserine and phosphothreonine. ANP and/or PMA addition caused substantial decreases in the 32P content of the receptor that coincided with reductions in hormone-dependent guanylyl cyclase activity. The specific PKC inhibitor, GF-109203X, completely blocked the PMA-dependent dephosphorylation and desensitization of GC-A but failed to inhibit either ANP-dependent process. Tryptic phosphopeptide maps of GC-A isolated from ANP- or PMA-treated cells were unique, suggesting that the sites that dephosphorylated in response to each agent were different. In contrast to previous reports, we conclude that PMA and ANP desensitization of GC-A are distinct events mediated by dephosphorylation of specific residues through PKC-dependent and -independent pathways, respectively.

Unique structural features and differential phosphorylation of the 280-kDa component (isozyme) of rat liver acetyl-CoA carboxylase

Rat liver acetyl-CoA carboxylase (ACC, EC 6.4.1.2) exhibits major and minor subunits (M(r) of 265,000 and 280,000 respectively), the structure and function of which are compared in this study. The two subunits copurified and each contained biotin as demonstrated by avidin reactivity and direct determination of biocytin. In agreement with previous studies, the ACC subunits could be distinguished with specific monoclonal antibodies and differential tissue expression. We now report extensive differences in primary structure revealed by peptide mapping, mass spectrometric analysis of peptides following reverse phase high performance liquid chromatography, and microsequencing of selected peptides. Four peptides derived from the 265-kDa subunit were sequenced and matched sequences within the predicted structure of rat 265-kDa ACC. Although one identical peptide sequence was detected within both subunits (residues 2009-2024 of the 265-kDa subunit), 12 peptides derived from the 280-kDa subunit exhibited entirely novel sequences or matched partially (average 70% identity) with sequences within the 265-kDa subunit. The 280-kDa subunit may also exhibit distinct functional properties, since the initial rate of phosphorylation was at least 10-fold greater than that of the 265-kDa subunit in the presence of cAMP-dependent protein kinase. Two-dimensional mapping demonstrated that the tryptic phosphopeptides released from the two ACC subunits are distinct. These structural studies suggest that the 265- and 280-kDa components (isozymes) of ACC are so distinct they may be encoded by separate genes, while the differential phosphorylation observed in vitro suggests a key role for the 280-kDa subunit in regulating enzyme activity within intact cells.

cAMP- and cGMP-dependent protein kinase phosphorylation sites of the focal adhesion vasodilator-stimulated phosphoprotein (VASP) in vitro and in intact human platelets

The vasodilator-stimulated phosphoprotein (VASP) is a major substrate for cAMP-dependent- (cAK) and cGMP-dependent protein kinase (cGK) in human platelets and other cardiovascular cells. To identify the VASP phosphorylation sites, purified VASP was phosphorylated by either protein kinase and subjected to trypsin, V8 and Lys-C proteolysis. The phosphorylated proteolytic fragments obtained were separated by reversed phase high performance liquid chromatography. Sequence analysis of the phosphorylated peptides and 32P measurement of the released 32P-labeled amino acids revealed three phosphorylation sites: a serine 1-containing site (LRKVSKQEEA), a serine 2-containing site (HIERRVSNAG), and a threonine-containing site (MNAVLARRRKATQVGE). Additional experiments with purified VASP demonstrated that both cAK and cGK phosphorylated serine 2 rapidly and the threonine residue slowly, whereas cGK phosphorylated the serine 1 residue more rapidly than the cAK. These differences in the phosphorylation rates of VASP by the two protein kinases were also observed with synthetic peptides corresponding to the sequences of the three identified phosphorylation sites. These experiments also established the synthetic peptide serine 1 as one of the best in vitro cGK substrates and the serine 2-containing site as the site responsible for the phosphorylation-induced mobility shift of VASP in sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Experiments with 32P-labeled platelets provided evidence that VASP is phosphorylated at the same three identified sites also in intact cells and that selective activation of cAK or cGK primarily increased the phosphorylation of both serine 2 and serine 1 but not threonine. Our results demonstrated overlapping substrate specificities of cAK and cGK in vitro and in intact cells. However, important quantitative and qualitative differences between cAK- and cGK-mediated phosphorylation of the focal adhesion protein VASP in human platelets were also observed, suggesting distinct functions of the two types of cyclic nucleotide-mediated VASP phosphorylation.

Backbone dynamics of a free and phosphopeptide-complexed Src homology 2 domain studied by 15N NMR relaxation

The backbone dynamics of the C-terminal SH2 domain of phospholipase C gamma 1 have been investigated. Two forms of the domain were studied, one in complex with a high-affinity binding peptide derived from the platelet-derived growth factor receptor and the other in the absence of this peptide. 2-D 1H-15N NMR methods, employing pulsed field gradients, were used to determine steady-state 1H-15N NOE values and T1 and T2 15N relaxation times. Backbone dynamics were characterized by the overall correlation time (tau m), order parameters (S2), effective correlation times for internal motions (tau e), and, if required, terms to account for motions on a microsecond-to-millisecond-time scale. An extended two-time-scale formalism was used for residues having relaxation data and that could not be fit adequately using a single-time-scale formalism. The overall correlation times of the uncomplexed and complexed forms of SH2 were found to be 9.2 and 6.5 ns, respectively, suggesting that the uncomplexed form is in a monomer-dimer equilibrium. This was subsequently confirmed by hydrodynamic measurements. Analysis of order parameters reveals that residues in the so-called phosphotyrosine-binding loop exhibited higher than average disorder in both forms of SH2. Although localized differences in order parameters were observed between the uncomplexed and complexed forms of SH2, overall, higher order parameters were not found in the peptide-bound form, indicating that on average, picosecond-time-scale disorder is not reduced upon binding peptide. The relaxation data of the SH2-phosphopeptide complex were fit with fewer exchange terms than the uncomplexed form. This may reflect the monomer-dimer equilibrium that exists in the uncomplexed form or may indicate that the complexed form has lower conformational flexibility on a microsecond-to-millisecond-time scale.

An approach to locate phosphorylation sites in a phosphoprotein: mass mapping by combining specific enzymatic degradation with matrix-assisted laser desorption/ionization mass spectrometry

A rapid, picomole-scale method is described to locate phosphorylation sites in phosphoproteins by using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) combined with enzymatic modification of the analyte. There are three steps to locate phosphorylation sites in a phosphoprotein: (i) degradation of the phosphoprotein into small peptides by specific enzymatic or chemical reactions; (ii) identification of the phosphopeptides by -80 (or multiples of -80)-Da mass shifts in the mass spectra after dephosphorylation with alkaline phosphatase; (iii) location of the phosphorylation sites by mass mapping. As the size of the protein increases, it is advantageous to fractionate the mixture by HPLC and analyze each fraction by MALDI-TOF-MS. To perform mass mapping, the primary structure of the protein must be known. Bovine beta-casein was analyzed by this method. The conclusions about the specific phosphorylation sites of bovine beta-casein from our data coincide with previously reported results. From calculations, it is found that a mass spectrometer with 0.1% mass accuracy is sufficient, for mass mapping, to identify completely or partially digested tryptic peptides in the mass range of 100-8000 Da from bovine beta-casein (MW 23,983).

Thermodynamic studies of tyrosyl-phosphopeptide binding to the SH2 domain of p56lck

The interaction between SH2 domains and tyrosine-phosphorylated proteins is essential in several cytosolic signal transduction pathways. Here we report thermodynamic studies of the interaction of the p56lck (lck) SH2 domain with several phosphopeptides, using the technique of isothermal titration calorimetry (ITC). This is the first report of the use of ITC to study SH2 domain binding reactions. The free energy of binding of the SH2 domain of lck to a phosphopeptide corresponding to the autoregulatory C-terminus of the protein (pY505) was found to be similar to that measured for a phosphopeptide modeled on the C-terminus of the epidermal growth-factor receptor (delta G degrees approximately -7.0 kcal mol-1 at pH 6.8), although significant differences in the enthalpy and entropy were observed. Binding of a phosphopeptide modeled on the C-terminus of p185neu was weaker (delta G degrees approximately -5.4 kcal mol-1 at pH 6.8). Lowering the pH to 5.5 reduced the binding affinity of pY505 by approximately 1 order of magnitude. We ascribe this to the protonation of a histidine side chain in the SH2 domain (H180), which is involved in a hydrogen-bonding network that optimizes the binding site geometry. No difference in affinity was observed between portions of lck corresponding to the SH3-SH2 (residues 63-228) and SH2 (residues 123-228) domains for the pY505 peptide. We also studied the effect upon pY505 peptide binding of mutations at two highly conserved arginine residues in the lck SH2 domain (R134 and R154).(ABSTRACT TRUNCATED AT 250 WORDS)

Phosphorylation of the duck hepatitis B virus capsid protein associated with conformational changes in the C terminus

The capsid protein of duck hepatitis B virus (DHBV) is phosphorylated at multiple sites during viral infection. A cluster of sites is located near the C terminus of the 262-amino-acid protein. We have used site-directed mutagenesis to show that three serines and one threonine serve as phosphate acceptor amino acids in the C terminus. An additional six potential phosphate acceptor sites in this region were apparently not utilized. Each serine or threonine that served as a phosphate acceptor was adjacent to a downstream proline, while all six serines that were not acceptors for phosphate residues lacked adjacent downstream prolines. Mutation of the downstream proline to glycine at each site had the same effect as mutating the serine itself, suggesting an SP or TP motif as an essential feature for capsid protein phosphorylation. Phosphorylation at these four sites resulted in complex shifts in electrophoretic mobility in sodium dodecyl sulfate gels of the capsid protein or of a C-terminal peptide containing the phosphorylated sites, suggesting that specific conformations of the C terminus are associated with different combinations of phosphorylated serines. We speculate that distinct functions of the C terminus may be associated with different phosphorylated domains on the intact capsid.

Mutants at Ser277 of Xenopus cdc2 protein kinase induce oocyte maturation in the absence of the positive regulatory phosphorylation site Thr161

The cdc2 protein kinase is an important regulatory protein for both meiosis and mitosis. Previously, we demonstrated that simultaneous mutation of Thr14-->Ala14 and Tyr15-->Phe15 in the Xenopus cdc2 protein results in an activated cdc2 mutant that induces maturation in resting oocytes. In addition, we confirmed the importance of the positive regulatory phosphorylation site, Thr161, by demonstrating that cdc2 mutants containing additional mutations of Thr161-->Ala161 or Glu161 are inactive in the induction of oocyte maturation. Here, we have analyzed the importance of an additional putative cdc2 phosphorylation site,Ser277. Single mutation of Ser277-->Asp277 or Ala277 had no effect on activity, and these mutants were unable to induce Xenopus oocyte maturation. However, the double mutant Ala161/Asp277 was capable of inducing oocyte maturation, suggesting that mutation of Ser277-->Asp277 could compensate for the mutation of Thr161-->Ala161. The Asp277 mutation could also compensate for the Ala161 mutation in the background of the activating mutations Ala14/Phe15. Although mutants containing the compensatory Ala161 and Asp277 mutations were capable of inducing oocyte maturation, these mutant cdc2 proteins lacked detectable in vitro kinase activity. Tryptic phosphopeptide mapping of mutant cdc2 protein and comparison with in vitro synthesized peptides indicated that Ser277 is not a major site of phosphorylation in Xenopus oocytes; however, we cannot rule out the possibility of phosphorylation at this site in a biologically active subpopulation of cdc2 molecules. The data presented here, together with prior reports of Ser277 phosphorylation in somatic cells, suggest an important role for Ser277 in the regulation of cdc2 activity. The regulatory role of Ser277 most likely involves its indirect effects on the nearby residue Arg275, which participates in a structurally important ion pair with Glu173, which lies in the same loop as Thr161 in the cdc2 protein.

Identification of the phosphorylation site in vitro for cAMP-dependent protein kinase on the rat adipocyte cGMP-inhibited cAMP phosphodiesterase

Rat adipocyte cGMP-inhibited cAMP phosphodiesterase (cGI-PDE) appears to be dually regulated in intact cells by serine phosphorylations induced by isoprenaline and insulin, respectively (Degerman, E., Smith, C. J., Tornqvist, H., Vasta, V., Belfrage, P., and Manganiello, V. C. (1990) Proc. Natl. Acad. Sci. U.S.A. 87, 533-537; Smith, C. J., Vasta, V., Degerman, E., Belfrage, P., and Manganiello, V. C. (1991) J. Biol. Chem. 266, 13385-13390). Since cAMP-dependent protein kinase (cAMP-PK) catalyzes the beta-adrenergic effects, the site in the isolated cGI-PDE phosphorylated by this kinase was explored. A peptide, LRRSSGASGLLTSEHHSR (P18), corresponding to the amino acid sequence Leu423-Arg440 in the putative regulatory domain of the rat adipocyte cGI-PDE was synthesized. It contains a consensus substrate sequence -RRXS- for cAMP-PK within two tryptic cleavage sites and was readily phosphorylated by cAMP-PK. Two phosphopeptides, identified as RS-[32P]SGASGLLTSEHHSR and S-[32P]SGASGLLTSEHHSR, were obtained after stoichiometric phosphorylation and trypsinization of the peptide. These two peptides and the two main tryptic phosphopeptides obtained from immunoisolated [32P]cGI-PDE phosphorylated with cAMP-PK in a solubilized crude adipocyte membrane fraction were immuno-precipitated by an affinity-purified polyclonal antibody raised against P18 and exhibited the same chromatographic and electrophoretic profiles in three different separation systems. Similar radiosequencing profiles indicated that the second most N-terminal serine, corresponding to Ser-427 in the intact cGI-PDE, was phosphorylated by cAMP-PK in both P18 and authentic cGI-PDE. It is concluded that serine 427 is the target for cAMP-PK phosphorylation of the rat adipocyte cGI-PDE in vitro.

Structure of the regulatory domains of the Src-family tyrosine kinase Lck

The kinase p56lck (Lck) is a T-lymphocyte-specific member of the Src family of non-receptor tyrosine kinases. Members of the Src family each contain unique amino-terminal regions, followed by Src-homology domains SH3 and SH2, and a tyrosine kinase domain. SH3 and SH2 domains mediate critical protein interactions in many signal-transducing pathways. They are small, independently folded modules of about 60 and 100 residues, respectively, and they are often but not always found together in the same molecule. Like all nine Src-family kinases (reviewed in ref. 3), Lck is regulated by phosphorylation of a tyrosine in the short C-terminal tail of its catalytic domain. There is evidence that binding of the phosphorylated tail to the SH2 domain inhibits catalytic activity of the kinase domain and that the SH3 and SH2 domains may act together to effect this regulation. Here we report the crystal structures for a fragment of Lck bearing its SH3 and SH2 domains, alone and in complex with a phosphotyrosyl peptide containing the sequence of the Lck C-terminal regulatory tail. The latter complex represents the regulatory apparatus of Lck. The SH3-SH2 fragment forms similar dimers in both crystals, and the tail peptide binds at the intermolecular SH3/SH2 contact. The two structures show how an SH3 domain might recognize a specific target and suggest how dimerization could play a role in regulating Src-family kinases.

Biochemical characterization of valosin-containing protein, a protein tyrosine kinase substrate in hematopoietic cells

Engagement of the T cell antigen receptor (TCR) leads to activation of multiple tyrosine kinases and rapid tyrosine phosphorylation of intracellular protein substrates. A number of these substrates have been identified and they include TCR subunits, phospholipase C-gamma 1, p95vav, and ezrin. In a recent study we have demonstrated that VCP (valosin-containing protein) becomes tyrosine phosphorylated upon TCR cross-linking. Analysis of the predicted amino acid sequence of this protein indicates that it is a member of a family of oligomeric proteins containing duplicated domains with predicted ATPase activity. In the current study we determine the site of tyrosine phosphorylation in VCP, demonstrate that murine VCP indeed is an oligomeric ATPase, and show that the tyrosine phosphorylation of the protein has no effect on VCP ATPase activity. Recent evidence suggests that VCP associates with clathrin. A possible role of tyrosine phosphorylation in regulating this protein-protein interaction is discussed.

Dephosphorylation of CTP-phosphocholine cytidylyltransferase is not required for binding to membranes

The sequence of the reversible phosphorylation and activation of CTP:phosphocholine cytidylyltransferase was investigated. Treatment of primary rat hepatocytes with oleic acid or phospholipase C caused a significant increase in the activity and amount of particulate cytidylyltransferase which correlated with decreased cytidylyltransferase activity and protein in the cytosol. The increase in membrane-associated cytidylyltransferase is accompanied by a decrease in the phosphorylation of the enzyme. Reversal of membrane association resulted in an increased amount of phosphorylated cytidylyltransferase in the cytosol. We wished to determine if dephosphorylation of the enzyme were a prerequisite for its translocation from the cytosol to the membranes. In vitro studies with membranes from oleic acid- or phospholipase C-treated cells showed that phosphorylated cytosolic cytidylyltransferase associated with these membranes with negligible dephosphorylation. Incubation of hepatocytes with oleic acid for different periods of time demonstrated that cytidylyltransferase associated with membranes in an active, phosphorylated form and was subsequently dephosphorylated. This result was supported by comparison of phosphopeptide maps of 32P-labeled cytidylyltransferase obtained from cytosolic, as well as membrane fractions of control, oleic acid-treated, or phospholipase C-treated cells. These studies revealed dephosphorylation on some sites and phosphorylation on other sites. Our data strengthen the hypothesis that a change in the lipid composition of membranes can mediate the initial binding of cytidylyltransferase to the membrane and that subsequently the enzyme becomes dephosphorylated.

Identification of substrate specificity determinants for the cell cycle-regulated NIMA protein kinase

NIMA is a cell cycle-regulated protein kinase required for the G2/M transition in the filamentous fungus Aspergillus nidulans. Previous biochemical characterization of the recombinant enzyme indicated that NIMA is a protein serine/threonine specific kinase with beta-casein being the best substrate from the many proteins and peptides tested (Lu, K.P., Osmani, S.A., and Means, A.R. (1993) J. Biol. Chem. 268, 8769-8776). However, substrate specificity or physiologically relevant substrates for NIMA remained unknown. In search for a peptide substrate for this enzyme, we screened an assembled library of synthetic peptides that each contained a phosphorylation site for a known protein kinase and found an excellent peptide substrate for NIMA, phospholemman 42-72 (PLM(42-72)). NIMA kinase phosphorylated PLM(42-72) uniquely and stoichiometrically on Ser63 with a Vmax of 1.4 mumol/min/mg and apparent Km of 20.0 microM. These kinetic constants were about 10-fold higher and 3-fold lower than those for beta-casein, respectively. A detailed analysis of substrate specificity determinants using synthetic peptide analogs of PLM(42-72) indicated that Phe-Arg-Xaa-Ser/Thr represents the optimal primary sequence for NIMA kinase phosphorylation. Replacement of the Arg at P-2 with Ala resulted in a 6-fold increase in Km and 2-fold decrease in Vmax, while substitution of the Phe at P-3 with Ala abolished NIMA phosphorylation. These results reveal the unique nature of substrate recognition by the NIMA kinase and should prove valuable in the search for biologically relevant NIMA substrates.

Isolation and characterization of a novel acetyl-CoA carboxylase kinase from rat liver

Acetyl-CoA carboxylase is regulated allosterically by citrate and covalently by a phosphorylation/dephosphorylation mechanism. We have isolated and purified from rat livers a novel kinase that phosphorylates and inactivates the carboxylase. This kinase is bound to the carboxylase and can be eluted in salt-rich solution. The native kinase exists as high molecular weight aggregates of a subunit that has a molecular weight of 40,000. The phosphorylation sites of the carboxylase were determined after tryptic and cyanogen bromide digestions of 32P-labeled carboxylase and separation of the peptides by various chromatographic procedures. Amino acid analyses of the phosphopeptides showed that the Ser77 and Ser1200 residues were the sites of phosphorylation. Treating the phosphorylated carboxylase with the Mn(2+)-dependent acetyl-CoA carboxylase phosphatase 2 removed the phosphate and reactivated the carboxylase. These results suggest that both this kinase and the acetyl-CoA carboxylase phosphatase 2 act at the same site(s) in the acetyl-CoA carboxylase molecule. Citrate dramatically inhibits the kinase-mediated phosphorylation of the carboxylase, suggesting that the allosteric modification and activation by citrate render the phosphorylation sites inaccessible to the kinase and therefore maintain high carboxylase activity. This observation indicates that there is a close interplay between the citrate effect on and phosphorylation of the carboxylase in regulating its activity.

Glucocorticoid receptors in ATP-depleted cells. Dephosphorylation, loss of hormone binding, HSP90 dissociation, and ATP-dependent cycling

Dependence of hormone binding to glucocorticoid receptors (GRs) on cellular ATP levels suggested that GRs traverse an ATP-dependent cycle, and without ATP accumulate in forms that cannot bind hormone. Such "null" receptors (NRs) were identified in ATP-depleted WEHI-7 cells, where they are tightly associated with the nuclear fraction and partly dephosphorylated. With WCL2 cells (Chinese hamster ovary cells with overexpressed GRs) depleted of ATP with azide, we have now identified dephosphorylated sites on NRs, studied possible roles of phosphorylation using GR mutants, and measured association with the 90-kDa heat shock protein (hsp90). Most NRs in WCL2 cells are dephosphorylated at serines 220 and 234, but GRs with those serines mutated to alanines do not resemble NRs since they bind hormone. They do not associate strongly with nuclei. On azide treatment, however, mutated GRs lose hormone binding capacity faster than normal GRs. Association of hsp90 (and presumably other heat shock proteins) with cytosolic GRs is drastically reduced by azide treatment, sufficient to account for decreased hormone binding. We conclude that: (a) dephosphorylation of GRs does not yield NRs, but may weaken association with hsp90. (b) The postulated ATP-dependent GR cycle can be accounted for by dissociation, and ATP-dependent reconstitution, of GR-hsp90 complexes. (c) ATP depletion blocks reconstitution of complexes. Uncomplexed GRs may accumulate as one form of NR; they are probably also the precursors for other forms of NR.

Analysis of cellular phosphoproteins by two-dimensional gel electrophoresis: applications for cell signaling in normal and cancer cells

Two-dimensional (2-D) gel electrophoresis has been used to map proteins from various cell types in an effort to eventually link such maps to the sequencing of the entire human genome. While this analysis indicates the cellular disposition and expression of proteins, another application of 2-D gels, the analysis of phosphoproteins, can provide much information as to the assembly and "wiring" of the signal transduction circuits within cells which appear to be enervated by phosphate exchange. The preparation and separation of 32P-labeled proteins is described, as well as various analytical methods, including: the variety of gel systems available for specialist types of analyses, comparing 33P- and 32P-labeling of proteins, imaging techniques, phosphoamino analysis, phosphopeptide separation, identifying the amino acid groups that are phosphorylated, and the identification of phosphoproteins on 2-D gels by immunoprecipitation, corunning of purified proteins, comparative mapping and microsequencing, and by Western blotting. Examples (in brackets) are given of applications in which 2-D phosphogels can be applied, which offer advantages over other techniques. These include: (i) identifying in vivo substrates for kinases (protein kinase C activated by phorbol myristate acetate), (ii) investigating cytokine signaling pathways (tumor necrosis factor and interleukin-1), (iii) investigating the effects of drugs on signaling pathways (okadaic acid, menadione and cyclooxygenase inhibitors), (iv) characterization of specific phosphoproteins (heat-shock protein Hsp27 and stathmin), (v) comparing normal and transformed cells (MRC-5 human lung fibroblasts and their SV-40-transformed counterparts, MRC-5 SV1 cells), (vi) purifying phosphoproteins, (vii) investigating the relationship of protein phosphorylation to stages in the cell cycle (stathmin), (viii) investigating protein/protein interactions, (ix) mapping in vitro kinase substrates (protein kinase C, protein kinase A, and mitogen activated protein kinase activated protein kinase 2), and (x) locating and identifying cellular phosphatases (Hsp27 phosphatase). It is possible that the mapping of phosphoproteins can be linked to other 2-D gel databases and that information derived from these can be used in the future to better understand the signaling mechanisms of normal and cancerous cells.

Phosphopeptide mapping and phosphoamino acid analysis by electrophoresis and chromatography on thin-layer cellulose plates

Identification of protein phosphorylation sites is essential in order to evaluate the contribution of individual sites to the regulation of a particular protein by phosphorylation. Here we review a method we have developed for the identification of phosphorylation sites based on digestion of 32P-labeled proteins with site-specific proteases and separation of the digestion products in two dimensions on thin-layer cellulose plates using electrophoresis in the first dimension followed by chromatography. This method is very sensitive, requiring only a few hundred 32P-disintegrations per minute to obtain reproducible phosphopeptide maps. We also report methods for the analysis of the phosphoamino acid content of both intact phosphoproteins and individual phosphopeptides recovered from two-dimensional separations, in which the material is subjected to partial acid hydrolysis, and the hydrolysis products are separated on thin-layer cellulose plates by electrophoresis in one or two dimensions. Finally, we describe methods for analyzing the structure of isolated phosphopeptides by secondary digestion with site-specific proteases, by manual Edman degradation, and by immunoprecipitation, and indicate how this information can be used in conjunction with the two-dimensional mobility of the peptide to deduce the identity of a phosphopeptide from the known sequence of a protein.

Cell cycle-dependent regulation of the phosphorylation and metabolism of the Alzheimer amyloid precursor protein

Accumulation of the amyloid A beta peptide, which is derived from a larger precursor protein (APP), and the formation of plaques, are major events believed to be involved in the etiology of Alzheimer's disease. Abnormal regulation of the metabolism of APP may contribute to the deposition of plaques. APP is an integral membrane protein containing several putative phosphorylation sites within its cytoplasmic domain. We report here that APP is phosphorylated at Thr668 by p34cdc2 protein kinase (cdc2 kinase) in vitro, and in a cell cycle-dependent manner in vivo. At the G2/M phase of the cell cycle, when APP phosphorylation is maximal, the levels of mature APP (mAPP) and immature APP (imAPP) do not change significantly. However, imAPP is altered qualitatively. Furthermore, the level of the secreted extracellular N-terminal domain (APPS) is decreased and that of the truncated intracellular C-terminal fragment (APPCOOH) is increased. These findings suggest the possibility that phosphorylation-dependent events occurring during the cell cycle affect the metabolism of APP. Alterations in these events might play a role in the pathogenesis of Alzheimer's disease.

Purification and identification of tyrosine-phosphorylated proteins from B lymphocytes stimulated through the antigen receptor

The activation of protein tyrosine kinase (PTKs) and subsequent tyrosine phosphorylation of cellular proteins is a critical initial signal in the response of eukaryotic cells to mitogens, differentiative signals, and other stimuli. A number of PTK substrates have been identified and many of these are components of signal transduction pathways that regulate cell function. However, the majority of proteins that are tyrosine-phosphorylated in response to receptor signaling remain unidentified. As some of these unidentified PTK substrates may also be signal-transducing proteins, their identification and functional characterization is an important objective towards understanding receptor signaling. We describe the development of a comprehensive and general process for the isolation and structural characterization of tyrosine-phosphorylated proteins. The method involves enrichment by anti-phosphotyrosine affinity chromatography, electrophoretic concentration and separation, and proteolytic fragmentation of individual purified phosphoproteins. Resulting peptide fragments are separated by microbore reverse-phase high performance liquid chromatography (RP-HPLC) and a portion of the eluted peptides are subjected to electrospray-mass spectrometry (ES/MS) for accurate determination of peptide masses. Proteolytic fragmentation of a protein produces a characteristic set of peptide masses that can be used to rapidly identify the protein by searching databases containing the peptide mass "fingerprints" for all known proteins. The identity of the protein established by this method can be confirmed by sequence analysis of selected peptides. We have applied this procedure to the analysis of PTK substrates from B lymphocytes that have been stimulated through the B cell antigen receptor (BCR). Signaling by this receptor is involved in the generation of antibodies against foreign molecules (antigens). The BCR activates multiple PTKs which phosphorylate at least 30 different proteins. We have identified several of these tyrosine-phosphorylated proteins, including Syk, a PTK that is known to be tyrosine-phosphorylated in activated B cells. Thus, the procedure described here can be used to identify regulatory proteins of low abundance. The process consists of a logical succession of compatible steps that avoids pitfalls inherent to prior attempts to characterize low abundance phosphoproteins and should find wide use for the identification of tyrosine-phosphorylated proteins in other cell types.

Cell-cycle-regulated phosphorylation of oncoprotein 18 on Ser16, Ser25 and Ser38

Oncoprotein 18 (Op18) has been independently identified due to its increased phosphorylation in response to external signals and its up-regulated expression in acute leukemia. We have identified two serine residues of Op18 that are phosphorylated after triggering by the T cell antigen receptor. One of these residues, Ser25, was shown to be a likely substrate for the mitogen-activated protein (MAP) kinase, while the other residue, Ser16, was shown to be phosphorylated in response to increased intracellular calcium. Our previous site-mapping studies of Op18 also revealed that basal phosphorylation of Op18 is mainly located on Ser38, which was found to be the primary in vitro phosphorylation site of p13suc1-precipitated cdc2 kinase activities. These findings raised the possibility that Op18 may be a substrate for both receptor-regulated calcium-induced protein kinases and the MAP kinase family, as well as being a substrate for the cell-cycle-regulated cdc2 kinase family. In the present report we have performed site-mapping studies of cell-cycle-regulated fluctuations of Op18 phosphorylation. The results reveal that S-phase progression of a synchronised leukemic T cell line is associated with increased phosphorylation of both the Ser25 and Ser38 residues. Moreover, during mitosis, a burst of phosphorylation was observed and at this stage of the cell cycle a major fraction of Op18 was phosphorylated at multiple sites. Phosphorylation of Op18 during mitosis was located primarily on Ser38 and to lesser extent on Ser25, Ser16 and at an unidentified C-terminal residue. In vitro phosphorylation experiments, employing two distinct members of the cdc2 kinase family, were consistent with involvement of both p34-cdc2 and p33-cdk2 in cell-cycle-regulated phosphorylation of Ser25 and Ser38 of Op18. Most importantly, the ratio of Ser25/Ser38 phosphorylation observed in vitro, using either p34-cdc2 or p33-cdk2, was found to be the same as the ratio observed in intact cells during all phases of the cell cycle. These findings suggest that Op18 may be a physiological substrate for several members of the cdc2 kinase family during both the S-phase and the mitotic phase of the cell cycle.

Production and characterization of antibodies against C-terminal peptide of protein F1: a novel phosphorylation at serine 209 of the peptide by protein kinase C

Protein F1 (GAP-43, B-50, neuromodulin, P-57), a neural tissue-specific phosphoprotein enriched in the growth cones of elongating neurites, is suggested to be involved in synaptic plasticity, neuronal development, and neurotransmitter release. In this study, a 21 amino acid polypeptide (AKPKES*ARQDEGKEDPEADQE) that corresponds to the C-terminus sequence of protein F1 (from position 204-224) was synthesized and used to produce anti-protein F1 antibodies. Immunoblot analysis has demonstrated that the prepared antibodies recognized intact protein F1. Protein F1 and the synthesized F1 peptide were phosphorylated in vitro by PKC. Furthermore, phosphorylated protein F1 was immunoprecipitated by anti-F1 peptide antibodies demonstrating that these antibodies recognized both native, non-phosphorylated and phosphorylated protein. The anti-protein F1 antibodies also stained the plasma membranes of cell bodies and neuritis of mouse neuronal cultures obtained from 14-day old spinal embryonic tissue. By contrast, no glial cells were stained. These data suggest that serine 209 at the C-terminus of protein F1 may be a substrate for PKC phosphorylation in vivo. In addition, antibodies raised against F1 peptide revealed protein F1 immunoreactivity that outlined all neurites of cultured mouse spinal neurons.

Multiple sites of phosphorylation within the alpha heavy chain of Chlamydomonas outer arm dynein

We have examined the phosphorylation of the alpha dynein heavy chain (DHC) from the outer arm of the Chlamydomonas flagellum. Quantitative analysis indicates that this DHC is phosphorylated at a minimum of six sites. Using previously identified proteolytic and photocleavage sites (King, S. M., and Witman, G. B. (1988) J. Biol. Chem. 263, 9244-9255), we have mapped two regions that are phosphorylated in vivo. One is located in a 20-kDa section immediately N-terminal to the site of V1 photocleavage. Thus, this region is close to the ATP hydrolytic site and also to the predicted junction between the head and stem domains of the particle. The second encompasses the 90-kDa C-terminal region of the molecule. In this latter section, at least one site is found in an approximately 2-kDa region close to domains that are predicted to adopt a coiled-coil structure in those DHCs that have been sequenced. The alpha DHC also is specifically labeled by endogenous kinases in demembranated, washed axonemes, suggesting that at least one alpha DHC kinase is located close to, or is a component of, the outer arm in situ.

Identification of TrkB autophosphorylation sites and evidence that phospholipase C-gamma 1 is a substrate of the TrkB receptor

The TrkB receptor protein-tyrosine kinase is a receptor for brain-derived neurotrophic factor and neurotrophin-3. In response to brain-derived neurotrophic factor and neurotrophin-3 treatment, TrkB expressed exogenously in Rat-2 cells is rapidly phosphorylated on tyrosine residues. At least 2 regions of TrkB contain phosphorylated tyrosines. The major sites of autophosphorylation are in the region containing Tyr-670, Tyr-674, and Tyr-675, which lies in the kinase domain and corresponds by sequence homology to the Tyr-416 autophosphorylation site in p60c-Src. Tyr-785, which lies just to the COOH-terminal side of the kinase domain in a relatively short tail characteristic of the Trk family of protein-tyrosine kinase receptors, is also phosphorylated in response to neurotrophin-3 treatment. The sequence around Tyr-785 fits a consensus sequence for binding phospholipase C-gamma 1. The simplest interpretation of these results is that, in response to neurotrophin binding, at least two and perhaps all three of the tyrosines in the Tyr-670/674/675 region are autophosphorylated independently, and Tyr-785 is autophosphorylated in vivo. Following activation of TrkB, phospholipase C-gamma 1 is phosphorylated on Tyr-783, Tyr-771, and Tyr-1254. Phospholipase C-gamma 1 also forms a complex with TrkB in response to neurotrophin-3 treatment, consistent with the possibility that one of the TrkB autophosphorylation sites provides a binding site for the phospholipase C-gamma 1 SH2 domains, as is the case for other receptor protein-tyrosine kinases. We conclude that phospholipase C-gamma 1 is directly phosphorylated by TrkB. Since phosphorylation of Tyr-783 and Tyr-1254 results in activation of phospholipase C-gamma 1, we predict that neurotrophin-3 leads to activation of phospholipase C-gamma 1 following binding to TrkB in Rat-2 cells.

Loss of cytotoxic effect of epidermal growth factor (EGF) on EGF receptor overexpressing cells is associated with attenuation of EGF receptor tyrosine kinase activity

The biological activity of epidermal growth factor (EGF) is mediated through the intrinsic tyrosine kinase activity of the EGF receptor (EGFR). In numerous cell types, binding of EGF to the EGFR stimulates the tyrosine kinase activity of the receptor eventually leading to cell proliferation. In tumor-derived cell lines, which overexpress the EGFR, however, growth inhibition is often seen in response to EGF. The mechanism for growth inhibition is unclear. To study the relationship between growth inhibition and EGFR kinase activity, we have used a cell line (PC-10) derived from a human squamous cell carcinoma that overexpresses EGFR. When exposed to 25 ng/ml EGF at low cell densities (1,300 cells/cm2), PC-10 cells exhibit cell death. In contrast, if EGF is added to high density cultures, no EGF mediated cell death is seen. When PC-10 cells were maintained at confluency in the presence of 25 ng/ml EGF for a period of 1 month, they were subsequently found competent to proliferate at low density in the presence of EGF. We designate these cells APC-10. The APC-10 cells exhibited a unique response to EGF, and no concentration of EGF tested could produce cell death. By 125I-EGF binding analysis and [35S]methionine labeling of EGFR, it was found that the total number of EGFR on the cell surface of APC-10 was not decreased relative to PC-10. No difference between PC-10 and APC-10 was seen in EGF binding affinity to the EGFR. Significantly, EGF stimulated autophosphorylation of the EGFR of APC-10 was 8-10-fold lower than that of PC-10. This reduced kinase activity was also seen in vitro in membrane preparations for EGFR autophosphorylation as well as phosphorylation of an exogenously added substrate. No difference between PC-10 and APC-10 in the overall pattern of EGFR phosphorylation in the presence or absence of EGF was detectable. However, the serine and threonine phosphorylation of the EGFR of APC-10 cells was consistently 2-3-fold lower than that seen in PC-10 cells. These results suggest a novel mechanism for EGFR overexpressing cells to survive EGF exposure, one that involves an attenuation of the tyrosine kinase activity of the EGFR in the absence of a change in receptor levels or receptor affinity.

Tyrosine phosphorylation of protein kinase C-delta in response to its activation

Retroviral vectors containing five different protein kinase C (PKC) isoenzymes (alpha, delta, epsilon, eta, zeta) were expressed in 32D hematopoietic cells and NIH-3T3 fibroblasts. In an effort to investigate signaling events regulated by PKC activation, we analyzed whether tyrosine phosphorylation of cellular proteins would occur after 12-O-tetradecanoylphorbol-13-acetate (TPA) treatment of the various transfectants. While no detectable tyrosine-specific phosphorylation was observed after treatment of the majority of the transfectants, pronounced TPA-dependent tyrosine phosphorylation of an 82-kDa protein was detected in the 32D/PKC-delta and NIH-3T3/PKC-delta lines. Interestingly, the 82-kDa substrate proved to be PKC-delta itself. Tyrosine phosphorylation of purified PKC-delta by src family or receptor tyrosine kinases in vitro enhanced PKC-delta activity, suggesting that tyrosine phosphorylation of PKC-delta may positively affect its function.

Phosphorylation of serine 985 negatively regulates the hepatocyte growth factor receptor kinase

The receptor for hepatocyte growth factor/scatter factor (HGF/SF) is an alpha beta tyrosine kinase of 190 kDa which mediates growth and motility in several cell types. We have previously shown that tyrosine autophosphorylation enhances the receptor kinase activity, while serine phosphorylation by protein kinase C or other Ca(2+)-dependent kinase(s) is inhibitory. We now identify Ser985 as the major phosphorylation site for the protein kinases responsible for such inhibition. Both phorbol esters or Ca2+ ionophore treatment induces phosphorylation of the same tryptic phosphopeptide corresponding to the sequence Leu983-Arg987 located in the juxta-membrane domain of the receptor beta chain. Purified protein kinase C phosphorylates in vitro a synthetic peptide (V14S) including Ser985. Trypsin digestion of the phosphorylated V14S generates a single phosphopeptide comigrating in reverse-phase high performance liquid chromatography with the tryptic peptide phosphorylated in vivo. Phorbol ester treatment of cultured cells inhibits the ligand-induced tyrosine autophosphorylation of the receptor. In vitro, Ser985 phosphorylation inhibits the receptor tyrosine kinase activity on exogenous substrates. Substitution of Ser985 by site-directed mutagenesis results in increased tyrosine phosphorylation of the receptor and abolishes down-modulation by protein kinase C. These data show that phosphorylation of Ser985 is a key mechanism for the negative regulation of HGF/SF receptor.

Phosphorylation-dependent interaction of adenovirus preterminal protein with the viral origin of DNA replication

Adenovirus preterminal protein (pTP) exists as a heterodimer with the viral DNA polymerase (AdPol) and becomes covalently linked to a dCMP residue during initiation of DNA replication. The in vivo phosphorylation of pTP could be demonstrated when pTP is overproduced using recombinant vaccinia viruses, or by a large scale metabolic labeling of adenovirus 2 (Ad2)-infected HeLa cells. Phosphoserine was the only phosphoamino acid obtained by acid hydrolysis of 32P-labeled pTP immunoprecipitated from metabolically labeled HeLa cells infected with either Ad2 or recombinant vaccinia virus. Tryptic peptide maps of pTP expressed using recombinant vaccinia virus system in HeLa cells revealed that phosphorylation of pTP occurred on multiple sites. Dephosphorylation of pTP with calf intestinal alkaline phosphatase resulted in a significant decrease in its activity in the in vitro DNA replication initiation assays. Further characterization of the phosphatase-treated pTP indicated that although dephosphorylation did not affect its interaction with AdPol, the specific recognition of the DNA replication origin by pTP was significantly reduced as determined by gel electrophoresis-based DNA mobility shift assays.

p56lck phosphorylation by Ca2+/calmodulin-dependent protein kinase type II

Ca2+/calmodulin-dependent protein kinases are implicated in regulating the Ca2+ signaling involved in T cell activation and in thymocyte selection. One of the earliest events in signaling through the T cell antigen receptor is activation of the protein tyrosine kinase p56lck. Following T cell activation or signaling through the IL-2 receptor, Ca(2+)-mediated phosphorylation of p56lck occurs on serine/threonine residues. Isoforms of the multifunctional Ca2+/calmodulin-dependent protein kinases, CaM kinase-II and CaM kinase-Gr are found in human T lymphocytes. CaM kinase-II, but not CaM kinase-Gr, phosphorylates the T cell tyrosine kinase p56lck in vitro. Tryptic phosphopeptide maps indicate that CaM kinase-II phosphorylates p56lck on multiple sites in vitro. Kinase assays of p56lck modified by CaM kinase-II indicate that CaM kinase-II modification does not appreciably affect p56lck phosphotransfer activity.

Role of the amino-terminal domain in regulating interactions of annexin I with membranes: effects of amino-terminal truncation and mutagenesis of the phosphorylation sites

Phosphorylation of the N-terminal tail by protein kinase C strongly inhibits the ability of bovine or human annexin I to aggregate chromaffin granules by increasing the calcium requirement 4-fold (Wang, W., & Creutz, C. E. (1992) Biochemistry 31, 9934-9936). In the present study three forms of human annexin I truncated in the amino terminus at residue Trp-12, Lys-26, or Lys-29 exhibit dramatic differences in their sensitivities to calcium in a chromaffin granule aggregation assay, while the [Ca2+](1/2)max values for binding of the truncated proteins to granule membranes are similar. Cleavage at Trp-12 causes a 3-fold decrease in calcium sensitivity in the membrane aggregation assay, while cleavage at Lys-26 causes a 4-fold enhancement of calcium sensitivity. In contrast, cleavage at Lys-29 results in virtually no change in calcium sensitivity. Mutagenic substitution with negatively charged amino acids of Ser-27, a site for phosphorylation by protein kinase C, or Tyr-21, a site for phosphorylation by the epidermal growth factor receptor kinase, mimics the inhibition of granule-aggregating activity seen with phosphorylation by protein kinase C. When bovine chromaffin cells are stimulated to secrete by nicotine, annexin I is phosphorylated in the amino terminus. Thr-24 and Ser-28, which are sites for phosphorylation by protein kinase C in vitro, are two of the sites phosphorylated in vivo in stimulated chromaffin cells. These data demonstrate that the ability of annexin I to promote membrane aggregation is highly sensitive to changes in the structure of the N-terminal domain of the protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Specificity in recognition of phosphopeptides by src-homology 2 domains

SH2 domains and SH3 domains, found in a number of protein-tyrosine kinases and substrates of protein-tyrosine kinases, provide specificity in downstream signaling. Both of these domains bind to relatively short linear sequences of peptides to provide specific interactions between proteins. The SH2 domains directly bind to phosphotyrosine residues of proteins in a specific sequence context. We have devised a phosphopeptide library technique that allows us to rapidly determine the sequence specificity of individual SH2 domains on the basis of amino acids selected at position +1, +2 and +3 C-terminal of the phosphotyrosine. The optimal motif for 22 distinct SH2 domains has been determined and used to predict likely sites of in vivo interaction. A second phosphopeptide library was devised in which the amino acids N-terminal of the phosphotyrosine were also varied. The residues N-terminal of phosphotyrosine had little influence on binding to the N-SH2 domain of the 85 kDa subunit of phosphoinositide 3-kinase. These results indicate that for this SH2 domain, specificity is determined by sequences carboxy-terminal of the phosphotyrosine moiety. Knowledge of the specificity of SH2 domains allows predictions about likely downstream targets on the basis of primary sequence of proteins. Some of these predictions will be discussed.

Identification of phosphorylated peptides from complex mixtures using negative-ion orifice-potential stepping and capillary liquid chromatography/electrospray ionization mass spectrometry

A rapid method for identifying and characterizing sites of phosphorylation of peptides and proteins is described. High-performance capillary liquid chromatography (HPLC) coupled with electrospray ionization mass spectrometry (ESI-MS) is used to distinguish non-phosphorylated and phosphorylated peptides originating from mixtures as complex as enzyme digests. The method relies on the ability to produce a fragment ion characteristic and unique to phosphopeptides (m/z 79, PO3) by stepping the orifice potential of the mass spectrometer as a function of mass. At low m/z values, a high orifice potential is applied to induce extensive fragmentation of the peptide, leading to the formation of the m/z 79 phosphate-derived ion. This method is analogous to that described by Carr et al. for the identification of glycopeptides from enzymatic digestion of glycoproteins (S.A. Carr, M.J. Huddleston, M.F. Bean, Protein Science 2, 183 (1993)). The method was first evaluated and validated for a mixture of non-, mono- and di-phosphorylated synthetic peptides. Both mono- and di-phosphorylated peptides were found to generate fragment ions characteristic of PO3 whereas the non-phosphorylated peptide did not. Application of the method was extended to identifying phosphopeptides generated from an endoprotease Lys-C digestion of beta-casein. Both the expected mono- and tetra-phosphorylated Lys-C peptides were observed and identified rapidly in the LC/SEI-MS analysis. The procedure was used additionally to identify the site(s) of phosphorylation of the cytosolic non-receptor tyrosine kinase, pp60(c-src).

FT-IR spectroscopy indicates that Ca(2+)-binding to phosphorylated C-terminal fragments of the midsized neurofilament protein subunit results in beta-sheet formation and beta-aggregation

The Fourier-transform infrared (FT-IR) spectra in trifluoroethanol (TFE) of phosphorylated peptides, NF-M17(Ser6P) and NF-M17(Ser11P) representing the C-terminal repeating domain of the midsized neurofilament protein subunit (NF-M) have been measured. In the absence of Ca2+ ions both phosphopeptides adopt predominantly aperiodic conformation with minor amounts of beta-turns, 3(10) helix or beta-pleated sheet. Addition of Ca(ClO4)2 results in opaque solutions, and in the case of the Ser6P peptide, precipitation. The infrared spectra of the supernatants reflect the presence of unordered and beta-sheet structure. The infrared spectrum of the solid Ca(2+)-complex of NF-M17 (Ser6P) in a KBr pellet shows amide component bands at 1654 (alpha-helix or loops) and 1626.5 cm-1 (beta-sheet). The high intensity of the beta-sheet component suggests extensive beta-aggregation. The data reported herein give an infrared spectroscopic support to our previous findings that upon Ca2+ binding, phosphorylated NF-M fragments undergo a marked conformational change which gives rise to partial beta-sheet formation and beta-aggregation. This observation may have relevance to the molecular events which lead to the accumulation of abnormal proteineous structures in Alzheimer's disease.

In vivo phosphorylation sites in fetal and adult rat tau

Fetal tau and tau in paired helical filaments show similar immunoreactivities with several phosphorylation-dependent paired helical filament-polyclonals and monoclonals, suggesting that the two molecules share several distinct phosphorylated epitopes. To make clear the similarities and differences between the two, we have undertaken work to identify the in vivo phosphorylation sites in fetal rat tau. We have approached this problem by identifying phosphopeptides by means of mass spectrometry and sequencing of those phosphopeptides after modification with ethanethiol. Although remarkable heterogeneity was present, fetal tau was found to bear at most 10 phosphates at Ser-189, Ser-190, Ser-193, Ser-226, Ser-387, Ser-395, Thr-172, Thr-222, and, presumably, Ser-391 and Thr-208 (numbering is according to the longest form of rat tau; Kosik, K. S., Orecchio, L. D., Bakalis, S., and Neve, R. L. (1989) Neuron 2, 1389-1397). In contrast, adult rat tau was much less phosphorylated; only Thr-172, Ser-190, Ser-193, Thr-222, and Ser-395 were phosphorylated to a slight-to-moderate extent. All these sites except for Ser-189 and Ser-391 were followed by Pro residues. Thus, tau is an in vivo substrate for proline-directed protein kinase(s), and its phosphorylation state is developmentally regulated.

Identification of the major regulatory phosphorylation site in sucrose-phosphate synthase

Sucrose-phosphate synthase (SPS; EC 2.4.1.14) is regulated in part by reversible protein phosphorylation. When dephospho-SPS is partially purified from illuminated spinach leaves and incubated with [gamma-32P]ATP the enzyme is phosphorylated by a copurifying protein kinase. In this report, 32P-phosphopeptides from tryptic digests of in vitro phosphorylated SPS were purified by metal-ion affinity chromatography and reversed-phase high-performance liquid chromatography. Three distinct 32P-phosphopeptides were resolved. Edman sequencing of the major phosphopeptide (which contained > 80% of the total 32P) identified the amino acid sequence as Ile-Ser-Ser(P)-Val-Glu-Met-Met-Asp-Asn-Trp-Ala-Asn-Thr-Phe-Lys. This sequence corresponds to residues 156 to 170 of the deduced amino acid sequence of spinach SPS [Klein, R. R., Crafts-Brandner, S. J., and Salvucci, M. E. (1993) Planta 190, 498-510, and Sonnewald, U., Quick, W. P., MacRae, E., Krause, K.-P., and Stitt, M. (1993) Planta 189, 174-181]. Identification of the phosphoseryl residue was accomplished by manual Edman sequencing. The two other phosphopeptides, which each contained less than 10% of the total 32P, were not sequenced. An Escherichia coli expressed, 26-kDa fragment of SPS which contains the major phosphorylation site was a substrate for the protein kinase which copurifies with SPS. Two-dimensional peptide mapping analysis of this fragment showed the major phosphopeptide was present but not the other site(s), suggesting that other peptides are derived from a site other than Ser158. These results provide additional indirect evidence for the presence of multiple phosphorylation sites in SPS.

Localization of the approximately 12 kDa M(r) discrepancy in gel migration of the mouse glucocorticoid receptor to the major phosphorylated cyanogen bromide fragment in the transactivating domain

The intact wild-type mouse glucocorticoid receptor has a theoretical molecular weight of approximately 96 kDa based on amino acid sequence, but on SDS-polyacrylamide gel electrophoresis it migrates as a protein of approximately 98 kDa. It is not known where the unusual primary structure or covalent modification responsible for this anomalous migration is located within the amino acid chain. In the course of examining the pattern of fragmentation of 32P-labeled glucocorticoid receptors from Chinese hamster ovary (CHO) cells containing amplified mouse receptor cDNA, we have found a localized region in the amino-terminal half of the receptor that accounts for this anomalous behavior. Cyanogen bromide treatment of the intact receptor produces a 23.4 kDa (theoretical) fragment consisting of residues 108-324 and containing all of the identified phosphorylated serines within the receptor. We find that the only large resolvable 32P-labeled receptor fragment produced after complete cyanogen bromide cleavage of intact receptors migrates with an apparent molecular weight of approximately 35 kDa. Because the apparent difference between the theoretical and the experimentally observed molecular weights of this cyanogen bromide fragment is essentially the same as the difference between the theoretical and experimental molecular weights of the intact mouse glucocorticoid receptor, we propose that some feature lying within this fragment accounts for slower migration. Although the existence of an additional phosphorylation site lying within the 15 kDa tryptic receptor fragment containing the DNA-binding domain has been contested, we also demonstrate that this fragment of the mouse glucocorticoid receptor is phosphorylated in vivo upon incubation of CHO cells in growth medium containing [32P]orthophosphate.

Sequence specificity in recognition of the epidermal growth factor receptor by protein tyrosine phosphatase 1B

Protein tyrosine phosphatases all contain a conserved cysteine that forms an intermediate thiophosphate ester bond during tyrosine phosphate hydrolysis. A bacterial glutathione S-transferase fusion protein containing rat brain phosphatase PTP1b was constructed in which this conserved cysteine was mutated to serine. The resulting catalytically inactive enzyme was labeled in vivo to high specific activity with 35S, and the binding of this labeled fusion protein to the immunoprecipitated epidermal growth factor (EGF) receptor was evaluated. The binding was ligand-dependent, and saturation analysis revealed a nonlinear Scatchard plot, with a Kd for high affinity binding of approximately 100 nM. A number of glutathione S-transferase fusion proteins containing src homology 2 (SH2) domains attenuated phosphatase binding in a concentration-dependent manner. Phospholipase C (PLC) gamma and the GTPase-activating protein of ras were the most potent inhibitors. Tyrosine-phosphorylated EGF receptor peptide fragments were evaluated for specific inhibition of PTP1b and PLC gamma SH2 binding to the activated receptor. One such peptide, modeled on EGF receptor tyrosine 992, blocked the binding of both fusion proteins. Another phosphopeptide, modeled on tyrosine 1148, inhibited the binding of PTP1b but not the PLC gamma fusion protein. This site specificity was confirmed by analysis of equilibrium binding of the fusion proteins to EGF receptors mutated in each of these phosphorylation sites. The results revealed clear sequence specificity in the binding of proteins involved in the regulation of intracellular signaling by receptor tyrosine kinases.

Preparation and functional characterization of a catalytically active fragment of phosphorylase kinase

Limited proteolysis of rabbit muscle phosphorylase kinase catalyzed by chymotrypsin generates a 33 kD product whose kinase activity is independent of both calcium and pH over the range of 6.8 to 8.3 (Malencik, D.A. & Fischer, E.H. Calcium and Cell Function III: 161-188, 1982). This active preparation consists of three related species containing residues 1-290, 1-296, and 1-298 of the 44.7 kD gamma-subunit of phosphorylase kinase (Harris, W.R., Malencik, D.A., Johnson, C.M., Carr, S.A., Roberts, G.D., Byles, C.E., Anderson, S.R., Heilmeyer, L.M.G., Fischer, E.H. & Crabb, J.W.J. Biol. Chem. 265:11740-11745, 1991). Good recoveries of catalytic activity--with varying degrees of calcium dependence--result upon the digestion of phosphorylase kinase with assorted proteases. However, especially high yields of the chymotryptic fragment are obtainable, with purification on an Ultrogel-34 column and a DEAE Sepharose CL-6B column giving 23% of the maximum possible protein. Physical characterization shows that the 33 kD chymotryptic fragment is globular, with S20,w = 2.9S, and that it has an isoelectric point of 5.3. Our continuous catalytic assay, based on differences in the binding of the fluorescent dye 1-anilinonaphthalene-8-sulfonate by phosphorylase a and b, shows that, on a molar basis, the activity of the fragment is 2.8 fold greater than that of phosphorylase kinase (Malencik, D.A., Zhao, Z. and Anderson, S.R. Biochem. Biophys. Res. Comm. 174: 344-350, 1991). The active fragment also undergoes autophosphorylation. Incubation with Mg[gamma-P32] ATP results in the reaction of 0.7 mol 32P/mol fragment. When the catalytic subunit of the cAMP-dependent protein kinase is also present, the amount of 32P incorporated increases to 1.1 mol/mol. In the former case, phosphorylation occurs primarily at Ser30 while in the latter an additional reaction takes place at Ser81. The phosphopeptides correspond to sequences occurring in the gamma-subunit of phosphorylase kinase.

Serine phosphorylation of protein tyrosine phosphatase (PTP1B) in HeLa cells in response to analogues of cAMP or diacylglycerol plus okadaic acid

The major intracellular protein tyrosine phosphatase (PTP1B) is a 50kDa protein, localized to the endoplasmic reticulum. This PTP is recovered in the particulate fraction of mammalian cells and can be solubilized as a complex of 150 kDa by extraction with non-ionic detergents. Previous work from this laboratory implicated phosphorylation of serine/threonine residues in the regulation of this PTP. Activity was several-fold higher in cells treated with activators of cAMP-dependent or Ca2+/phospholipid-dependent protein kinases or inhibitors of protein phosphatase 2A. Here we show that these treatments result in more than an 8-fold increase in the phosphorylation of the 50 kDa PTP catalytic subunit within the 150kDa form of the phosphatase in HeLa cells. The phosphorylation occurred exclusively on serine residues, and the same tryptic and cyanogen bromide 32P-phosphopeptides were recovered in the PTP from control and stimulated cells. Either multiple kinases phosphorylate a common site in the PTP1B, or a single kinase is activated 'downstream' of cAMP- and Ca2+/phospholipid-dependent kinases. The results indicate that phosphorylation of a serine residue in the segment 283-364, probably serine 352 in the sequence Lys-Gly-Ser-Pro-Leu, occurs in response to cell stimulation. Phosphorylation in this region of PTP1B, between the N-terminal catalytic domain and the C-terminal membrane localization segment, is proposed to regulate phosphatase activity.

The synthesis and use of pp60src-related peptides and phosphopeptides as substrates for enzymatic phosphorylation studies

A series of peptides and phosphopeptides corresponding to the auto-phosphorylation site of pp60src, -Asn-Glu-Tyr416-Thr-Ala-, were prepared by either Boc/solution or Fmoc/solid phase peptide synthesis and used as substrates to study their enzymatic phosphorylation by various casein kinases. The Tyr(P)-containing peptide, Asn-Glu-Tyr(P)-Thr-Ala, was prepared by the use of Fmoc-Tyr(PO3Bzl2)-OH in Fmoc/solid phase peptide synthesis followed by acidolytic treatment of the peptide-resin with 5% anisole/CF3CO2H. Both Asn-Glu-Tyr-Thr-Ala and Asn-Glu-Ser(P)-Thr-Ala were prepared by the Boc/solution phase peptide synthesis and employed hydrogenolytic deprotection of the protected peptides. Enzymatic phosphorylation studies established that (A) the Tyr residue acted as an unusual positive determinant for directing phosphorylation to the Thr-residue, (B) the rate of Thr-phosphorylation was markedly facilitated by a change from the Tyr-residue to the Tyr(P)-residue, and (C) a Ser(P)-residue was as effective as the Tyr(P)-residue in facilitating Thr-phosphorylation. A subsequent structure-function study using Asn-Glu-Phe-Thr-Ala, Asn-Glu-Tyr(Me)-Thr-Ala (prepared by Fmoc/solid phase peptide synthesis) and Asn-Glu-Cha-Thr-Ala (prepared by hydrogenation of Asn-Glu-Tyr-Thr-Ala) established that the rate of Thr-phosphorylation was influenced by the extent of hydrophobic-hydrophobic interactions by the aralkyl side-chain group (either aromatic or aliphatic) of the 416-residue with casein kinase-2; the rate of Thr-phosphorylation being decreased by the introduction of methyl or hydroxyl groups at the 4-position of the aromatic group (i.e. Tyr(Me) and Tyr respectively) but enhanced by the introduction of the hydrophilic phosphate group (i.e. as Tyr(P)).

Protein kinase FA/GSK-3 phosphorylates tau on Ser235-Pro and Ser404-Pro that are abnormally phosphorylated in Alzheimer's disease brain

Previously, we identified protein kinase FA/glycogen synthase kinase-3 (GSK-3) as a microtubule-associated protein tau kinase that can incorporate 4 mol of phosphates into 1 mol of tau protein and cause its electrophoretic mobility shift in sodium dodecyl sulfate gels, a unique property characteristic of paired helical filament-associated pathological tau (PHF-tau) in Alzheimer's disease brains. In this report, we identified TPPKS(p)PSAAK and SPVVSGDTS(p)PR as two phosphorylation site sequences phosphorylated by kinase FA/GSK-3 in tau using peptide sequence analysis and sequential manual Edman degradation for radiosequencing. When mapping with human brain tau sequence, we further identified Ser235-Pro and Ser404-Pro as the two major phosphorylation sites according to the numbering of the longest tau isoform. Ser235 and Ser404 have been reported as two of the major abnormal phosphorylation sites in PHF-tau. Taken together, the results provide initial evidence that protein kinase FA/GSK-3 may represent one of the Ser-Pro motif-directed tau kinases involved in the abnormal phosphorylation of pathological PHF-tau in Alzheimer's disease brain.

The phosphorylation of stathmin by MAP kinase

Stathmin, a ubiquitous cytosolic phosphoprotein which may play a role in integrating the effects of diverse signals regulating proliferation, differentiation and other cell functions, was found to be phosphorylated rapidly and stoichiometrically by mitogen-activated protein (MAP) kinase in vitro. Ser-25 was identified as the major site and Ser-38 as a minor site of phosphorylation, while the p42 and p44 isoforms of MAP kinase were the only significant stathmin kinases detected in PC12 cells after stimulation by nerve growth factor (NGF). The results suggest that MAP kinases are the enzymes responsible for increasing the level of phosphorylation of Ser-25, which has been observed previously in PC12 cells following stimulation by NGF.

Phosphorylation of a 62 kd porcine alpha-internexin, a newly identified intermediate filament protein

A 62 kd protein was purified from the Triton-insoluble fraction of porcine brain white matter. This protein formed 10nm filaments, in vitro. The phosphorylation of the 62 kd protein by cAMP-dependent protein kinase caused electrophoretic mobility to shift to 66 kd on SDS-PAGE and a complete loss of the filament forming ability ensued. Amino acid sequences of four peptide fragments obtained from the 62 kd protein by lysylendopeptidase were identical with that of a 66 kd rat brain alpha-internexin. Amino acid analyses of the phosphopeptide fragment derived from phosphorylated porcine alpha-internexin revealed that the phosphorylation sites by cAMP-dependent protein kinase located in the amino-terminal head domain of this protein. These results strongly suggest that alpha-internexin polymerizes into 10nm filaments in vitro and that phosphorylation of the amino-terminal domain of alpha-internexin controls its polymerizability.