Phosphorylation of synthetic peptide analogs of rabbit cardiac troponin inhibitory subunit by the cyclic AMP-dependent protein kinase

Long-range periodic sequence of the cement/silk protein of Stenopsyche marmorata: purification and biochemical characterisation

The long-range periodic amino acid sequence of the bifunctional silk/cement protein from larvae of the caddisfly, Stenopsyche marmorata, is discussed in this study. The protein, named the S. marmorata silk protein (Smsp-1), was first purified to electrophoretic homogeneity. The results of Edman-based sequencing of Smsp-1 tryptic digests were consistent with the amino acid sequence deduced from a cDNA clone of the Smsp-1 gene. All undetected amino acids in the Edman-based sequencing were encoded as Ser, suggesting the presence of O-phospho-Ser. (31)P-NMR and an O-phospho-amino acid analysis successfully showed that the O-phospho-Ser residue occurred in a clustered manner, serving a cement function for Smsp-1. Two patterns of non-phosphorylated repeats, -SLGPYGDPRGDXLGPYGG- (X = V, G or D) and -GVGPYGDGLGPYGG-, were enriched in Smsp-1 compared with the O-phospho-Ser cluster, and have fibre-forming functions.

Docking-based Substrate Recognition by the Catalytic Domain of a Protein Tyrosine Kinase, C-terminal Src Kinase (Csk)

Protein tyrosine kinases are key enzymes of mammalian signal transduction. Substrate specificity is a fundamental property that determines the specificity and fidelity of signaling by protein tyrosine kinases. However, how protein tyrosine kinases recognize the protein substrates is not well understood. C-terminal Src kinase (Csk) specifically phosphorylates Src family kinases on a C-terminal Tyr residue, which down-regulates their activities. We have previously determined that Csk recognizes Src using a substrate-docking site away from the active site. In the current study, we identified the docking determinants in Src recognized by the Csk substrate-docking site and demonstrated an interaction between the docking determinants of Src and the Csk substrate-docking site for this recognition. A similar mechanism was confirmed for Csk recognition of another Src family kinase, Yes. Although both Csk and MAP kinases used docking sites for substrate recognition, their docking sites consisted of different substructures in the catalytic domain. These results helped establish a docking-based substrate recognition mechanism for Csk. This model may provide a framework for understanding substrate recognition and specificity of other protein tyrosine kinases.

Phosphorylation of the catalytic subunit of protein kinase A. Autophosphorylation versus phosphorylation by phosphoinositide-dependent kinase-1

The identification of phosphoinositide-dependent kinase-1 (PDK-1) as an activating kinase for members of the AGC family of kinases has led to its implication as the activating kinase for cAMP-dependent protein kinase. It has been established in vitro that PDK-1 can phosphorylate the catalytic (C) subunit (), but the Escherichia coli-expressed C-subunit undergoes autophosphorylation. To assess which of these mechanisms occurs in mammalian cells, a set of mutations was engineered flanking the site of PDK-1 phosphorylation, Thr-197, on the activation segment of the C-subunit. Two distinct requirements appeared for autophosphorylation and phosphorylation by PDK-1. Autophosphorylation was disrupted by mutations that compromised activity (Thr-201 and Gly-200) or altered substrate recognition (Arg-194). Conversely, only residues peripheral to Thr-197 altered PDK-1 phosphorylation, including a potential hydrophobic PDK-1 binding site at the C terminus. To address the in vivo requirements for phosphorylation, select mutant proteins were transfected into COS-7 cells, and their phosphorylation state was assessed with phospho-specific antibodies. The phosphorylation pattern of these mutant proteins indicates that autophosphorylation is not the maturation mechanism in the eukaryotic cell; instead, a heterologous kinase with properties resembling the in vitro characteristics of PDK-1 is responsible for in vivo phosphorylation of PKA.

An alanine residue in the M3-M4 linker lines the glycine binding pocket of the N-methyl-D-aspartate receptor

While attempting to map a central region in the M3-M4 linker of the N-methyl-D-aspartate receptor NR1 subunit, we found that mutation of a single position, Ala-714, greatly reduced the apparent affinity for glycine. Proximal N-glycosylation localized this region to the extracellular space. Glycine affinities of additional Ala-714 mutations correlated with side chain volume. Substitution of alanine 714 with cysteine did not alter glycine sensitivity, although this mutant was rapidly inhibited by dithionitrobenzoate. Glycine protected the A714C mutant from modification by dithionitrobenzoate, whereas the co-agonist L-glutamate was ineffective. These experiments place Ala-714 in the glycine binding pocket of the N-methyl-D-aspartate receptor, a determination not predicted by previous structural models based on bacterial periplasmic binding protein homology.

Phosphorylation of the R domain by cAMP-dependent protein kinase regulates the CFTR chloride channel

CFTR, the protein associated with cystic fibrosis, is phosphorylated on serine residues in response to cAMP agonists. Serines 660, 737, 795, and 813 were identified as in vivo targets for phosphorylation by protein kinase A. The SPQ fluorescence assay revealed that mutagenesis of any one of these sites did not affect Cl- channel activity. Indeed, concomitant mutagenesis of three of the four sites still resulted in cAMP-responsive Cl- channel activity. However, mutagenesis of all four sites abolished the response. One interpretation of these results is that the CFTR Cl- channel is blocked by the R domain and that phosphorylation on serines by protein kinase A electrostatically repels the domain, allowing passage of Cl-. The four phosphorylation events appear to be degenerate: no one site is essential for channel activity, and, at least in the case of serine 660, phosphorylation at one site alone is sufficient for regulation of Cl- channel activity.

Evidence that Ser-82 is a unique phosphorylation site on vimentin for Ca2(+)-calmodulin-dependent protein kinase II

We identified the sites on vimentin that are phosphorylated by Ca2(+)-calmodulin-dependent protein kinase II (CaM-kinase II). Sequential analysis of the purified phosphopeptides demonstrated that the sites are -Thr-Arg-Thr-Tyr-Ser(PO4)38-Leu-Gly-Ser-Ala- and -Val-Arg-Leu-Leu-Gln-Asp-Ser(PO4)82-Val-Asp-, which are located within the amino-terminal head domain of vimentin. For Ser-82 but not Ser-38, the proposed CaM-kinase II recognition amino acid sequence (Arg-X-X-Ser/Thr) was not found. Studies with a series of synthetic peptide analogs corresponding to Ser-82 and its surrounding amino acid sequence indicate that Asp-84 acts as an essential substrate specificity determinant for the Ser-82 phosphorylation by CaM-kinase II. The CaM-kinase II recognition site may be more extensive than heretofore determined.

The substrate specificity of protein kinases which phosphorylate the alpha subunit of eukaryotic initiation factor 2

The alpha subunit of eukaryotic protein synthesis initiation factor (eIF-2 alpha) is phosphorylated at a single serine residue (Ser51) by two distinct and well-characterized protein kinase, the haem-controlled repressor (HCR) and the double-stranded RNA-activated inhibitor (dsI). The sequence adjacent to Ser51 is rich in basic residues (Ser51-Arg-Arg-Arg-Ile-Arg) suggesting that they may be important in the substrate specificity of the two kinases, as is the case for several other protein kinases. A number of proteins and synthetic peptides containing clusters of basic residues were tested as substrates for HCR and dsI. Both kinases were able to phosphorylate histones and protamines ar multiple sites as judged by two-dimensional mapping of the tryptic phosphopeptides. These data also showed that the specificities of the two kinases were different from one another and from the specificities of two other protein kinases which recognise basic residues, cAMP-dependent protein kinase and protein kinase C. In histones, HCR phosphorylated only serine residues while dsI phosphorylated serine and threonine. Based on phosphoamino acid analyses and gel filtration of tryptic fragments, dsI was capable of phosphorylating both 'sites' in clupeine Y1 and salmine A1, whereas HCR acted only on the N-terminal cluster of serines in these protamines. The specificities of HCR and dsI were further studied using synthetic peptides with differing configurations of basic residues. Both kinases phosphorylated peptides containing C-terminal clusters of arginines on the 'target' serine residue, provided that they were present at positions +3 and/or +4 relative to Ser51. However, peptides containing only N-terminal basic residues were poor and very poor substrates for dsI and HCR, respectively. These findings are consistent with the disposition of basic residues near the phosphorylation site in eIF-2 alpha and show that the specificities of HCR and dsI differ from other protein kinases whose specificities have been studied.

Identification of phosphorylation sites for adenosine 3',5'-cyclic phosphate dependent protein kinase on the voltage-sensitive sodium channel from Electrophorus electricus

The voltage-sensitive sodium channel from the electroplax of Electrophorus electricus is selectively phosphorylated by the catalytic subunit of cyclic-AMP-dependent protein kinase (protein kinase A) but not by protein kinase C. Under identical limiting conditions, the protein was phosphorylated 20% as rapidly as the synthetic model substrate kemptamide. A maximum of 1.7 +/- 0.6 equiv of phosphate is incorporated per mole. Phosphoamino acid analysis revealed labeled phosphoserine and phosphothreonine at a constant ratio of 3.3:1. Seven distinct phosphopeptides were identified among tryptic fragments prepared from radiolabeled, affinity-purified protein and resolved by HPLC. The three most rapidly labeled fragments were further purified and sequenced. Four phosphorylated amino acids were identified deriving from three consensus phosphorylation sites. These were serine 6, serine 7, and threonine 17 from the amino terminus and a residue within 47 amino acids of the carboxyl terminus, apparently serine 1776. The alpha-subunits of brain sodium channels, like the electroplax protein, are readily phosphorylated by protein kinase A. However, these are also phosphorylated by protein kinase C and exhibit a markedly different pattern of incorporation. Each of three brain alpha-subunits displays an approximately 200 amino acid segment between homologous repeat domains I and II, which is missing from the electroplax and skeletal muscle proteins [Noda et al. (1986) Nature (London) 320, 188; Kayano et al. (1988) FEBS Lett. 228, 1878; Trimmer et al. (1989) Neuron 3, 33]. Most of the phosphorylation of the brain proteins occurs on a cluster of consensus phosphorylation sites located in this segment. This contrasts with the pattern of highly active sites on the amino and carboxyl termini of the electroplax protein. The detection of seven labeled tryptic phosphopeptides compared to the maximal labeling stoichiometry of approximately 2 suggests that many of the acceptor sites on the protein may be blocked by endogenous phosphorylation.

Evidence for negative control in protein kinase C substrate specificity

The peptide Leu-Asp-Asp-Ser-Lys-Arg-Val-Ala-Lys-Arg-Lys-Leu-Ile-Glu, which corresponds to sequence 124 to 137 of c-erb-A protein, was synthesized and tested as substrate for protein kinase C (PKC). Although a typical recognition sequence for PKC, consisting of a cluster of basic residues, is found on the C-terminus side of serine, its phosphorylation was totally prevented by the presence of the two acidic residues on the amino-terminus side. Three analogs in which aspartyl residues were successively replaced with alanine were studied and the influence of the acidic side chain in modulating phosphorylation by PKC was thus possible to determine. The results show that the presence of a single aspartyl residue located in positions i-1 or i-2 with respect to the phosphorylable residue can almost totally abolish the positive effect of a highly favorable cluster of basic residues. These observations highlight the role of negative substrate specificity determinants in settling the protein substrate profile of protein kinase C.

Partial sequence of MAP2 in the region of a shared epitope with Alzheimer neurofibrillary tangles

A 3.3-kilobase DNA complementary to human microtubule-associated protein 2 (MAP2) was sequenced by the dideoxy method. The 3' end terminates at an internal EcoRI site before the polyA tail. Due to the arrangement of the cDNA insert in the lambda gt11 vector, the MAP2 fragment is not fused to beta-galactosidase when expressed. The Chou Fasman algorithm for the initial 58 amino acids from the first in-frame methionine predicts an alpha helix. Beyond this point, a series of turns is predicted until amino acid 160. The frequent presence of basic residues in proximity to serines or threonines is consistent with multiple phosphorylation sites. The minimum specificity determinant for Ca2+/calmodulin-dependent kinase is repeated 13 times. The sequence of a region containing a MAP2 epitope that is shared with the Alzheimer neurofibrillary tangle was determined by DNase treatment of the cDNA and antibody selecting the small resultant clones in a lambda gt11 sublibrary. Likewise, a MAP2 epitope that is not shared with the neurofibrillary tangle also has been located. Both epitopes are in the projection portion of the molecule. A bovine MAP2 cyanogen bromide fragment, which contains the epitope shared with the neurofibrillary tangle, is partially insoluble under aqueous conditions, probably due to the aggregation of oppositely charged residues. Thus, rapid cleavage of MAP2 to small peptides is probably necessary in vivo to prevent the aggregation of larger cleavage fragments.

Hydroxyamino acid specificity of smooth muscle myosin light chain kinase

Synthetic peptides corresponding to the phosphorylation site in the myosin regulatory light chain from smooth muscle, Lys-Lys-Arg-Ala-Arg-Ala-Thr-Ser-Asn-Val-Phe-Ala ([Ala14,15]MLC(11-23] and containing a variety of hydroxyamino acid analogs at position 19, were tested as substrates for the smooth muscle myosin light chain kinase. Peptide analogs containing either D-serine or cis-hydroxyproline were not phosphorylated. The corresponding trans-hydroxyproline containing peptide was poorly phosphorylated with a Km of 2.3 microM and a Vmax of 3 X 10(-3) mumol.min-1.mg-1 compared to a Km of 12.5 microM and a Vmax of 1.43 mumol.min-1.mg-1 for the parent peptide. All three hydroxyamino acid analog peptides acted as relatively potent inhibitors of myosin light chain phosphorylation with Ki values in the range 7.5-10 microM, comparable to 7 microM for the parent peptide. Thus the failure of the hydroxyamino acid analog peptides to act as effective substrates was not the result of poor binding to the enzyme. In contrast, the same substitutions made in the peptide substrate for the cAMP-dependent protein kinase resulted in poor inhibitors. It is likely that the hydroxyl group of the substituting amino acids in the myosin light chain peptide analogs is not presented in the correct orientation in the active site for transfer of the phosphate group.

NMR of a synthetic peptide spanning the triphosphate binding site of adenosine 5'-triphosphate in actin

The amino acid residues 114-118 in actin were found to be implicated strongly in the binding of nucleotide, and as would be expected for such an important binding site, they are located in a completely conserved region of the actin sequence. A 19-residue peptide with the actin sequence 106-124 was synthesized in order to span the putative triphosphate binding site. Proton NMR spectra of the actin peptide 114-118 in the presence and absence of ATP indicated that Arg-116 and Lys-118 are particularly involved in binding ATP. A strong binding of ATP to the peptide 106-124 also was measured. Tripolyphosphate bound to the peptide 106-124 somewhat more weakly than ATP. Binding involved residues 115-118 and 121-124, indicating the presence of a reverse turn between these segments. Proton resonances were assigned by using two-dimensional double quantum correlated spectroscopy, one-dimensional spin decoupling techniques, one-dimensional nuclear Overhauser enhancement difference spectroscopy, and pH titration. The alpha CH resonances of Ala-3 and Asn-6 are markedly shifted downfield with respect to values in small unstructured peptides due to their close proximity to the side chains of Pro-4 and Pro-7, respectively. Several other resonances display chemical shifts which are indicative of a structured environment. Assignment of the amide proton resonances in H2O and measurements of the coupling constant 3JHNCH and the chemical shifts of the amide protons reveal that much of the synthetic peptide, particularly the backbone, exhibits a highly structured environment and represents a good model for the triphosphate binding site in actin.

Selective retention of organic phosphate esters and phosphonates on aluminium oxide

Compounds containing the -PO3H2 function, such as monoesters of phosphoric acid and phosphonic acids, specifically bind to aluminium oxide in aqueous solution under experimental conditions where non-phosphorylated compounds are completely desorbed. The bound organic phosphate can be specifically displaced by aqueous solution of inorganic phosphates thus allowing their separation or detection by a technique similar to that of affinity chromatography. The consequences of this finding for phosphate compound biochemistry are discussed.

Molecular basis for substrate specificity of protein kinases and phosphatases

Regulation of various metabolic processes occurs by the phosphorylation/dephosphorylation of enzymes. Both the protein kinases that catalyze the phosphorylations and the protein phosphatases that catalyze the dephosphorylations display relatively broad specificity, reacting with a number of distinct sites in target enzymes. In this way changes in the activity of a particular kinase or phosphatase can cause coordinated and pleiotropic responses. However, the kinases and phosphatases do not exhibit a one-to-one correspondence in their reactions. Residues at different positions may be phosphorylated by a single kinase, yet dephosphorylated by different individual phosphatases. Conversely, sites which are substrates for different individual kinases may be dephosphorylated by a single phosphatase. In exploring the molecular basis for these differences this article shows that whereas kinases react with specific primary structures that often times appear as beta bends, the phosphatases recognize higher order structure, less strictly ruled by amino acid sequence surrounding the phosphorylated site. The differences, seen in the ability of these enzymes to utilize synthetic peptide substrates, might be rationalized in terms of function. Kinases need protruding segments of structure that can be enwrapped to exclude water, thereby minimizing ATP hydrolysis and enhancing phosphotransferase activity. On the other hand phosphatases are hydrolytic enzymes that may operate especially well on protein interfaces. Hydrolytic action often measured with p-nitrophenylphosphate is not necessarily indicative of a protein phosphatase and consideration of the mechanism reveals why this substrate can be misleading.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of substrate specificity of myosin kinase and cyclic AMP-dependent protein kinase

A series of synthetic peptides corresponding to the amino-terminal region of chicken gizzard myosin light chain (Mr 20 000) have been tested for their capacity to act as substrates for the cAMP-dependent protein kinase. The 18-residue peptide, K6AKTTK11 K12R13PQRATS19NVFS , was stoichiometrically phosphorylated on serine-19 by the cAMP-dependent protein kinase. This is the same residue phosphorylated by the myosin light chain kinase. The cAMP-dependent protein kinase phosphorylated this peptide with an apparent Km of 120 microM and Vmax of 0.29 mumol . min .-1 mg-1. The Km is 17-fold higher and the Vmax 10-fold lower than the corresponding values obtained with this peptide as substrate for the myosin light chain kinase. The kinetics of phosphorylation of shortened peptides corresponding to this 18-residue sequence together with those of another related sequence, RPQRAKAKTTKATSNVFS , indicated that the myosin light chain kinase had a relatively stronger dependence on lysine residues, whereas the cAMP-dependent protein kinase depends more on arginine residues. Although both the cAMP-dependent protein kinase and the myosin light chain kinase phosphorylate the same serine in the myosin light chain peptides, these enzymes are influenced by different nearby basic residues.

Synthetic peptide substrates for the membrane tyrosine protein kinase stimulated by epidermal growth factor

The substrate specificity of the epidermal-growth-factor-stimulated tyrosine protein kinase of A431 cell membranes has been studied using a series of synthetic peptide analogs of the sequence around the phosphorylated tyrosine-419 of pp60src. The nine-residue peptide Leu-Ile-Glu-Asp-Ala-Glu-Tyr-Thr-Ala was phosphorylated on tyrosine with an apparent Km of 0.4 mM and a V of 5.7 nmol X min-1 X mg-1. Synthetic peptide tyrosine phosphorylation was stimulated by epidermal growth factor but not by insulin or relaxin. Extension of the nine-residue peptide to include the basic residues, arginine-412, arginine-422 and lysine-423 led to an increased apparent Km. Substitution of glutamic-418 by leucine also increased the apparent Km. In the model peptide Ile-Xaa-Xaa-Ala-Ala-Tyr-Thr-Ala a lower apparent Km was obtained when Xaa was glutamic rather than aspartic acid. Poly(aspartic acid) and poly(glutamic acid) had only weak effects on peptide tyrosine phosphorylation. The results support the concept that acidic residues and not basic residues are important specificity determinants for the epidermal-growth-factor-stimulated tyrosine protein kinase.

Near- and far-ultraviolet circular dichroism of the catalytic subunit of adenosine cyclic 5'-monophosphate dependent protein kinase

The circular dichroism spectrum of the catalytic subunit of cAMP-dependent protein kinase was measured in the far-UV (190-240 nm) and near-UV (250-300 nm) region. Data from the far-UV spectra were processed with the CONTIN program for estimation of globular protein secondary structure [ Provencher , S. W. (1982) CONTIN (Version 2) User's Manual, European Molecular Biology Laboratory, Heidelberg, West Germany]. The composition of the protein determined by this method was 49 +/- 2% alpha-helix, 20 +/- 4% beta-sheet, and 31 +/- 3% remainder. This composition changes when the protein is allowed to bind Kemptide , a synthetic peptide substrate, with more than half of the disordered portion of the protein taking the form of beta-sheet. A certain portion of the alpha-helical structure also appears to move into a beta-sheet form. The near-UV CD spectrum of catalytic subunit shows changes in aromatic amino acid dichroism associated with substrate binding. These changes can be ascribed with a fair degree of certainty to alterations in the orientation of a tyrosine residue at the surface of the protein. These findings are discussed in terms of previous work on induced dichroism in this enzyme with regard to control mechanisms operating at the active site.

In vitro phosphorylation of a synthetic collagen peptide by cyclic AMP-dependent protein kinase

A synthetic tridecapeptide that corresponds closely to amino acid residues 98 to 110 in chick collagen alpha 1(I) contains several determinants of specificity required for recognition and phosphorylation by the cyclic AMP-dependent protein kinase. The peptide Gly-Leu-Hyp-Gly-Nle-Lys-Gly-His-Arg-Gly-Phe-Ser-Gly was predicted to be a substrate for the cyclic AMP-dependent protein kinase because it contained multiple basic amino acids NH2-terminal to a potentially phosphorylatable seryl residue. When tested as a substrate for the enzyme, the peptide was stoichiometrically phosphorylated. Phosphoserine was identified as the only phosphoamino acid in a partial hydrolysate of the phosphorylated peptide. The peptide and several of its analogs were phosphorylated by the cyclic AMP-dependent protein kinase with Km values from 1 to 6 mM and Vmax values from 1 to 3 mumol of phosphate/min/mg of enzyme. Although the Km of the kinase for the collagen peptide was high, these results confirmed the prediction made from knowledge of the substrate specificity of cyclic AMP-dependent protein kinase. The potential for such a phosphorylation reaction to occur in vivo is discussed.

Mechanistic studies of cAMP-dependent protein kinase action

The details of the process by which protein kinase catalyzes phosphoryl group transfers are beginning to be understood. Early work that explored the primary specificity of cAMP-dependent protein kinase action enabled the synthesis of small peptide substrates for the enzyme. Enzyme-peptide interactions seem simpler to understand than protein-protein interactions, so peptide substrates have been used in most protein kinase studies. In most investigations the kinetics for the phosphorylation of small peptides have been interpreted as being consistent with mechanisms which do not invoke phospho-enzyme intermediates (see, for example, Bolen et al.). Protein kinase has been shown to bind two metal ions in the presence of a nucleotide. Using magnetic resonance techniques the binding of these ions has been utilized to elucidate the conformation of nucleotide and peptide substrates or inhibitors when bound in the enzymic active site. Also, two new peptides with the form Leu-Arg-Arg-Ala-Ser-Y-Gly, where Y was either Pro or (N-methyl)Leu, were synthesized and found not to be substrates, within the limits of detection, for protein kinase. The striking lack of affinity that protein kinase has for such peptides which are unlikely to form a beta 3-6 turn has not been reported before. Our results may indicate that this type of turn is a requirement for protein kinase catalyzed phosphorylation or that these peptides lack the ability to form a particular hydrogen bond with the enzyme. Magnetic resonance techniques have indicated that the distance between the phosphorous in the gamma-phosphoryl group of MgATP and the hydroxyl oxygen of serine in the peptide Leu-Arg-Arg-Ala-Ser-Leu-Gly is 5.3 +/- 0.7 A. This, together with certain kinetic evidence, suggests that the mechanism by which protein kinase catalyzes phosphoryl group transfer has considerable dissociative character. Chemical modifications, including one using a peptide-based affinity label, have identified two residues at or near the active site, lysine-72 and cysteine 199. While neither of these groups has been shown to be catalytically essential, similar studies may help to identify groups that are directly involved in the catalytic process. Finally, a spectrophotometric assay for cAMP-dependent protein kinase has been described. Using this assay the preliminary results of an in-depth study of the pH dependence of protein kinase catalyzed phosphoryl group transfer have been obtained. This study shall aid in the identification of active site residues and should contribute to the elucidation of the enzyme's catalytic mechanism.(ABSTRACT TRUNCATED AT 400 WORDS)

The use of synthetic peptides for defining the specificity of typrosine protein kinases

The tyrosine protein kinases are a large family of enzymes that may be involved in regulating cell growth and differentiation. An important property of these enzymes is their substrate specificity. Defining the specificity of these enzymes will contribute to a greater understanding of their biological functions. Synthetic peptides provide a useful means for studying the specificities of protein kinases. The utility of synthetic peptides for specificity studies is exemplified by the results obtained with the cyclic nucleotide-dependent protein kinases. A large number of synthetic peptides have been tested as substrates for these enzymes. There are three important conclusions from this work. First, in terms of primary sequence, the two cyclic nucleotide-dependent protein kinases both require the presence of a pair of basic residues on the N-terminal side of the phosphorylatable residue. Second, recognition of a specific secondary structure in the substrate is an equally important factor in the substrate specificities of these enzymes. Third, the specificities of the two cyclic nucleotide-dependent protein kinases are remarkably similar in terms of both primary and secondary structure recognition. Sequences at the sites of tyrosine phosphorylation often show the presence of numerous acidic residues on the N-terminal side of the tyrosine. This result led to the suggestion that these acidic residues might be important for the recognition of these sites by the tyrosine protein kinases. Specificity studies using synthetic peptides have provided some experimental verification of this concept. In the case of four tyrosine protein kinases, LSTRA cell tyrosine protein kinase, epidermal growth factor receptor kinase, insulin receptor kinase and pp60gag-yes tyrosine protein kinase, the presence of acidic residues on the N-terminal side of the tyrosine in a synthetic peptide was a favorable determinant. In the case of a fifth member of the tyrosine protein kinase family, namely pp60src kinase, the data are less clear that the presence of acidic residues is involved in substrate recognition. This enzyme readily phosphorylated several peptides that did not have acidic residues on the N-terminal side of the tyrosine. Although there is some indication that the presence of acidic residues on the N-terminal side of the tyrosine is a factor in substrate recognition by several of the tyrosine protein kinases, the changes in kinetic parameters with the various peptide substrates were rather small. In addition, some sites phosphorylated in proteins do not have any acidic residues on the N-terminal side of the phosphorylated tyrosine residue.(ABSTRACT TRUNCATED AT 400 WORDS)

Phosphorylation of gastrin-17 by epidermal growth factor-stimulated tyrosine kinase

Tyrosine phosphorylation seems to be a key event in the control of cellular growth. Several viral transforming proteins, including the src protein of Rous sarcoma virus, the p120 protein of Abelson leukaemia virus and the middle T antigen of polyoma virus, are phosphorylated by associated tyrosine kinases. The levels of kinase activity correlate with the transforming efficiency of the virus. The receptors for epidermal growth factor (EGF), platelet-derived growth factor (PDGF) and insulin are also phosphorylated by associated tyrosine kinase activities, which are stimulated by EGF, PDGF and insulin, respectively. The EGF-stimulated kinase and the src protein share similar substrate specificity for tyrosines immediately C-terminal to a sequence of acidic amino acids. Such a sequence is also found adjacent to the phosphotyrosine of middle T antigen, and in the homologous region of the hormone gastrin, adjacent to a tyrosine which is sulphated in approximately half the gastrin isolated from gastric mucosa. Reports that gastrin acts as a growth factor for cells of the gastrointestinal tract suggested that phosphorylation of this tyrosine might be physiologically more relevant than sulphation. We report here that synthetic human gastrin 17 is phosphorylated by the EGF-stimulated tyrosine kinase of A431 cell membranes. The Km values of 53-87 and 223-547 microM obtained in the presence and absence of EGF, respectively, are the lowest reported so far for this enzyme.