Solid-phase synthesis of a range of O-phosphorylated peptides by post-assembly phosphitylation and oxidation

One-pot chemical pyro- and tri-phosphorylation of peptides by using diamidophosphate in water

Protein (pyro)phosphorylation is emerging as a post-translational modification (PTM) in signalling pathways involved in many cellular processes. However, access to synthetic pyrophosphopeptides that can serve as tools for understanding protein pyrophosphorylation is quite limited. Herein, we report a chemical phosphorylation method that enables the synthesis of pyrophosphopeptides in aqueous medium without the need for protecting groups. The strategy employs diamidophosphate (DAP) in a one-pot sequential phosphorylation-hydrolysis of mono-phosphorylated peptide precursors. This operationally simple method exploits the intrinsic nucleophilicity of a phosphate moiety installed on serine, threonine or tyrosine residues in complex peptides with excellent chemoselectivity and good yields under mild conditions. We demonstrate the installation of the pyrophosphate group within a wide range of model peptides and showcase the potential of this methodology by selectively pyrophosphorylating the highly functionalized Nopp140 peptide fragment. The potential to produce higher (poly)phosphorylated peptides was demonstrated as a proof-of-principle experiment where we synthesized the triphosphorylated peptides using this one-pot strategy.

Dissecting the role of protein phosphorylation: a chemical biology toolbox

Protein phosphorylation is a crucial regulator of protein and cellular function, yet, despite identifying an enormous number of phosphorylation sites, the role of most is still unclear. Each phosphoform, the particular combination of phosphorylations, of a protein has distinct and diverse biological consequences. Aberrant phosphorylation is implicated in the development of many diseases. To investigate their function, access to defined protein phosphoforms is essential. Materials obtained from cells often are complex mixtures. Recombinant methods can provide access to defined phosphoforms if site-specifically acting kinases are known, but the methods fail to provide homogenous material when several amino acid side chains compete for phosphorylation. Chemical and chemoenzymatic synthesis has provided an invaluable toolbox to enable access to previously unreachable phosphoforms of proteins. In this review, we selected important tools that enable access to homogeneously phosphorylated protein and discuss examples that demonstrate how they can be applied. Firstly, we discuss the synthesis of phosphopeptides and proteins through chemical and enzymatic means and their advantages and limitations. Secondly, we showcase illustrative examples that applied these tools to answer biological questions pertaining to proteins involved in signal transduction, control of transcription, neurodegenerative diseases and aggregation, apoptosis and autophagy, and transmembrane proteins. We discuss the opportunities and challenges in the field.

The role of phosphopeptides in the mineralisation of silica

We describe the synthesis of hyperphosphorylated peptides and the investigation of theirin vitrosilicification activity in combination with long-chain polyamines (LCPA) at high dilution and mildly acidic conditions.

Multiphosphorylated peptides: importance, synthetic strategies, and applications for studying biological mechanisms

Advances in the synthesis of multiphosphorylated peptides and peptide libraries: tools for studying the effects of phosphorylation patterns on protein function and regulation.

Strong Uranium(VI) Binding onto Bovine Milk Proteins, Selected Protein Sequences, and Model Peptides

Hexavalent uranium is ubiquitous in the environment. In view of the chemical and radiochemical toxicity of uranium(VI), a good knowledge of its possible interactions in the environment is crucial. The aim of this work was to identify typical binding and sorption characteristics of uranium(VI) with both the pure bovine milk protein β-casein and diverse related protein mixtures (caseins, whey proteins). For comparison, selected model peptides representing the amino acid sequence 13-16 of β-casein and dephosphorylated β-casein were also studied. Complexation studies using potentiometric titration and time-resolved laser-induced fluorescence spectroscopy revealed that the phosphoryl-containing proteins form uranium(VI) complexes of higher stability than the structure-analog phosphoryl-free proteins. That is in agreement with the sorption experiments showing a significantly higher affinity of caseins toward uranium(VI) in comparison to whey proteins. On the other hand, the total sorption capacity of caseins is lower than that of whey proteins. The discussed binding behavior of milk proteins to uranium(VI) might open up interesting perspectives for sustainable techniques of uranium(VI) removal from aqueous solutions. This was further demonstrated by batch experiments on the removal of uranium(VI) from mineral water samples.

Advances in Fmoc solid-phase peptide synthesis

Today, Fmoc SPPS is the method of choice for peptide synthesis. Very-high-quality Fmoc building blocks are available at low cost because of the economies of scale arising from current multiton production of therapeutic peptides by Fmoc SPPS. Many modified derivatives are commercially available as Fmoc building blocks, making synthetic access to a broad range of peptide derivatives straightforward. The number of synthetic peptides entering clinical trials has grown continuously over the last decade, and recent advances in the Fmoc SPPS technology are a response to the growing demand from medicinal chemistry and pharmacology. Improvements are being continually reported for peptide quality, synthesis time and novel synthetic targets. Topical peptide research has contributed to a continuous improvement and expansion of Fmoc SPPS applications.

Incorporation of two modified nucleosides allows selective platination of an oligonucleotide making it suitable for duplex cross-linking

Platinated oligonucleotides are promising tools for the control of gene expression, since they may target and cross-link nucleic acid chains. Here we describe a method for the preparation of platinated oligonucleotides that has proved able to selectively cross-link complementary sequences, making use of 5-methylcytidine analogs with thioether or imidazole groups attached to the 4-position. These nucleoside analogs were derivatized as phosphoramidites and introduced in oligonucleotide chains using standard phosphite triester chemistry. Different oligonucleotide sequences containing either one or two analogs appending from the 5'-end were synthesized and used in preliminary platination studies. The reaction of transplatin with oligonucleotides containing the thioether-modified nucleobase was fast, but generally afforded unstable adducts and complex reaction mixtures. The imidazole-containing oligonucleotides reacted with transplatin much more slowly, in particular at slightly basic pH, and it was found that the imidazole-modified cytosine was less reactive than the natural nucleobases. In contrast, transplatin selectively reacted with the thioether and imidazole groups of oligonucleotides containing the two cytosine analogs in neighboring positions, even in the presence of the four nucleobases and particularly three guanines, affording platinated oligonucleotides suitable for cross-linking.

Structural basis for high-affinity peptide inhibition of human Pin1

Human Pin1 is a key regulator of cell-cycle progression and plays growth-promoting roles in human cancers. High-affinity inhibitors of Pin1 may provide a unique opportunity for disrupting oncogenic pathways. Here we report two high-resolution X-ray crystal structures of human Pin1 bound to non-natural peptide inhibitors. The structures of the bound high-affinity peptides identify a type-I beta-turn conformation for Pin1 prolyl peptide isomerase domain-peptide binding and an extensive molecular interface for high-affinity recognition. Moreover, these structures suggest chemical elements that may further improve the affinity and pharmacological properties of future peptide-based Pin inhibitors. Finally, an intramolecular hydrogen bond observed in both peptide complexes mimics the cyclic conformation of FK506 and rapamycin. Both FK506 and rapamycin are clinically important inhibitors of other peptidyl-prolyl cis-trans isomerases. This comparative discovery suggests that a cyclic peptide polyketide bridge, like that found in FK506 and rapamycin or a similar linkage, may significantly improve the binding affinity of structure-based Pin1 inhibitors.

The adapter protein APS associates with the multifunctional docking sites Tyr-568 and Tyr-936 in c-Kit

The adapter protein APS has previously been shown to be involved in recruiting the ubiquitin E3 ligase c-Cbl to the insulin receptor, the platelet-derived growth factor beta-receptor and the erythropoietin receptor, leading to increased degradation of the receptors and inhibition of mitogenesis. Here we demonstrate, by use of immobilized synthetic phosphopeptides corresponding to various autophosphorylated tyrosine residues in the receptor for stem-cell factor (c-Kit), that APS preferentially associates with phosphorylated Tyr-568 and Tyr-936. Tyr-568 has previously been identified as the binding site of the Src family of tyrosine kinases, the Csk-homologous kinase CHK, and the protein tyrosine phosphatase SHP-2. We have recently demonstrated that Tyr-936 is an autophosphorylation site involved in binding the adapter proteins Grb2 and Grb7. We could further demonstrate that the critical determinant for binding of APS is the presence of either a leucine or an isoleucine residue in the position +3 to the phosphorylated tyrosine. This allowed us to design mutants that selectively failed to associate with APS, while still associating with Src family members, SHP-2 and Grb2, respectively.

Synthesis and immunogenic properties of phosphopeptides related to the human insulin receptor

Two phosphoserine tetradecapeptides corresponding to sequences 987-1000 (peptide pSer994) and 1017-1030 (peptide pSer1023/1025) from the human insulin receptor involved in the regulation of its activity were successfully synthesized using Fmoc-based chemistry. Phosphorylation was performed by post-assembly phosphitylation followed by oxidation. The selective phosphorylation of Ser residues was achieved incorporating into the peptide chain the Ser (Trt) derivative and t-Bu blocking groups at sites other than those intended to be phosphorylated. The Trt group was selectively removed with dichloroacetic acid while under this condition t-Bu protecting groups remained unaltered. Following conjugation to keyhole limpet hemocyanin phosphopeptides were used as immunogens to generate sequence-specific phosphoserine antibodies. Peptide pSer994 induced antibodies in New Zealand white rabbits which discriminated between the phosphorylated and nonphosphorylated forms of the peptide, thus representing promising candidates to recognize signaling pathways associated to the regulation of the human insulin receptor.

SHP-2 is involved in heterodimer specific loss of phosphorylation of Tyr771 in the PDGF beta-receptor

We have previously shown that the binding site for GTPase activating protein of Ras (RasGAP) in the PDGF beta-receptor, Tyr771, is phosphorylated to a much lower extent in the heterodimeric configuration of PDGF alpha- and beta-receptors, compared to the PDGF beta-receptor homodimer. The decreased recruitment of the RasGAP to the receptor leads to prolonged activation of the Ras/MAP kinase pathway, which could explain the increase in mitogenicity seen upon induction of heterodimers. The molecular mechanism underlying these differences was investigated. We could show that the loss of phosphorylation of Tyr771 was dependent on presence of intact binding sites for the protein tyrosine phosphatase SHP-2 on the PDGF beta-receptor. Thus, in PDGF receptor mutants in which binding of SHP-2 was lost, a higher degree of phosphorylation of Tyr771 was seen, while other phosphorylation sites in the receptor remained virtually unaffected. Thus, SHP-2 appears to play an important role in modulating phosphorylation of Y771, thereby controlling RasGAP recruitment and Ras/MAP kinase signaling in the heterodimeric configuration of the PDGF receptors.

The H-phosphonate approach to the synthesis of phosphopeptides on solid phase

[reaction: see text]. Ammonium tert-butyl H-phosphonate was used for the phosphorylation of Tyr- and Ser-containing peptides synthesized by an Fmoc strategy. This reaction, leading to a monoprotected peptide phosphate, was found to be highly efficient and generally applicable. Moreover, the method employed avoids undesired side reactions during chain elongation (pyrophosphate formation and beta-elimination catalyzed by piperidine).

The synthesis of phosphopeptides

Phosphorylation and dephosphorylation are key events in protein expression and regulation and in signal transduction. Phosphopeptides are very useful reagents for the study of these processes, and have been used to great advantage in the study of phosphatase substrate specificity, SH2 domain ligand specificity, and protein-protein interactions. Furthermore, the advent of cell-permeable peptide carriers, such as those from the antennapedia homeodomain and the HIV TAT transcription factor, has allowed the study of intracellular events, thus underscoring the utility of these reagents. In this paper we review methods for the synthesis of phosphopeptides with the emphasis on the preparation of phosphoamino acid building blocks.

1H-NMR analysis of CD3-epsilon reveals the presence of turn-helix structures around the ITAM motif in an otherwise random coil cytoplasmic tail

The conformation adopted in solution by the cytoplasmic tail of CD3-epsilon has been analyzed by 1H-nmr. The cytoplasmic tail is mostly random coil expect for the amino acids conforming the immunoreceptor tyrosine-based activation motif (ITAM), YxxL/IxxxxxxxY xxL. Although the N-terminal Y xxL sequence of the motif is poorly folded, adopting 6-residue turn-like conformations with the Tyr side chain in two different orientations, the C-terminal Y xxL sequence is placed in a more complex structure involving a set of nonclassical alpha-helix turns and beta-turns that comprises 11 amino acids. This structure is not modified by phosphorylation of the tyrosine residue. The differences in the conformation adopted around the two tyrosines of the ITAM motif suggest that they may play different roles pertaining to either binding signal transducing proteins or, alternatively, proteins involved in other processes such as endoplasmic reticulum location.

SHP-2 binds to Tyr763 and Tyr1009 in the PDGF beta-receptor and mediates PDGF-induced activation of the Ras/MAP kinase pathway and chemotaxis

Activation of the beta-receptor for platelet-derived growth factor (PDGF) by its ligand leads to autophosphorylation on a number of tyrosine residues. Here we show that Tyr763 in the kinase insert region is a novel autophosphorylation site, which after phosphorylation binds the protein tyrosine phosphatase SHP-2. SHP-2 has also previously been shown to bind to phosphorylated Tyr1009 in the PDGF beta-receptor. Porcine aortic endothelial (PAE) cells transfected with a PDGF beta-receptor in which Tyr763 and Tyr1009 were mutated to phenylalanine residues failed to associate with SHP-2 after ligand stimulation. Moreover, PDGF-BB-induced Ras GTP-loading and Erk2 activation were severely compromised in the receptor mutant. Whereas the mitogenic response to PDGF-BB remained at the same level as in cells expressing wild-type PDGF beta-receptor, chemotaxis induced by PDGF-BB was significantly decreased in the case of the Y763F/Y1009F mutant cells, suggesting an important role for SHP-2 in chemotactic signaling.

Transcription factor AP-2 activity is modulated by protein kinase A-mediated phosphorylation

We recently reported that APOE promoter activity is stimulated by cAMP, this effect being mediated by factor AP-2 [Garcia et al. (1996) J. Neurosci. 16, 7550-7556]. Here, we study whether cAMP-induced phosphorylation modulates the activity of AP-2. Recombinant AP-2 was phosphorylated in vitro by protein kinase A (PKA) at Ser239. Mutation of Ser239 to Ala abolished in vitro phosphorylation of AP-2 by PKA, but not the DNA binding activity of AP-2. Cotransfection studies showed that PKA stimulated the effect of AP-2 on the APOE promoter, but not that of the S239A mutant. Therefore, cAMP may modulate AP-2 activity by PKA-induced phosphorylation of this factor.

Adducin is an in vivo substrate for protein kinase C: phosphorylation in the MARCKS-related domain inhibits activity in promoting spectrin-actin complexes and occurs in many cells, including dendritic spines of neurons

Adducin is a heteromeric protein with subunits containing a COOH-terminal myristoylated alanine-rich C kinase substrate (MARCKS)-related domain that caps and preferentially recruits spectrin to the fast-growing ends of actin filaments. The basic MARCKS-related domain, present in alpha, beta, and gamma adducin subunits, binds calmodulin and contains the major phosphorylation site for protein kinase C (PKC). This report presents the first evidence that phosphorylation of the MARCKS-related domain modifies in vitro and in vivo activities of adducin involving actin and spectrin, and we demonstrate that adducin is a prominent in vivo substrate for PKC or other phorbol 12-myristate 13-acetate (PMA)-activated kinases in multiple cell types, including neurons. PKC phosphorylation of native and recombinant adducin inhibited actin capping measured using pyrene-actin polymerization and abolished activity of adducin in recruiting spectrin to ends and sides of actin filaments. A polyclonal antibody specific to the phosphorylated state of the RTPS-serine, which is the major PKC phosphorylation site in the MARCKS-related domain, was used to evaluate phosphorylation of adducin in cells. Reactivity with phosphoadducin antibody in immunoblots increased twofold in rat hippocampal slices, eight- to ninefold in human embryonal kidney (HEK 293) cells, threefold in MDCK cells, and greater than 10-fold in human erythrocytes after treatments with PMA, but not with forskolin. Thus, the RTPS-serine of adducin is an in vivo phosphorylation site for PKC or other PMA-activated kinases but not for cAMP-dependent protein kinase in a variety of cell types. Physiological consequences of the two PKC phosphorylation sites in the MARCKS-related domain were investigated by stably transfecting MDCK cells with either wild-type or PKC-unphosphorylatable S716A/S726A mutant alpha adducin. The mutant alpha adducin was no longer concentrated at the cell membrane at sites of cell-cell contact, and instead it was distributed as a cytoplasmic punctate pattern. Moreover, the cells expressing the mutant alpha adducin exhibited increased levels of cytoplasmic spectrin, which was colocalized with the mutant alpha adducin in a punctate pattern. Immunofluorescence with the phosphoadducin-specific antibody revealed the RTPS-serine phosphorylation of adducin in postsynaptic areas in the developing rat hippocampus. High levels of the phosphoadducin were detected in the dendritic spines of cultured hippocampal neurons. Spectrin also was a component of dendritic spines, although at distinct sites from the ones containing phosphoadducin. These data demonstrate that adducin is a significant in vivo substrate for PKC or other PMA-activated kinases in a variety of cells, and that phosphorylation of adducin occurs in dendritic spines that are believed to respond to external signals by changes in morphology and reorganization of cytoskeletal structures.

The solid-phase synthesis of side-chain-phosphorylated peptide-4-nitroanilides

Peptide-4-nitroanilides can be quickly synthesised using an Fmoc-based approach on 2-chlorotritylchloride resin. Preformed building blocks Fmoc-Xaa-NH-Np (Xaa = Cit, Cys, Gln, His, Lys, Orn, Ser, Thr, Tyr, Trp) can be attached via side chain to the 2-chlorotritylchloride linker of the resin. N-terminal elongation yields the respective peptide-4-nitroanilides after detachment from the solid support. We synthesised a set of tetrapeptide-4-nitroanilides with the general structure Suc-Ala-Phe-Pro-Xaa-NH-Np (Xaa = Asp, Cit, Cys, Glu, Gln, His, Lys, Orn, Ser, Thr, Tyr, Trp). Even peptidyl-arginine-4-nitroanilides are available by a slightly modified procedure. First, the appropriate ornithine-containing peptide was synthesised. After detachment of the peptide from the resin the side-chain primary amino group was transformed to the guanidino function of arginine using 1-guanyl-3,5-dimethylpyrazole. A further application of this method is the convenient synthesis of phosphorylated peptide-4-nitroanilides. Five phosphopeptides with the general structure Ac-Ala-Xaa(PO3H2)-Pro-Yaa-NH-Np (Xaa = Ser, Thr, Tyr; Yaa = Tyr, Lys) and their nonphosphorylated analogues were prepared. Global phosphorylation was carried out on the resin-bound peptides using dibenzyl-N, N-diisopropyl-phosphoramidite/tetrazole followed by oxidation with tert-butyl hydroperoxide.

Association of coatomer proteins with the beta-receptor for platelet-derived growth factor

The nonreceptor tyrosine kinase Src binds to and is activated by the beta-receptor for platelet-derived growth factor (PDGF). The interaction leads to Src phosphorylation of Tyr934 in the kinase domain of the receptor. In the course of the functional characterization of this phosphorylation, we noticed that components of 136 and 97 kDa bound to a peptide from this region of the receptor in a phosphorylation-independent manner. These components have now been purified and identified as alpha- and beta'-coatomer proteins (COPs), respectively. COPs are a family of proteins involved in the regulation of intracellular vesicle transport. In order to explore the functional significance of the interaction between alpha- and beta'-COP and the PDGF receptor, a receptor mutant was made in which the conserved histidine residue 928 was mutated to an alanine residue. The mutant receptor, which was unable to bind alpha- or beta'-COP, showed a normal ligand-induced autophosphorylation. The mutant receptor also behaved like the wildtype receptor with regard to biosynthesis and maturation, and mediated a mitogenic signal. The possible functional importance of the interaction between the PDGF beta-receptor and alpha- and beta'-COP, is discussed.

Combined Fmoc-Alloc strategy for a general SPPS of phosphoserine peptides; preparation of phosphorylation-dependent tau antisera

A block method for the solid phase synthesis (SPPS) of serine phosphopeptides has been developed using a combination of Fmoc and Alloc strategies. Alloc-Ser[PO(OCH2CH CH2)2] OH2, prepared in a one pot procedure from Alloc-Ser-OH, was introduced at the N-terminus of a sequence prepared by standard Fmoc-SPPS. Global cleavage of the allyl ester based protecting groups, followed by coupling of a tripeptide fragment, led to the tau phosphopeptide, 1. Using tau phosphopeptides a series of phosphorylation state-dependent antisera to human tau protein have been raised. These antisera are valuable tools for studying the tau protein which is found in an abnormal, hyperphosphorylated form in Alzheimer's disease brain.

Synthesis of phospho-urodilatin by combination of global phosphorylation with the segment coupling approach

The chemical synthesis of biologically active phosphorylated urodilatin (CDD/ANP-95-126) was achieved by using a strategy of coupling protected peptide segments in solution. Three protected peptide segments corresponding to urodilatin (1-14) with side chain-unprotected Ser10, (15-24) and (25-32) were prepared manually using Fmoc chemistry on an aminopropyl polystyrene resin with the super acid-labile HMPB linker. For the coupling of segments, the carboxy group of the C-terminal segment (25-32) was converted into the tert-butyl ester by treatment with TBTA. The protected peptide segments were coupled in the presence of EDC/HOOBt or TBTU/HOBt to yield fully protected urodilatin with a free hydroxy function at Ser10. Introduction of the phosphate was performed with Et2NP(OtBu)2 and tetrazole followed by oxidation of the phosphite. Alternatively, a prephosphorylated protected segment (1-14) was used in the segment condensation. Our investigations indicate that both pathways, phosphorylation of protected urodilatin in solution and use of a prephosphorylated building block, are suitable methods to obtain a large phosphopeptide of high purity without formation of H-phosphonates or other by-products.

Solid phase synthesis of pp60src-related phosphopeptides via 'global' phosphorylation and their use as substrates for enzymatic phosphorylation by casein kinase-2

The seven phosphopeptide derivatives based on the native -NEYTA- sequence of the pp60src protein kinase family, Asn-Glu-Tyr(P)-Ser-Ala, Ala-Glu-Tyr(P)-Ser-Ala, Ala-Ser-Tyr(P)-Ser-Ala, Ala-Ser(P)-Tyr-Ser-Ala, Ala-Thr-Tyr(P)-Ser-Ala, Ala-Thr(P)-Tyr-Ser-Ala and Ala-Ser(P)-Tyr(P)-Ser-Ala, were prepared in good yield using the "global' "phosphite-triester' phosphorylation method. The peptide resins were assembled using the Fmoc mode of solid phase peptide synthesis (PyBOP coupling method) with specific Ser-, Thr-, or Tyr-residues incorporated as their side chain free Fmoc-derivatives. The final "global' phosphorylation of the peptide resins was accomplished using di-tert-butyl N, N-diethylphosphoramidite followed by m-chloroperoxybenzoic acid oxidation of the resultant di-t-butyl phosphite triester intermediate. Subsequent resin cleavage and deprotection of the phosphorylated peptide resins was effected by treatment with 5% anisole: TFA and gave the seven phosphopeptides in high yield and purity. The use of the seven synthetic phosphopeptides in enzymatic (casein kinase-2) phosphorylation studies showed that, (A) the change of the target Thr site to Ser resulted in markedly improved phosphorylation of the peptide substrates, (B) that the Tyr(P) residue in the - 1 position was significantly more important than the Ser(P)/Thr(P) residue in the - 2 position for efficient seryl phosphorylation, and (C) that an acidic residue in the - 2 position relative to the target site facilitated phosphorylation of the downstream seryl residue irrespective of the nature of the acidic residue in the -Xxx-Tyr(P)-Ser- and -Xxx-Tyr-Ser- sequences {Xxx = Ser(P), Thr(P), Glu}. In addition to the Tyr(P) residue directing phosphorylation to the +1 position, the good phosphorylation of both ASY(P)SA and ATY(P)SA by casein kinase-2 indicated that the Tyr(P) residue was also able to direct phosphorylation to a Ser/Thr in the - 1 position.

Dynamics of human keratin 18 phosphorylation: polarized distribution of phosphorylated keratins in simple epithelial tissues

Phosphorylation of keratin polypeptides 8 and 18 (K8/18) and other intermediate filament proteins results in their reorganization in vitro and in vivo. In order to study functional aspects of human K18 phosphorylation, we generated and purified a polyclonal antibody (termed 3055) that specifically recognizes a major phosphorylation site (ser52) of human K18 but not dephosphorylated K18 or a ser52-->ala K18 mutant. Pulse-chase experiments followed by immunoprecipitation and peptide mapping of in vivo 32PO4-labeled K8/18 indicated that the overall phosphorylation turnover rate is faster for K18 versus K8, and that ser52 of K18 is a highly dynamic phosphorylation site. Isoelectric focusing of 32PO4 labeled K18 followed by immunoblotting with 3055 showed that the major phosphorylated K18 species contain ser52 phosphorylation but that some K18 molecules exist that are preferentially phosphorylated on K18 sites other than ser52. Immunoblotting of total cell lysates obtained from cells at different stages of the cell cycle showed that ser52 phosphorylation increases three to fourfold during the S and G2/M phases of the cell cycle. Immunofluorescence staining of cells at different stages of mitosis, using 3055 or other antibodies that recognize the total keratin pool, resulted in preferential binding of the 3055 antibody to the reorganized keratin fraction. Staining of human tissues or tissues from transgenic mice that express human K18 showed that the phospho-ser52 K18 species are located preferentially in the basolateral and apical domains in the liver and pancreas, respectively, but no preferential localization was noted in other simple epithelial organs examined. Our results support a model whereby phosphorylated intermediate filaments are localized in specific cellular domains depending on the tissue type and site(s) of phosphorylation. In addition, ser52 of human K18 is a highly dynamic phosphorylation site that undergoes modulation during the S and G2/M phases of the cell cycle in association with filament reorganization.

Solution structure of the C-terminal SH2 domain of the human tyrosine kinase Syk complexed with a phosphotyrosine pentapeptide

BACKGROUND

Recruitment of the intracellular tyrosine kinase Syk to activated immune-response receptors is a critical early step in intracellular signaling. In mast cells, Syk specifically associates with doubly phosphorylated immunoreceptor tyrosine-based activation motifs (ITAMs) that are found within the IgE receptor. The mechanism by which Syk recognizes these motifs is not fully understood. Both Syk SH2 (Src homology 2) domains are required for high-affinity binding to these motifs, but the C-terminal SH2 domain (Syk-C) can function independently and can bind, in isolation, to the tyrosine-phosphorylated IgE receptor in vitro. In order to improve understanding of the cellular function of Syk, we have determined the solution structure of Syk-C complexed with a phosphotyrosine peptide derived from the gamma subunit of the IgE receptor.

RESULTS

The Syk-C:peptide structure is compared with liganded structures of both the SH2 domain of Src and the C-terminal SH2 domain of ZAP-70 (the 70 kDa zeta-associated protein). The topologies of these domains are similar, although significant differences occur in the loop regions. In the Syk-C structure, the phosphotyrosine and leucine residues of the peptide ligand interact with pockets on the protein, and the intervening residues are extended.

CONCLUSIONS

Syk-C resembles other SH2 domains in its peptide-binding interactions and overall topology, a result that is consistent with its ability to function as an independent SH2 domain in vitro. This result suggests that Syk-C plays a unique role in the intact Syk protein. The determinants of the binding affinity and selectivity of Syk-C may reside in the least-conserved structural elements that comprise the phosphotyrosine- and leucine-binding sites. These structural features can be exploited for the design of Syk-selective SH2 antagonists for the treatment of allergic disorders and asthma.

The synthesis and compositional analysis of phosphopeptides

Protein phosphorylation plays a critical role in the regulation of cell growth and differentiation. There is considerable interest, therefore, in the facile synthesis of peptides that possess selectively phosphorylated residues for use as molecular probes in mechanistic studies of the biological consequences of phosphorylation. This work will review the various synthetic protocols used in the generation of phosphopeptides and will discuss their characterization by amino acid compositional analysis.

ApoE3 binding to tau tandem repeat I is abolished by tau serine262 phosphorylation

The risk of Alzheimer's disease is determined, in part, by inheritance of specific alleles of ApoE. Isoform specific interactions of ApoE have been shown with the microtubule-associated protein tau, which forms the neurofibrillary tangle in this disease. Synthetic peptides representing each of the four microtubule-binding domains of tau more avidly bind ApoE3 than ApoE4. Phosphorylation of serine262 in domain I of tau decreases tau binding to microtubules and also abolishes binding by ApoE3. Understanding the molecular mechanisms of the high avidity, isoform-specific interactions of ApoE with tau may help in developing approaches for disease intervention.

Demonstration of functionally different interactions between phospholipase C-gamma and the two types of platelet-derived growth factor receptors

Phosphorylated tyrosine residues in receptor tyrosine kinases serve as binding sites for signal transduction molecules. We have identified two autophosphorylation sites, Tyr-988 and Tyr-1018, in the platelet-derived growth factor (PDGF) alpha-receptor carboxyl-terminal tail, which are involved in binding of phospholipase C-gamma (PLC-gamma). The capacities of the Y988F and Y1018F mutant PDGF alpha-receptors, expressed in porcine aortic endothelial cells, to bind PLC-gamma are 60 and 5% of that of the wild-type receptor, respectively. Phosphorylated but not unphosphorylated peptides containing Tyr-1018 are able to compete with the intact receptor for binding to immobilized PLC-gamma SH2 domains; a phosphorylated Tyr-988 peptide competes 10 times less efficiently. The complex between PLC-gamma and the PDGF alpha-receptor is more stable than that of PLC-gamma and the PDGF beta-receptor. However, PDGF stimulation results in a smaller fraction of tyrosine-phosphorylated PLC-gamma and a smaller accumulation of inositol trisphosphate in cells expressing the alpha-receptor as compared with cells expressing the beta-receptor. We conclude that phosphorylated Tyr-988 and Tyr-1018 in the PDGF alpha-receptor carboxyl-terminal tail bind PLC-gamma, but this association leads to only a relatively low level of tyrosine phosphorylation and activation of PLC-gamma.

Variations in in vivo phosphorylation at the proline-rich domain of the microtubule-associated protein 2 (MAP2) during rat brain development

Microtubule-associated protein 2 (MAP2) is an in vitro substrate for MAP kinase. Part of the phosphorylation occurs at the C-terminal microtubule-binding domain of the molecule which contains a cluster of putative consensus sites for MAP kinase on a proline-rich region. A peptide with the sequence RTPGTPG-TPSY, located at this region of the molecule, is efficiently phosphorylated by MAP kinase in vitro. An antibody (972) raised against this non-phosphorylated peptide has been used to test for in vivo phosphorylation at the proline-rich domain of the MAP2 molecule. The reaction of purified MAP2 with antibody 972 diminishes after in vitro phosphorylation by MAP kinase and is enhanced after in vitro dephosphorylation by alkaline phosphatase. A fraction of brain MAP2 isolated by iron-chelation affinity chromatography appears to be phosphorylated in vivo at the site recognized by antibody 972. There is some variation in the phosphorylation of MAP2 at the proline-rich region throughout rat brain development. MAP2C is more highly phosphorylated in the developing rat brain, whereas high-molecular-mass MAP2 is more extensively phosphorylated in the adult rat brain.

The purification and characterization of a human dual-specific protein tyrosine phosphatase

An expression and purification method was developed to obtain the recombinant human dual-specific protein tyrosine phosphatase (PTPase) VHR in quantities suitable for both kinetic studies and crystallization. Physical characterization of the homogeneous recombinant protein verified the mass to be 20,500 +/- 100 by matrix-assisted laser desorption mass spectrometry, confirmed the anticipated NH2-terminal amino acid sequence and demonstrated that the protein exists as a monomer. Conditions were developed to obtain crystals which were suitable for x-ray structure determination. Using synthetic diphosphorylated peptides corresponding to MAP177-189 (mitogen-activated protein) kinase (DHTG-FLpTEpYVATR), an assay was devised which permitted the determination of the rate constants for dephosphorylation of the diphosphorylated peptide on threonine and tyrosine residues. The diphosphorylated peptides are preferred over the singly phosphorylated on tyrosine by 3-8-fold. The apparent second-order rate constant kcat/Km for dephosphorylation of phosphotyrosine on DHTGFLpTEpYVATR was 32,000 M-1 S-1 while dephosphorylation of phosphothreonine was 14 M-1 S-1 (pH 6). The reaction of DHTGFLpTEpYVATR with VHR is ordered, with rapid dephosphorylation on tyrosine occurring first followed by slow dephosphorylation on threonine. Similar results were obtained with F(NLe)(N-Le)pTPpYVVTR, a peptide corresponding to a MAP kinase-like protein (JNK1(180-189)) which is involved in the stress response signaling pathway.

Interaction of p72syk with the gamma and beta subunits of the high-affinity receptor for immunoglobulin E, Fc epsilon RI

Activation of protein tyrosine kinases is one of the initial events following aggregation of the high-affinity receptor for immunoglobulin E (Fc epsilon RI) on RBL-2H3 cells, a model mast cell line. The protein tyrosine kinase p72syk (Syk), which contains two Src homology 2 (SH2) domains, is activated and associates with phosphorylated Fc epsilon RI subunits after receptor aggregation. In this report, we used Syk SH2 domains, expressed in tandem or individually, as fusion proteins to identify Syk-binding proteins in RBL-2H3 lysates. We show that the tandem Syk SH2 domains selectively associate with tyrosine-phosphorylated forms of the gamma and beta subunits of Fc epsilon RI. The isolated carboxy-proximal SH2 domain exhibited a significantly higher affinity for the Fc epsilon RI subunits than did the amino-proximal domain. When in tandem, the Syk SH2 domains showed enhanced binding to phosphorylated gamma and beta subunits. The conserved tyrosine-based activation motifs contained in the cytoplasmic domains of the gamma and beta subunits, characterized by two YXXL/I sequences in tandem, represent potential high-affinity binding sites for the dual SH2 domains of Syk. Peptide competition studies indicated that Syk exhibits a higher affinity for the phosphorylated tyrosine activation motif of the gamma subunit than for that of the beta subunit. In addition, we show that Syk is the major protein in RBL-2H3 cells that is affinity isolated with phosphorylated peptides corresponding to the phosphorylated gamma subunit motif. These data suggest that Syk associates with the gamma subunit of the high-affinity receptor for immunoglobulin E through an interaction between the tandem SH2 domains of SH2 domains of Syk and the phosphorylated tyrosine activation motif of the gamma subunit and that Syk may be the major signaling protein that binds to Fc epsilon RI tyrosine activation motif of the gamma subunit and that Syk may be the major signaling protein that binds to Dc epsilon tyrosine activation motifs in RBL-2H3 cells.

Peptides as active probes

The use of peptides as probes of peptide binding sites of neuropeptide receptors, and of peptidases and proteases is discussed. The rapidly expanding use of peptide antigens as probes of protein structure and valuable diagnostics and vaccines is described. We also discuss the use of synthetic peptide motifs in studies on the molecular details of protein-protein and protein-nucleic acid interactions. Covalently modified peptides such as phosphopeptides exemplifies the use of synthetic peptides in the study of posttranslational modifications of proteins.

Rhodopsin kinase: studies on the sequence of and the recognition motif for multiphosphorylations

Peptides of 10-12 amino acids in length, which overlapped with the sequence of the last 20 amino acids in the C-terminal tail of rhodopsin, were synthesized and used as substrates for rhodopsin kinase. In all cases the phosphorylation of the peptides was found to be greatly stimulated (> 20-fold) by the presence of light-activated rhodopsin (Rho*). The incorporation of 32P at seven Ser/Thr residues that are the potential sites of phosphorylation was quantified, and the results were analyzed in terms of two parameters. First, a global comparison of phosphorylation at each site was made when the propensity for the modification was found to be in the order: Ser 343 > Ser 338 > Thr 336 > Ser 334, Thr 342 > Thr 335, Thr 340. Second, the peptides were aligned on a hypothetical template with the residue to be phosphorylated occupying the P-position, and the manner in which the nature of the surrounding residues effected the phosphorylation was assessed. It was found that the optimal phosphorylation of the P-site Ser/Thr occurs if it has at least one residue on the amino side and five on the acyl side and also contains a neutral residue, preferably small (A, P, S, T) at the P+4 position.(ABSTRACT TRUNCATED AT 250 WORDS)