Chemical synthesis of phosphorylated peptides of the carboxy-terminal domain of human p53 by a segment condensation method

Unremitting progresses for phosphoprotein synthesis

Phosphorylation, one of the important protein post-translational modifications, is involved in many essential cellular processes. Site-specifical and homogeneous phosphoproteins can be used as probes for elucidating the protein phosphorylation network and as potential therapeutics for interfering their involved biological events. However, the generation of phosphoproteins has been challenging owing to the limitation of chemical synthesis and protein expression systems. Despite the pioneering discoveries in phosphoprotein synthesis, over the past decade, great progresses in this field have also been made to promote the biofunctional exploration of protein phosphorylation largely. Therefore, in this review, we mainly summarize recent advances in phosphoprotein synthesis, which includes five sections: 1) synthesis of the nonhydrolyzable phosphorylated amino acid mimetic building blocks, 2) chemical total and semisynthesis strategy, 3) in-cell and in vitro genetic code expansion strategy, 4) the late-stage modification strategy, 5) nonoxygen phosphoprotein synthesis.

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.

Synthesis of complex phosphopeptides as mimics of p53 functional domains

A complete set of mono-, di- and triphosphorylated peptides comprising amino acids 10-27, the Mdm2 and p300 binding site(s) of p53, with and without a fluorescein label at the N-terminus, was synthesized by step-by-step solid phase synthesis. Fluorescence polarization analysis revealed that phosphorylation at Thr18 decreased binding to recombinant Mdm2 protein compared with the unphosphorylated and the two other single phosphorylated analogues. Unlabelled multiply phosphorylated peptides corresponding to this amino-terminal transactivation domain proved to be powerful tools in analysing the phosphate specificity of existing anti-p53 monoclonal and polyclonal antibodies using direct ELISA. The tetramerization domain of human p53 protein was modelled with a 53 residue-long unlabelled unphosphorylated and Ser315-phosphorylated peptide pair. CD analysis showed similar alpha-helical structures for both peptides and no major difference in the secondary structure could be observed upon phosphorylation. Size-exclusion HPLC indicated that these synthetic oligomerization domain mimics underwent a pH-dependent tetramerization process, but the presence of a phosphate group at Ser315 did not modify the oligomeric state of the 308-360 p53 fragments. Nevertheless, the fluorescein-labelled Ser315 phosphorylated peptide bound to the downstream signalling ligand DNA topoisomerase I protein with slightly higher affinity than did the unphosphorylated analogue.

Synthesis and biological properties of amino acid amide ligand-based pyridinioalkanoyl thioesters as anti-HIV agents

Hyper-mutable retroviruses such as HIV can become rapidly resistant to drugs used to treat infection. Strategies for coping with drug-resistant strains of virus include combination therapies, using viral protease and reverse transcriptase inhibitors. Another approach is the development of antiviral agents that attack mutationally nonpermissive targets that have functions essential for viral replication. Thus, the highly conserved nucleocapsid protein, NCp7, was chosen as a prime target in our search for novel anti-HIV agents that can overcome the problem of viral drug resistance. Recently, we reported (J. Med. Chem. 1999, 42, 67) a novel chemotype, the pyridinioalkanoyl thioesters (PATEs), based on 2-mercaptobenzamides as the thiol component and having its amide nitrogen substituted with various phenylsulfonyl moieties. These compounds were identified as relatively nontoxic anti-HIV agents in the XTT cytoprotection assay. In this study, we wish to report a separate genre of active PATEs wherein the thiol component consists of an N-2-mercaptobenzoyl-amino acid derivative. Active derivatives (EC(50) < 10 microM) reported herein were confined to amino acid primary amides or methyl amides having side chains no larger than isobutyl. Amino acids terminating in free carboxyl or carboxylic acid ester groups were mostly inactive. Selected compounds were shown to be active on chronically infected CEM/SK-1, TNFalpha-induced U1, ACH-2 cells and virucidal on cell-free virus, latently infected U1 cells and acutely infected primary peripheral blood mononuclear cells (PBMCs).

Inactivation of HIV-1 nucleocapsid protein P7 by pyridinioalkanoyl thioesters. Characterization of reaction products and proposed mechanism of action

The synthesis and antiviral properties of pyridinioalkanoyl thioester (PATE) compounds that target nucleocapsid p7 protein (NCp7) of the human immunodeficiency virus type 1 (HIV-1) have been described previously (Turpin, J. A., Song, Y., Inman, J. K., Huang, M., Wallqvist, A., Maynard, A., Covell, D. G., Rice, W. G., and Appella, E. (1999) J. Med. Chem. 42, 67-86). In the present study, fluorescence and electrospray ionization-mass spectrometry were employed to determine the mechanism of modification of NCp7 by two lead compounds, N-[2-(5-pyridiniovaleroylthio)benzoyl]sulfacetamide bromide and N-[2-(5-pyridiniovaleroylthio)benzoyl]-4-(4-nitrophenylsulfonyl )anili ne bromide (compounds 45 and 47, respectively). Although both compounds exhibit antiviral activity in cell-based assays, we failed to detect appreciable ejection of zinc from NCp7 under conditions in which previously described NCp7-active disulfides readily eject zinc. However, upon "activation" by Ag(+), compound 45 reacted with NCp7 resulting in the zinc ejection from both zinc fingers. The reaction followed a two-step mechanism in which zinc was ejected from the carboxyl-terminal zinc finger faster than from the amino-terminal zinc finger. Both compounds covalently modified the protein with pyridinioalkanoyl groups. Compound 45 modified cysteines 36 and 49 of the carboxyl-terminal zinc finger. The results obtained herein demonstrate that PATE compounds can be constructed that selectively target only one of the two zinc fingers of NCp7, thus providing an impetus to pursue development of highly selective zinc finger inhibitors.

Chemical synthesis, antibacterial activity and conformation of diptericin, an 82-mer peptide originally isolated from insects

The small amounts of antibacterial peptides that can be isolated from insects do not allow detailed studies of their range of activity, side-chain sugar requirements, or their conformation, factors that frequently play roles in the mode of action. In this paper, we report the solid-phase step-by-step synthesis of diptericin, an 82-mer peptide, originally isolated from Phormia terranovae. The unglycosylated peptide was purified to homogeneity by conventional reversed-phase high performance liquid chromatography, and its activity spectrum was compared to that of synthetic unglycosylated drosocin, which shares strong sequence homology with diptericin's N-terminal domain. Diptericin appeared to have antibacterial activity for only a limited number of Gram-negative bacteria. Diptericin's submicromolar potency against Escherichia coli strains indicated that, in a manner similar to drosocin, the presence of the carbohydrate side chain is not necessary to kill bacteria. Neither the N-terminal, drosocin-analog fragment, nor the C-terminal, glycine-rich attacin-analog region was active against any of the bacterial strains studied, regardless of whether the Gal-GalNAc disaccharide units were attached. This suggested that the active site of diptericin fell outside the drosocin or attacin homology domains. In addition, the conformation of diptericin did not seem to play a role in the antibacterial activity, as was demonstrated by the complete lack of ordered structure by two-dimensional nuclear magnetic resonance spectroscopy and circular dichroism. Diptericin completely killed bacteria within 1 h, considerably faster than drosocin and the attacins; unlike some other, fast-acting antibacterial peptides, diptericin did not lyse normal mammalian cells. Taken together, these data suggest diptericin does not belong to any known class of antibacterial peptides.

A monoclonal antibody to a multiphosphorylated, conformational epitope at the carboxy-terminus of p53

Mutations of the gene encoding the tumor suppressor protein p53 are the most common molecular alterations of cancer cells found in about half of all human tumors. Mutations which cluster in well-defined hot spots change the structure of the protein thus affecting its ability to bind to DNA. Post-translational modifications, primarily phosphorylation, might also influence how p53 binds to DNA or folds to its active tetrameric form. However, the lack of appropriate biochemical markers to characterize the status of phosphorylation in different cell types and in cells at different stages of tumor progression has prohibited such investigations. To generate a sensitive and phosphorylation-specific monoclonal antibody (mAb), we chemically synthesized the C-terminal 23 amino acid stretch of human p53 in a double-phosphorylated form. The peptide 371-393, carrying phosphate groups on Ser378 and Ser392, was co-synthesized with a turn-inducing spacer and peptide 31D, an immunodominant T-helper cell epitope in mice of the H-2k haplotype. After immunization and fusion of splenocytes with myeloma cells, a number of mAbs were obtained, from which mAb p53-18 emerged as a highly sensitive reagent. By enzyme-linked immunosorbent assay, p53-18, a mAb of the IgM isotype, recognized phosphorylated p53, expressed in insect cells infected with a recombinant baculovirus but not p53 expressed in Escherichia coli. Moreover, murine p53 from insect cells could be immune purified with mAb p53-18. Mass spectrometry following tryptic digestion of the purified protein and liquid chromatography of the fragments verified the presence of phosphate groups at both Ser375 and Ser389. From the corresponding human protein fragments, mAb p53-18 bound to the immunizing peptide phosphorylated on Ser378 and on Ser392, but failed to cross-react with the unphosphorylated peptide, or peptides phosphorylated individually on either Ser378 or Ser392. The binding to the unphosphorylated peptide could be restored, however, if the peptide conformation was stabilized to that of an alpha-helix. The immunogenic nature of the multiphosphorylated C-terminus of p53 is indicated by the finding that human sera, mostly from cancer patients, preferentially recognized the double-phosphorylated peptide over the monophosphorylated or unphosphorylated analogs. Antibody p53-18 appears to be a highly useful biochemical marker to detect low levels of p53 protein in different tissues, and to be a key tool to characterize the phosphorylation status of the C-terminus of p53 protein originated from various sources.

DNA damage activates p53 through a phosphorylation-acetylation cascade

Activation of p53-mediated transcription is a critical cellular response to DNA damage. p53 stability and site-specific DNA-binding activity and, therefore, transcriptional activity, are modulated by post-translational modifications including phosphorylation and acetylation. Here we show that p53 is acetylated in vitro at separate sites by two different histone acetyltransferases (HATs), the coactivators p300 and PCAF. p300 acetylates Lys-382 in the carboxy-terminal region of p53, whereas PCAF acetylates Lys-320 in the nuclear localization signal. Acetylations at either site enhance sequence-specific DNA binding. Using a polyclonal antisera specific for p53 that is phosphorylated or acetylated at specific residues, we show that Lys-382 of human p53 becomes acetylated and Ser-33 and Ser-37 become phosphorylated in vivo after exposing cells to UV light or ionizing radiation. In vitro, amino-terminal p53 peptides phosphorylated at Ser-33 and/or at Ser-37 differentially inhibited p53 acetylation by each HAT. These results suggest that DNA damage enhances p53 activity as a transcription factor in part through carboxy-terminal acetylation that, in turn, is directed by amino-terminal phosphorylation.

Effect of phosphorylation on tetramerization of the tumor suppressor protein p53

Human tumor suppressor protein p53 is a 393-amino acid phosphoprotein that enhances transcription in response to DNA damage from several genes that regulate cell cycle progression. The tetrameric state of p53 is critical to wild-type function; the p53 tetramerization element is located in the C-terminal region of the protein. This region is phosphorylated at several evolutionarily conserved serines, suggesting that phosphorylation may be an important regulator of p53 function. In order to determine the effect of phosphorylation on tetramer formation, we synthesized phosphopeptides corresponding to p53(Ser303-Asp393) with phosphate incorporated at Ser315, Ser378, or Ser392, and at both Ser315 and Ser392. Equilibrium ultracentrifugation analysis showed that phosphorylation at Ser392 increased the association constant for tetramer formation nearly ten-fold. By itself, phosphorylation at Ser315 or Ser378 had little effect on tetramer formation, but Ser315 largely reversed the effect of phosphorylation at Ser392. Analysis by calorimetry suggests that phosphorylation may influence subunit affinity by an enthalpy driven process.