Synthesis and characterization of zinc di(O-2,2-dimethylpentan-3-yl dithiocarbonates) bearing pyridine or tetramethylethylenediamine coligands and investigation of their thermal conversion mechanisms towards nanocrystalline zinc sulfide

Metal xanthates are versatile single source precursors for the preparation of various metal sulfides. In this study, we present the synthesis of the two novel zinc xanthate complexes bis(O-2,2-dimethylpentan-3-yl-dithiocarbonato)(N,N,N',N'-tetramethylethylenediamine)zinc(ii) and bis(O-2,2-dimethylpentan-3-yl-dithiocarbonato)(pyridine)zinc(ii). A thorough investigation of these compounds revealed distinct differences in their structural and thermal properties. While in the complex containing the chelating tetramethylethylenediamine, the xanthate groups coordinate in a monodentate way, they are bidentally coordinated to the zinc atom in the pyridine containing complex. Both compounds show a two-step thermal decomposition with an onset temperature of 151 °C and 156 °C for the tetramethylethylenediamine and pyridine containing complex, respectively. Moreover, different mechanisms are revealed for the two phases of the decomposition based on high resolution mass spectrometry investigations. By the thermal conversion process nanocrystalline zinc sulfide is produced and the coligand significantly influences its primary crystallite size, which is 4.4 nm using the tetramethylethylenediamine and 11.4 nm using the pyridine containing complex for samples prepared at a temperature of 400 °C.

Vascular endothelial growth factor (VEGF) receptor II-derived peptides inhibit VEGF

Vascular endothelial growth factor (VEGF) directly stimulates endothelial cell proliferation and migration via tyrosine kinase receptors of the split kinase domain family. It mediates vascular growth and angiogenesis in the embryo but also in the adult in a variety of physiological and pathological conditions. The potential binding site of VEGF with its receptor was identified using cellulose-bound overlapping peptides of the extracytosolic part of the human vascular endothelial growth factor receptor II (VEGFR II). Thus, a peptide originating from the third globular domain of the VEGFR II comprising residues 247RTELNVGIDFNWEYP261 was revealed as contiguous sequence stretch, which bound 125I-VEGF165. A systematic replacement with L-amino acids within the peptide representing the putative VEGF-binding site on VEGFR II indicates Asp255 as the hydrophilic key residue for binding. The dimerized peptide (RTELNVGIDFNWEYPAS)2K inhibits VEGF165 binding with an IC50 of 0.5 microM on extracellular VEGFR II fragments and 30 microM on human umbilical vein cells. VEGF165-stimulated autophosphorylation of VEGFR II as well as proliferation and migration of microvascular endothelial cells was inhibited by the monomeric peptide RTELNVGIDFNWEYPASK at a half-maximal concentration of 3-10, 0.1, and 0.1 microM, respectively. We conclude that transduction of the VEGF165 signal can be interrupted with a peptide derived from the third Ig-like domain of VEGFR II by blockade of VEGF165 binding to its receptor.

Construction and functional characterization of scFv(14E1)-ETA - a novel, highly potent antibody-toxin specific for the EGF receptor

Epidermal growth factor (EGF) receptor-overexpression is characteristic of many human tumours of epithelial origin and has been correlated with unfavourable patient prognosis. Its involvement in the malignant process, its elevated expression in tumours and its accessibility on the tumour cell surface make the EGF receptor a potential target for directed tumour therapy. We have previously characterized a recombinant antibody - Pseudomonas exotoxin A fusion protein, scFv(225)-ETA, which displayes antitumoral activity towards EGF receptor-overexpressing tumour cells but is less potent in tumour cell killing than TGF-alpha-ETA, a recombinant toxin using the natural EGF receptor ligand transforming growth factor alpha (TGF-alpha) as a targeting domain. Here, we describe the construction and functional characterization in vitro of a novel single-chain antibody-toxin, scFv(14E1)-ETA, based on the independently isolated EGF receptor-specific monoclonal antibody 14E1. ScFv(14E1)-ETA binds to an EGF receptor epitope that is very similar or identical to that of scFv(225)-ETA with nine times higher affinity than the latter and displays more than tenfold higher cytotoxic activity on EGF receptor-overexpressing tumour cells. ScFv(14E1)-ETA cell killing activity was very similar to that of TGF-alpha-ETA on receptor-overexpressing cells but, in contrast to the latter, scFv(14E1)-ETA was much more selective and did not display significant cytotoxic activity on cells expressing moderate EGF receptor levels.

A general route to fingerprint analyses of peptide-antibody interactions using a clustered amino acid peptide library: comparison with a phage display library

We provide a general route to fingerprint analyses of peptide-antibody interactions using a novel chemically synthesized peptide library. A combinatorial clustered amino acid peptide library XO1O2O3O4X (O = one of six amino acid clusters [APG], [DE], [HKR], [NQST], [FYW] and [ILVM]; X = randomized position) bound to a continuous cellulose membrane support was designed to overcome the problem of combinatorial explosion in the synthesis of peptide libraries. This library served as the starting point for the identification and detailed characterization of a TGF alpha peptide epitope recognized by the antibody Tab2. By analysing 1728 hexapeptide mixtures and 1600 single hexapeptides we identified a large number of structurally different high affinity Tab2 binding molecules. Our data provide a detailed picture of the structural basis of this antibody-peptide interaction. In addition to the detection of key amino acids involved in Tab2 binding we observed a high variability of Tab2 binding sequences supporting an induced fit mechanism in antibody-peptide recognition. In contrast, a phage display hexapeptide library led to the detection of only one dominant Tab2 binding peptide. The data obtained also demonstrate the influence of phage proteins on the interaction between the antibody and the displayed peptide. Comparing both approaches with regard to ease of handling and identified sequences, the chemical libraries are clearly favored to study antibody-peptide interactions.

The M(r) 35,000 beta-adrenergic receptor mRNA-binding protein induced by agonists requires both an AUUUA pentamer and U-rich domains for RNA recognition

Delineating the molecular basis for agonist-induced destabilization of mRNA of G-protein-linked receptors that contributes to receptor down-regulation is fundamental to our understanding of long-term regulation of receptors by agonist. Previously we identified a prominent, M(r) 35,000 cytosolic RNA-binding protein that (i) binds selectively to beta 1 and beta 2-adrenergic receptor mRNAs, both of which undergo agonist-induced down-regulation; (ii) does not bind either to alpha 1b-adrenergic receptor mRNA, which does not undergo agonist-induced down-regulation, or to beta-globin mRNA; (iii) displays binding to beta 2-adrenergic receptor mRNA that is selectively competed by poly(U) RNA, but not poly(A),-(C), or -(G) RNA; and (iv) its abundance varies inversely with the level of receptor mRNA, being induced by agonists that down-regulate receptor mRNA (Port, J. D., Huang, L.-y., and Malbon (1992) J. Biol. Chem. 267, 24103-24108). We demonstrate here that the binding of beta-adrenergic receptor mRNA by this protein, termed beta-ARB protein, is sensitive to competition by AU-rich domains of the 3'-untranslated regions of c-fos, c-myc, and human granulocyte-macrophage colony-stimulating factor. Using the AU-rich 3'-untranslated regions of wild-type adenovirus IVa2 mRNA and variants with defined mutations in the AUUUApentamer, AU-rich, and U-rich domains, we were able to define sequences critical to the binding of the beta 2-receptor mRNA by the beta-ARB protein. Recognition of beta-ARB protein requires not only an AUUUA destabilization pentamer, but also a flanking U-rich domain(s). Using radiolabeled 3'-untranslated regions of short-lived mRNA, we were able to identify this same M(r) 35,000 cytosolic RNA-binding protein(s), beta-ARB protein, as selective for beta 2-adrenergic receptor mRNA.

A 32-kilodalton protein binds to AU-rich domains in the 3' untranslated regions of rapidly degraded mRNAs

An AU-rich sequence present within the 3' untranslated region has been shown to mark some short-lived mRNAs for rapid degradation. We demonstrate by label transfer and gel shift experiments that a 32-kDa polypeptide, present in nuclear extracts, specifically interacts with the AU-rich domains present within the 3' untranslated region of human granulocyte-macrophage colony-stimulating factor, c-fos, and c-myc mRNAs and a similar domain downstream of the poly(A) addition site of the adenovirus IVa2 mRNA. Competition experiments and partial protease analysis indicated that the same polypeptide interacts with all four RNAs. A single AUUUA sequence in a U-rich context was sufficient to signal binding of the 32-kDa polypeptide. Insertion of three copies of this minimal recognition site led to markedly reduced accumulation of beta-globin RNA, while the same insert carrying a series of U-to-G changes had little effect on RNA levels. Steady-state levels of beta-globin-specific nuclear RNA, including incompletely processed RNA, and cytoplasmic mRNA were reduced. Cytoplasmic mRNA containing the AU-rich recognition sites for the 32-kDa polypeptide exhibited a half-life shorter than that of mRNA with a mutated insert. We suggest that binding of the 32-kDa polypeptide may be involved in the regulation of mRNA half-life.

Adenovirus E1A protein activates transcription of the E1A gene subsequent to transcription complex formation

The mechanism of transcriptional activation of the adenovirus E1A and E3 genes by E1A protein during infection was examined by using transcription-competition assays. Infection of HeLa cells with one virus led to inhibition of mRNA accumulation from a superinfecting virus. Synthesis of the E1A 289R protein by the first virus to infect reduced inhibition of transcription of the superinfecting virus, indicating that the E1A 289R protein was limiting for E1A-activated transcription. Infection with an E1A- virus, followed 6 h later by superinfection with a wild-type virus, led to preferential transcriptional activation of the E1A gene of the first virus, suggesting that a host transcription component(s) stably associated with the E1A promoter in the absence of E1A protein and that this complex was the substrate for transcriptional activation by E1A protein. The limiting host transcription component(s) bound to the E1A promoter to form a complex with a half-life greater than 24 h in the absence of E1A 289R protein, as demonstrated in a challenge assay with a large excess of superinfecting virus. In the presence of the E1A 289R protein, the E1A gene of the superinfecting virus was gradually activated with a reduction in E1A mRNA accumulation from the first virus. The kinetics of the activation suggest that this was due to an indirect effect rather than to destabilization of stable transcription complexes by the 289R protein.

Allosteric control of simian virus 40 T-antigen binding to viral origin DNA

Simian virus 40 (SV40) large tumor antigen (T antigen) possesses several biochemical activities localized in different domains of the protein. These activities include sequence-specific binding to two major sites, I and II, in the SV40 control region, ATPase, and nucleotide-binding activity. In the present communication, we present evidence that specific binding of immunopurified T antigen to SV40 DNA is markedly inhibited by low concentrations of ATP, dATP, GTP, and dGTP. The inhibition is reversible after removal of the nucleotide, suggesting that simple nucleotide binding rather than a covalent modification of T antigen in the presence of ATP is responsible for the inhibition. The results suggest that T antigen may assume two conformations, one active and one inactive in binding to the SV40 origin of replication. In the presence of purine nucleoside triphosphates, the inactive conformation is favored.

Identification and biochemical analysis of DNA replication-defective large T antigens from SV40-transformed cells

Nine commonly studied Simian virus 40 (SV40)-transformed rodent cell lines were screened for tumor (T) antigens defective in SV40 DNA replication using a simple polyethylene glycol-mediated cell fusion assay. Each line contained a functional origin of SV40 DNA replication, as shown by fusion with Cos 1 cells. Fusion with uninfected monkey cells revealed that T antigens from two lines lacked detectable replicative activity, while T antigens from five other lines exhibited only very weak replicative activity. One line, and a tumor cell line derived from it, expressed T antigen with wild-type replication activity. Biochemical analysis of these proteins revealed defects in DNA binding activity and ATPase activity. One line expressed large T antigen defective in both activities. All of the lines contained complexes of T antigen with the cellular protein p53 and all of the T antigens exhibited nucleotide-binding activity. The results indicate that some of these lines may constitute a useful source of new replication-defective T antigens.

Seventeen base pairs of region I encode a novel tripartite binding signal for SV40 T antigen

Three sequence components direct high affinity binding of dimeric SV40 T antigen to SV40 origin region I. Two signals are encoded by two directly repeated 5'-GAGGC-3' pentanucleotides. Approximately equal contributions to binding stability are made by each pentanucleotide, and both spacing and orientation of the pentanucleotides are important for binding affinity. The third vital component is contained in a 5'-TTTTTTG-3' spacer sequence that separates the pentanucleotides. Sequence-specific features of the spacer stabilize binding to the adjacent pentanucleotides. The asymmetry of the spacer suggests that a novel binding mechanism is involved. Because the alignment of T antigen on mutant and wild-type DNAs is similar, we propose that any two of the three sequence signals are sufficient to determine the unique arrangement of a bound protein dimer.

Localization and activities of nitrogenase, glutamine synthetase and glutamate synthase in Azotobacter vinelandii grown in oxygen-controlled continuous culture

Azotobacter vinelandii was grown in oxygen-controlled continuous cultures under conditions of dinitrogen fixation. Different oxygen concentrations were adjusted with air. Cell-free extracts were employed to study the oxygen dependency of the intracellular distribution and activity of the following enzymes: nitrogenase, glutamine synthetase and glutamate synthase. Nitrogenase was localized exclusively in the soluble fraction. Its activity increased steeply when the oxygen concentration employed in growing the organism decreased from about 30% close to 0% air saturation. Glutamine synthetase was identified exclusively as a soluble enzyme. The degree of adenylylation of the enzyme increased from about one to about four parallel to nitrogenase activity when the oxygen concentration in the culture was lowered. Glutamate synthase was detected in both a soluble and a membrane-bound form. The sum of specific activities of both forms stayed constant irrespective of changes in the oxygen concentration. However, with increasing oxygen concentration, the proportion of the membrane-bound form increased up to two-thirds of the total activity.