Magnesium restriction induces granulocytic differentiation and expression of P27Kip1 in human leukemic HL-60 cells

When cultured in Mg restricted medium, human leukemic HL-60 cells develop morphological and functional granulocytic differentiation. In 0.03 mM Mg, cells display the distinctive features of differentiation, without appreciable inhibition of proliferation. In 0.01 mM Mg, cells show terminal differentiation, accompanied by clear inhibition of proliferation. Such cells accumulate in the G0/G1 phase and subsequently die via apoptosis, similar to HL-60 cells that have been induced to differentiate by DMSO. These phenotypic changes are associated with a marked increase in the expression level of the cyclin dependent kinase inhibitor p27^Kip1. Cyclin E expression is also slightly increased in Mg restricted cells, whereas no changes are observed in the expression level of cyclin D1. We also show that during differentiation cell total Mg decreases, whereas [Mg²⁺]_i increases in both Mg-depleted and DMSO-treated cells. These data suggest that the maturation process is paralleled by a redistribution of intracellular Mg, leading to a shift from the bound to the free form. These changes could modulate the kinetics of Mg-dependent enzyme(s) that are involved in the control of the differentiation pathway. We propose that this model may represent an useful tool for the study of the mechanisms of cell differentiation and related events, such as aging and death. J. Cell. Biochem. 70:313–322, 1998. © 1998 Wiley-Liss, Inc.     

Determination of drug levels in human serum by circular dichroism

A study of the applicability of circular dichroism (CD) for the determination of drug levels in human serum is described and a new method for the quantitative determination of optically active absorbing drugs having Cotton effects at wavelengths above 250 nm in human serum and/or plasma is proposed. The principal advantages of this method are speed, economy, and simplicity, no derivatization or chromatographic separation steps being needed. The validity of the CD determination was confirmed by analysis of variance, β-lactam antibiotics being chosen as model drugs. In addition, the validation studies performed confirm the accuracy and precision of the proposed method. For β-lactam antibiotics lacking Cotton effects above 250 nm, an alternative method based on the extraction of the drug from serum is considered. Chirality 10:507–512, 1998. © 1998 Wiley-Liss, Inc.     

Spectroscopic studies of 9-hydroxyellipticine binding to DNA

The binding of 9-hydroxyellipticine to calf thymus DNA, poly[d(A-T)]₂, and poly-[d(G-C)]₂ has been studied in detail by means of CD, linear dichroism, resonance light scattering, and molecular dynamics. The transition moment polarizations of 9-hydroxyelliptiycine were determined in polyvinyl alcohol stretched film. Spectroscopic solution studies of the DNA/drug complex are combined with theoretical CD calculations using the final 50 ps of a series of molecular dynamics simulations as input. The spectroscopic data shows 9-hydroxyellipticine to adopt two main binding modes, one intercalative and the other a stacked binding mode involving the formation of drug oligomers in the DNA major groove. Analysis of the intercalated binding mode in poly[d(A-T)]₂ suggests the 9-hydroxyellipticine hydroxyl group lies in the minor groove and hydrogen bonds to water with the pyridine ring protruding into the major groove. The stacked binding mode was examined using resonance light scattering and it was concluded that the drug was forming small oligomer stacks rather than extended aggregates. Reduced linear dichroism measurements suggested a binding geometry that precluded a minor groove binding mode where the plane of the drug makes a 45° angle with the plane of the bases. Thus it was concluded that the drug stacks in the major groove. No obvious differences in the mode of binding of 9-hydroxyellipticine were observed between different DNA sequences; however, the stacked binding mode appeared to be more favorable for calf thymus DNA and poly[d(G-C)]₂ than for poly[d(A-T)]₂, an observation that could be explained by the slightly greater steric hindrance of the poly[d(A-T)]₂ major groove. A strong concentration dependence was observed for the two binding modes where intercalation is favored at very low drug load, with stacking interactions becoming more prominent as the drug concentration is increased. Even at DNA : drug mixing ratios of 70:1 the stacked binding mode was still important for GC-rich DNAs. © 1998 John Wiley & Sons, Inc. Biopoly 46: 127–143, 1998     

Biological and conformational studies on analogues of a synthetic peptide enhancing HIV-1 infection

We have previously demonstrated that a 23-amino acid peptide derived from the V3 loop of the surface glycoprotein of the HIV-1 strain MN is able to bind CD4 and to enhance HIV-1 infection. Further studies have suggested that the peptide/CD4 interaction induces an increase in both CD4 expression and CD4/gp120 binding affinity. This paper describes the biological and physico-chemical characterization of three analogues of reduced sequence that have been designed in order to identify the minimum active sequence of this peptide corresponding to the MN-HIV-1 principal neutralizing domain. Biological studies indicate that the entire sequence is required for biological activity and that the sequence 1–18 presents an inhibitory activity. CD and FT-IR absorption data are discussed here in order to identify possible structure-function correlations. © 1998 European Peptide Society and John Wiley & Sons, Ltd.     

Structure and dynamics of the M13 coat signal sequence in membranes by multidimensional high-resolution and solid-state NMR spectroscopy

The polypeptide corresponding to the signal sequence of the M13 coat protein and the five N-terminal residues of the mature protein was prepared by solid-phase peptide synthesis with a ¹⁵N isotopic label at the alanine-12 position. Multidimensional solution NMR spectroscopy and molecular modeling calculations indicate that this polypeptide assumes helical conformations between residues 5 and 20, in the presence of sodium dodecylsulfate micelles. This is in good agreement with circular dichroism spectroscopic measurement, which shows an α-helix content of approximately 42%. The α-helix comprises an uninterrupted hydrophobic stretch of ≤12 amino acids, which is generally believed to be too short for a stable transmembrane alignment in a biological bilayer. The monoexponential proton-deuterium exchange kinetics of this hydrophobic helical region is characterized by half-lives of 15–75 minutes (pH 4.2, 323 K). When the polypeptide is reconstituted into phospholipid bilayers, the broad anisotropy of the proton-decoupled ¹⁵N solid-state NMR spectroscopy indicates that the hydrophobic helix is immobilized close to the lipid bilayer surface at the time scale of ¹⁵N solid-state NMR spectroscopy (10⁻⁴ seconds). By contrast, short correlation times, immediate hydrogen-deuterium exchange as well as nuclear Overhauser effect crosspeak analysis suggest that the N and C termini of this polypeptide exhibit a mobile random coil structure. The implications of these structural findings for possible mechanisms of membrane insertion and translocation as well as for membrane protein structure prediction algorithms are discussed. © 1997 Wiley-Liss Inc.     

Construction and analysis of a detailed three-dimensional model of the ligand binding domain of the human B cell receptor CD40

The interaction between the human B cell receptor CD40 and its ligand on T cells is critical for B cell proliferation and the regulation of humoral immune responses. CD40 is a member of the tumor necrosis factor receptor (TNFR) family. We report here the construction and analysis of a detailed three-dimensional model of the TNFR-homologous extracellular region of CD40. This study provides an example for structure-based model building in the presence of low sequence similarity. The assessment of model quality and sequence-structure compatibility is emphasized, and limitations of the model are discussed. The current CD40 model predicts structural details beyond the backbone level. Features of the CD40 ligand binding site are discussed in conjunction with the results of a previous mutagenesis study. Proteins 27:59–70 © 1997 Wiley-Liss, Inc.     

Interactions of DNA with a new electron-deficient tentacle porphyrin: meso-Tetrakis[2,3,5,6-tetrafluoro-4-(2-trimethylammoniumethylamine)phenyl]porphyrin

A new electron-deficient tentacle porphyrin meso-tetrakis[2,3,5,6-tetrafluoro-4-(2-trimethylammoniumethylamine)phenyl]porphyrin (TθF₄TAP) has been synthesized. The binding interactions of TθF₄TAP with DNA polymers were studied for comparison to those of an electron-deficient tentacle porphyrin and an electron-rich tentacle porphyrin; these previously studied porphyrins bind to DNA primarily by intercalative and outside-binding modes, respectively. The three tentacle porphyrins have similar size and shape. The basicity of TθF₄TAP indicated that it has electronic characteristics similar to those of the intercalating electron-deficient tentacle porphyrin. However, TθF₄TAP binds to calf thymus DNA, [poly(dA-dT)]₂, and [poly(dG-dC)]₂ in a self-stacking, outside-binding manner under all conditions. Evidence for this binding mode included a significant hypochromicity of the Soret band, a conservative induced CD spectrum, and the absence of an increase in DNA solution viscosity. As found previously for the electron-rich porphyrin, the results suggest that combinations of closely related self-stacked forms coexist. The mix of forms depended on the DNA and the solution conditions. There are probably differences in the detailed features of the self-stacking adducts for the two types of tentacle porphyrins, especially at high R (ratio of porphyrin to DNA). At low R values, the induced CD signal of TθF₄TAP/CT DNA resembled that of TθF₄TAP/[poly(dA-dT)]₂, suggesting that TθF₄TAP binds preferentially at AT regions. Competitive binding experiments gave evidence that TθF₄TAP binds preferentially to [poly(dA-dT)]₂ over [poly(dG-dC)]₂. Thus, despite the long, positively charged, flexible substituents on the porphyrin, the binding of TθF₄TAP is significantly affected by base-pair composition. Similar characteristics were found previously for the electron-rich tentacle porphyrin. Thus, significant changes in electron richness have relatively minor effects on this outside binding selectivity for AT regions. TθF₄TAP is the first porphyrin with electron deficiency and shape similar to intercalating porphyrins that does not appear to intercalate. All porphyrins reported to intercalate have had pyridinium substituents. Thus, the electronic distribution in the porphyrin ring, not just the overall electron richness, may play a role in facilitating intercalation. © 1997 John Wiley & Sons, Inc. Biopoly 42: 203–217, 1997     

The effect of mutations on peptide models of the DNA binding helix of p53: Evidence for a correlation between structure and tumorigenesis

The tumor suppresser protein p53 has been called the “guardian of the genome.” DNA damage induces p53 to either halt the cell cycle, allowing for repair, or initiate apoptosis. P53 is mutated in over 50% of human tumors and it has been proposed that many tumorigenic mutations are deleterious to p53 because they induce local unfolding. To explore this hypothesis, peptide models have been developed to study tumorigenic mutations in the H2 helix of the p53 core domain. This helix is rich with charged residues and is a key component of the DNA binding region. A 16‐residue peptide corresponding to the H2 wild‐type sequence extended with an Ala‐rich C‐terminus was synthesized and studied by ¹H‐nmr (500 MHz) and CD. The nmr studies demonstrate that this peptide adopts helical structure in solution. Six additional peptides corresponding to subtle tumorigenic mutations were synthesized and CD was used to assess the relative stability of these “mutant analogues.” All six mutations studied are destabilizing relative to the wild type, with ΔΔG values in the range of 0.26 to 1.35 kcal mol⁻¹. Surprisingly, substitution of Asp 281 with Ala resulted in a peptide with the greatest destabilization even though Ala possesses the largest helix propensity of the common 20 amino acids. Because this helix appears to be stabilized mainly by local electrostatics, we conclude that its structure is susceptible to even the most conservative mutations. These results provide support for the hypothesis that tumorigenic mutations induce local unfolding of p53. © 1999 John Wiley & Sons, Inc. Biopoly 49: 215–224, 1999     

Light scattering,CD, and ligand binding studies of ferrihemoglobin–polyelectrolyte complexes

Quasi‐elastic light scattering (QELS), electrophoretic light scattering (ELS), CD spectroscopy, and azide binding titrations were used to study the complexation at pH 6.8 between ferrihemoglobin and three polyelectrolytes that varied in charge density and sign. Both QELS and ELS show that the structure of the soluble complex formed between ferrihemoglobin and poly(diallyldimethylammonium chloride) [PDADMAC] varies with protein concentration. At fixed 1.0 mg/mL polyelectrolyte concentration, protein addition increases complex size and decreases complex mobility in a tightly correlated manner. At 1.0 mg/mL or greater protein concentration, a stable complex is formed between one polyelectrolyte chain and many protein molecules (i.e., an intra‐polymer complex) with apparent diameter approximately 2.5 times that of the protein‐free polyelectrolyte. Under conditions of excess polyelectrolyte, each of the three ferrihemoglobin–polyelectrolyte solutions exhibits a single diffusion mode in QELS, which indicates that all protein molecules are complexed. CD spectra suggest little or no structural disruption of ferrihemoglobin upon complexation. Azide binding to the ferrihemoglobin–poly(2‐acrylamide‐2‐methylpropanesulfonate) [PAMPS] complex is substantially altered relative to the polyelectrolyte‐free protein, but minimal change is induced by complexation with an AMPS‐based copolymer of reduced linear charge density. The change in azide binding induced by PDADMAC is intermediate between that of PAMPS and its copolymer. © 1999 John Wiley & Sons, Inc. Biopoly 50: 153–161, 1999