RNA-protein complexes.

RNA-binding proteins are an extremely diverse group of proteins, reflecting the diverse functional requirements of cellular RNAs. Whereas the number of structures of RNA-binding proteins or modules is increasing at a reasonable rate, that of protein-RNA complexes increments by only a few each year. The recently determined structure of a complex from the U2 small nuclear ribonucleoprotein particle shows the subtleties of RNA stem-loop recognition by ribonucleoprotein modules. A second structure provides the first direct information on double-stranded RNA recognition by the double-stranded RNA-binding module that occurs in a variety of functionally distinct proteins. Another two new complexes concern proteins interacting with tRNA. The first is methionyl-tRNAf(Met) transformylase, which has to compete with elongation factor Tu for charged initiator tRNAMet and does so by recognising specific features of the acceptor stem of tRNAf(Met). The second is prolyl-tRNA synthetase, complexed with its cognate tRNA, that has to specifically recognise the two guanines common to all tRNA anticodons specific for proline.     

Spectroscopic properties of complexes of acridine orange with glycosaminoglycans. I. Soluble complexes.

The interaction of acridine orange with dermatan and chondrotin sulfates results in the formation of complexes containing bound dye molecules ordered into dissymmetric arrays. Complexes containing an excess of available disaccharide residues compared to dye are completely soluble, and exhibit biphasic circular dichroism bands. Analysis of the dependence of the extrinsic circular dichrosim on dye aggregation indicates the presence of extended dye stacks bound to the glycosaminoglycan. Complexes formed in solutions containing an excess of dye are only partially soluble, and exhibit circular dichroism spectra having band shifts and intensity changes relative to the soluble complexes. The latter complexes show a sharp drop in induced circular dichroism with temperature, due to a cooperative change in the structure of the complex. The structural order of the dye–glycosaminoglycan complex may differ from the intrinsic structure of the glycosaminoglycan itself in dilute solution.     

Platinum complexes of nucleotides.

The reaction of [Pt(dien)Cl1Cl (dien = NH2CH2CH2NHCH2CH2NH2) with nucleotides has been studied by nuclear magnetic resonance. It has been found that the CMP (cytidine 5'-monophosp-ate) and GMP (guanosine 5'-monophosphate/coordinate to the platinum atom through N3 and N7, respectively. The reaction of the platinum salt with the nucleotide is complete when one to one ratio of platinum to nucleotide is used and no evidence of phosphate group binding to platinum has been found. No additional binding sites have been detected except the N7 site on the guanylic group of GMP even in the presence of a large excess of [Pt(dien) Cl1Cl. The AMP (adenosine 5'monophosphate] coordinates to the platinum at the N1 and/or N7 sites. The reaction of AMP and platinum is complete is complete at a ratio of four platinum to one AMP.     

Formation of alpha-tocopherol complexes with fatty acids. Nature of complexes

Using ultraviolet spectrophotometry and 1H-NMR high-resolution spectroscopy, it has been demonstrated that the formation of alpha-tocopherol complexes with free fatty acids occurs via two types of interaction, namely formation of a hydrogen bond between the alpha-tocopherol chromanol nucleus hydroxyl and the carboxyl group of a fatty acid, and interaction of the fatty acid acyl chains with the chromanol nucleus methyl groups. The second interaction is significantly enhanced by an increase in the number of double bonds in the fatty acid molecule, which results in restriction of the molecular mobility of alpha-tocopherol. The proposed structural model of alpha-tocopherol-fatty acid complexes has been confirmed by the use of molecular models. It has been assumed that the efficiency of complex formation of natural tocopherols with fatty acids is correlated with their biological activity.     

Biological complexes of poly-beta-hydroxybutyrate.

Short-chain complexed poly-beta-hydroxybutyrate, 130-170 monomer units, is a ubiquitous constituent of cells, wherein it is usually associated with other macromolecules by multiple coordinate bonds, or by hydrogen bonding and hydrophobic interactions. This conserved PHB has been isolated from the plasma membranes of bacteria, from a variety of plant tissues, and from the plasma membranes, mitochondria, and microsomes of animal cells. In bacterial membranes, PHB has been found complexed to the calcium salts of inorganic polyphosphates, and to single-stranded DNAs. The ability of PHB to solvate salts, consisting of cations having high solvation energies and large delocalized anions, is in accordance with its molecular characteristics, that of a flexible linear molecule possessing a large number of electron-donating ester oxygens suitably spaced to replace the hydration shell of cations. In turn, PHB may be rendered soluble in aqueous media by complexation to water-soluble proteins, such as serum lipoproteins and albumin. Such solvates are highly resistant to hydrolytic enzymes. These findings suggest that the physiological roles of this unique biopolymer may include the solvation of salts of polymeric anions to facilitate their movement through hydrophobic barriers, and the protection of cellular polymers from enzymatic degradation.     

Transition-metal complexes of isatin-beta-thiosemicarbazone. X-ray crystal structure of two nickel complexes

Manganese, iron, cobalt, nickel, copper and zinc complexes of isatin-beta-thiosemicarbazone (H2L) have been synthesized and spectroscopically characterized The X-ray crystal structures of two nickel complexes, namely [Ni(HL)2]. EtOH (1) and [Ni(HL)2]. 2DMF (2), reveal a distorted octahedral coordination with the monodeprotonated ligand that behaves as an O,N,S terdentate. Different packing interactions are determined by the presence of different crystallization solvents, i.e., ethanol in 1 and dimethylformamide (DMF) in 2. 1H and 13C NMR studies of the ligand and zinc complexes in solution were carried out and a complete assignment for the ligand was made by homodecoupling, gradient assisted 2D 1H-13C HMQC and HMBC NMR spectroscopy. Biological studies, carried out in vitro on human leukaemic cell lines U937, have shown that the free ligand and the copper (II) complex are more active in the inhibition of cell proliferation than the nickel complexes. No compound was able to induce apoptosis.     

Photothermal spectroscopy of bacteriochlorophyll-lipoprotein complexes.

Photoacoustic spectra at room and 85 K temperatures as well as photothermal beam deflection spectra of bacteriochlorophyll-lipoprotein complexes from purple bacterium Chromatium minutissimum were measured. Spectra were compared and obtained differences were tentatively explained by various inertion of these two methods. Photothermal beam deflection method measure the heat which is generated in close surroundings of absorbing pigment molecule, whereas usage of more inert photoacoustic signal is averaged over contributions from various pigments located in a larger sample volume and therefore is similar to absorption spectra.     

Synaptonemal complexes of Xenopus laevis.

Synaptonemal complexes (SCs) have been analyzed in spread Xenopus spermatocytes and oocytes. They showed all the usual features of animal SCs in addition to a high incidence of centromere mismatching. A centriole pair is visible throughout zygotene and pachytene. At zygotene the ends of SCs are markedly thickened and are clustered at the nuclear periphery.     

Kinetic studies of drug-dinucleotide complexes.

Three classes of kinetic behavior are observed in the complexes of actinomycin or ethidium with deoxydinucleotides. First, the initial dinucleotide binding to form a 1:1 complex is a rapid bimolecular process, whose rate could be measured for combination of actinomycin with d(pTpG) d(pGpT), d(pGpA), d(pGpG) d(pCpGpG), and d(pCpG) andfor combination of ethidium with d(pGpC). Second, with one exception, all reactions in which a second dinucleotide is added to form a 2:1 dinucleotide-drug complex are limited by a first-order step at high concentration. This class includes the combination of actinomycin with all dinucleotides tested except d(pGpC), and the reaction of ethidium with nucleotides of complementary sequence pyrimidine-purine, such as d(pCpG). The final class is the special case of d(pGpC) interacting to form a 2:1 complex with actinomycin. Third-order kinetics is observed, with no evidence for a first-order, rate-limiting step.     

Infrared spectroscopy of heparin-cation complexes.

Hydrated and partially hydrated films and aqueous solutions of heparin, heparans and N-desulphated preparations of these polymers were studied by near- and fundamental-region-i.r. spectroscopy in the presence of a range of countercations. The results suggest that ion binding is not explicable solely in terms of simple electrostatic theory, and that specific cation effects, and the hydration pattern of the polymer-cation complex need to be taken into account.     

Stoichiometry of anthrax toxin complexes.

After being proteolytically activated, the protective antigen (PA) moiety of anthrax toxin self-associates to form symmetric, ring-shaped heptamers. Heptameric PA competitively binds the enzymatic moieties of the toxin, edema factor and lethal factor, and translocates them across the endosomal membrane by a pH-dependent process. We used two independent approaches to determine how many of the seven identical EF/LF binding sites of the PA heptamer can be occupied simultaneously. We measured isotope ratios in complexes assembled from differentially radiolabeled toxin subunits, and we determined the molecular masses of unlabeled complexes by multiangle laser light scattering. Both approaches yielded the same value: the PA heptamer in solution binds three molecules of protein ligand under saturating conditions. This suggests that each bound ligand sterically occludes the binding sites of two PA subunits. According to this model, a ligand-saturated heptamer is asymmetric, with the sites of six of the seven subunits occluded. These results contribute to the conceptual framework for understanding the mechanism of membrane translocation by anthrax toxin.