Preservation of native conformation during aluminium-induced aggregation of tau protein

ALUMINIUM exposure has been shown to result in aggregation of microtubule-associated protein tau in vitro. In the light of recent observations that the native random structure of tau protein is maintained in its monomeric and dimeric states as well as in the paired helical filaments characteristic of Alzheimer's disease, it is likely that factors playing a causative role in neurofibrillary pathology would not drastically alter the native conformation of tau protein. We have studied the interaction of tau protein with aluminium using circular dichroism (CD) and 27(Al) NMR spectroscopy. The CD studies revealed a five-fold increase in the observed ellipticity of the tau-aluminium assembly. The increase in elipticity was not associated with a change in the general conformation of the protein and was most likely due to an aggregation of the tau protein induced by aluminium. Al-27 NMR spectroscopy confirmed the binding of aluminium to tau protein. Hyperphosphorylation of tau in Alzheimer's disease is known to be associated with defective microtubule assembly in this condition. Abnormally phosphorylated tau exists in a polymerized form in the paired helical filaments (PHF) which constitute the neurofibrillary tangles found in Alzheimer's disease. While it is hypothesized that its altered biophysical characteristics render abnormally phosphorylated tau resistant to proteolysis, causing the formation of stable deposits,the sequence of events resulting in the polymerization of tau are little understood, as are the additional factors or modifications required for tills process. Based on the results of our spectroscopic studies, a model for the sequence of events occurring in neurofibrillary pathology is proposed.

Simplified theory of optical nonlinearities in spin-polarized semiconductors

The spin degree of freedom is largely disregarded in existing theories of the density-dependent optical properties of an interacting electron-hole plasma in quasiequilibrium. Here, we extended the pair equation, which is applicable to a bulk semiconductor at elevated temperatures, to calculate optical nonlinearities due to a spin-polarized plasma. We obtained agreement with recent circular dichroism data in laser-excited GaAs by using the plasma density alone as the fitting parameter. The simplicity of our theory, based on the analytical pair-equation formula, makes it ideal for conveniently modelling absorption in a carrier spin-polarized semiconductor.

Acetylation of Transition Protein 2 (TP2) by KAT3B (p300) Alters Its DNA Condensation Property and Interaction with Putative Histone Chaperone NPM3

The hallmark of mammalian spermiogenesis is the dramatic chromatin remodeling process wherein the nucleosomal histones are replaced by the transition proteins TP1, TP2, and TP4. Subsequently these transition proteins are replaced by the protamines P1 and P2. Hyperacetylation of histone H4 is linked to their replacement by transition proteins. Here we report that TP2 is acetylated in vivo as detected by anti-acetylated lysine antibody and mass spectrometric analysis. Further, recombinant TP2 is acetylated in vitro by acetyltransferase KAT3B (p300) more efficiently than by KAT2B (PCAF). In vivo p300 was demonstrated to acetylate TP2. p300 acetylates TP2 in its C-terminal domain, which is highly basic in nature and possesses chromatin-condensing properties. Mass spectrometric analysis showed that p300 acetylates four lysine residues in the C-terminal domain of TP2. Acetylation of TP2 by p300 leads to significant reduction in its DNA condensation property as studied by circular dichroism and atomic force microscopy analysis. TP2 also interacts with a putative histone chaperone, NPM3, wherein expression is elevated in haploid spermatids.Interestingly, acetylation of TP2 impedes its interaction with NPM3. Thus, acetylation of TP2 adds a new dimension to its role in the dynamic reorganization of chromatin during mammalian spermiogenesis.

Synthesis and Evaluation of a Novel Class of G-Quadruplex-Stabilizing Small Molecules Based on the 1,3-Phenylene-Bis(piperazinyl benzimidazole) System

Achieving stabilization of telomeric DNA in G-quadruplex conformation by Various organic compounds has been an important goal for the medicinal chemists seeking to develop new anticancer agents. Several compounds are known to stabilize G-quadruplexes. However, relatively few are known to induce their formation and/or alter the topology, of the preformed quadruplex DNA. Herein, four compounds having the 1,3-phenylene-bis(piperazinyl benzimidazole) unit as a basic skeleton have been synthesized, and their interactions with the 24-mer telomeric DNA sequences from Tetrahymena thermophilia d(T(2)G(4))(4) have been investigated using high-resolution techniques Such as circular dichroism (CD) spectropolarimetry, CD melting, emission spectroscopy, and polyacrylamide gel electrophoresis. The data obtained, in the presence of one of three ions (Li+, Na+, or K+), indicate that all the new compounds have a high affinity for G-quadruplex DNA, and the strength of the binding with G-quadruplex depends on (1) phenyl ring substitution, (ii) the piperazinyl side chain, and (iii) the type of monovalent cation present in the buffer. Results further Suggest that these compounds are able to abet the conversion of the Intramolecular quadruplex into parallel stranded intermolecular G-quadruplex DNA. Notably, these compounds are also capable of inducing and stabilizing the parallel stranded quadruplex from randomly structured DNA in the absence of any stabilizing cation. The kinetics of the structural changes Induced by these compounds could be followed by recording the changes in the CD signal as a function of time. The implications of the findings mentioned above are discussed in this paper.

DNA- and chromatin-condensing properties of rat testes H1a and H1t compared to those of rat liver H1bdec: H1t is a poor condenser of chromatin

Histones H1a and H1t are two major linker histone variants present at the pachytene interval of mammalian spermatogenesis. The DNA- and chromatin-condensing properties of these two variants isolated from rat testes were studied and compared with those from rat liver. For this purpose, the histone H1 subtypes were purified from the respective tissues using bath acid and salt extraction procedures, Circular dichroism studies revealed that acid exposure during isolation affects the alpha-helical structure of both the globular domain (in the presence of 1 M NaCl) and the C-terminal lambda-tail (in the presence of 60% trifluoroethanol). The condensation of rat oligonucleosomal DNA, as measured by circular dichroism spectroscopy, by the salt-extracted histone H1 was at least 10 times more efficient than condensation by the acid-extracted histone H1. A site size of 16-20 base pairs was calculated for the salt-extracted histone H1. Among the different histone H1 subtypes, somatic histone H1bdec had the highest DNA-condensing property, followed by histone H1a and histone H1t. All the salt-extracted histones condensed rat oligonucleosomal DNA more efficiently than linear pBR-322 DNA, Histones H1bdec and H1a condensed histone H1-depleted chromatin, prepared from rat liver nuclei, with relatively equal efficiency. On the other hand, there was no condensation of histone H1-depleted chromatin with the testes specific histone H1t. A comparison of the amino acid sequences of histone H1d (rat) and histone H1t (rat) revealed several interesting differences in the occurrence of DNA-binding motifs at the C-terminus. A striking observation is the presence of a direct repeat of an octapeptide motif K(A)T(S)PKKA(S)K(T)K(A) in histone H1d that is absent in histone H1t.

DNA-condensing and chromatin-condensing properties of rat testes hla and hit compared to those of rat-liver hlbdec - hlt is a poor condenser of chromatin

Histones H1a and H1t are two major linker histone variants present at the pachytene interval of mammalian spermatogenesis. The DNA- and chromatin-condensing properties of these two variants isolated from rat testes were studied and compared with those from rat liver. For this purpose, the histone H1 subtypes were purified from the respective tissues using bath acid and salt extraction procedures, Circular dichroism studies revealed that acid exposure during isolation affects the alpha-helical structure of both the globular domain (in the presence of 1 M NaCl) and the C-terminal lambda-tail (in the presence of 60% trifluoroethanol). The condensation of rat oligonucleosomal DNA, as measured by circular dichroism spectroscopy, by the salt-extracted histone H1 was at least 10 times more efficient than condensation by the acid-extracted histone H1. A site size of 16-20 base pairs was calculated for the salt-extracted histone H1. Among the different histone H1 subtypes, somatic histone H1bdec had the highest DNA-condensing property, followed by histone H1a and histone H1t. All the salt-extracted histones condensed rat oligonucleosomal DNA more efficiently than linear pBR-322 DNA, Histones H1bdec and H1a condensed histone H1-depleted chromatin, prepared from rat liver nuclei, with relatively equal efficiency. On the other hand, there was no condensation of histone H1-depleted chromatin with the testes specific histone H1t. A comparison of the amino acid sequences of histone H1d (rat) and histone H1t (rat) revealed several interesting differences in the occurrence of DNA-binding motifs at the C-terminus. A striking observation is the presence of a direct repeat of an octapeptide motif K(A)T(S)PKKA(S)K(T)K(A) in histone H1d that is absent in histone H1t.

Use of structure-directed DNA ligands to probe the binding of recA protein to narrow and wide grooves of DNA and on its ability to promote homologous pairing

We have used circular dichroism and structure-directed drugs to identify the role of structural features, wide and narrow grooves in particular, required for the cooperative polymerization, recognition of homologous sequences, and the formation of joint molecules promoted by recA protein. The path of cooperative polymerization of recA protein was deduced by its ability to cause quantitative displacement of distamycin from the narrow groove of duplex DNA. By contrast, methyl green bound to the wide groove was retained by the nucleoprotein filaments comprised of recA protein-DNA. Further, the mode of binding of these ligands and recA protein to DNA was confirmed by DNaseI digestion. More importantly, the formation of joint molecules was prevented by distamycin in the narrow groove while methyl green in the wide groove had no adverse effect. Intriguingly, distamycin interfered with the production of coaggregates between nucleoprotein filaments of recA protein-M13 ssDNA and naked linear M13 duplex DNA, but not with linear phi X174 duplex DNA. Thus, these data, in conjunction with molecular modeling, suggest that the narrow grooves of duplex DNA provide the fundamental framework required for the cooperative polymerization of recA protein and alignment of homologous sequences. These findings and their significance are discussed in relation to models of homologous pairing between two intertwined DNA molecules.

DNA-induced conformational changes in RecA protein. Evidence for structural heterogeneity among nucleoprotein filaments and implications for homologous pairing

We have used circular dichroism as a probe to characterize the solution conformational changes in RecA protein upon binding to DNA. This approach revealed that RecA protein acquires significant amounts of alpha-helix upon interaction with DNA. These observations, consistent with the data from crystal structure (Story, R. M., Weber, I., and Steitz, T. (1992) Nature 355, 318-325), support the notion that some basic domains including the DNA binding motifs of RecA protein are unstructured and might contribute to the formation of alpha-helix. A comparison of nucleoprotein filaments comprised of RecA protein and a variety of DNA substrates revealed important structural heterogeneity. The most significant difference was observed with poly(dG). poly(dC) and related polymers, rich in GC sequences, which induced minimal amounts of alpha-helix in RecA protein. The magnitude of induction of alpha-helix in RecA protein, which occurred concomitant with the production of ternary complexes, was 2-fold higher with homologous than heterologous duplex DNA. Most importantly, the stimulation of ATP hydrolysis by high salt coincided with that of the induction of alpha-helix in RecA protein. These conformational differences provide a basis for thinking about the biochemical and structural transitions that RecA protein experiences during the formal steps of presynapsis, recognition, and alignment of homologous sequences.

Design, Synthesis, and DNA Binding Properties of Photoisomerizable Azobenzene−Distamycin Conjugates: An Experimental and Computational Study

Here, we present the synthesis, photochemical, and DNA binding properties of three photoisomerizable azobenzene−distamycin conjugates in which two distamycin units were linked via electron-rich alkoxy or electron-withdrawing carboxamido moieties with the azobenzene core. Like parent distamycin A, these molecules also demonstrated AT-specific DNA binding. Duplex DNA binding abilities of these conjugates were found to depend upon the nature and length of the spacer, the location of protonatable residues, and the isomeric state of the conjugate. The changes in the duplex DNA binding efficiency of the individual conjugates in the dark and with their respective photoirradiated forms were examined by circular dichroism, thermal denaturation of DNA, and Hoechst displacement assay with poly[d(A-T).d(T-A)] DNA in 150 mM NaCl buffer. Computational structural analyses of the uncomplexed ligands using ab initio HF and MP2 theory and molecular docking studies involving the conjugates with duplex d[(GC(AT)10CG)]2 DNA were performed to rationalize the nature of binding of these conjugates.

Investigation of the local Fe magnetic moments at the grain boundaries of the Ca2FeReO6 double perovskite

The local Fe ferromagnetic (FM) moment at the grain boundaries of a ceramic sample of Ca2FeReO6 double perovskite was investigated by means of x-ray magnetic circular dichroism spectroscopy at the Fe L-2,L-3 edges and compared to the overall bulk magnetization. We found that, at the grain boundaries, the Fe FM moments at H=5 T are much smaller than expected and that the MxH curve is harder than in the bulk magnetization. These results suggest a larger degree of Fe/Re antisite disorder at the grain boundaries of this sample, shedding light into the intriguing nonmetallic resistivity behavior despite the reported presence of free carriers. (c) 2007 American Institute of Physics.

High-level expression of biologically active glycoprotein hormones in Pichia pastoris strains—selection of strain GS115, and not X-33, for the production of biologically active N-glycosylated 15N-labeled phCG

The methylotrophic yeast Pichia pastoris is widely used for the production of recombinant glycoproteins. With the aim to generate biologically active 15N-labeled glycohormones for conformational studies focused on the unravelling of the NMR structures in solution, the P. pastoris strains GS115 and X-33 were explored for the expression of human chorionic gonadotropin (phCG) and human follicle-stimulating hormone (phFSH). In agreement with recent investigations on the N-glycosylation of phCG, produced in P. pastoris GS115, using ammonia/glycerol-methanol as nitrogen/carbon sources, the N-glycosylation pattern of phCG, synthesized using NH4Cl/glucose–glycerol–methanol, comprised neutral and charged, phosphorylated high-mannose-type N-glycans (Man8–15GlcNAc2). However, the changed culturing protocol led to much higher amounts of glycoprotein material, which is of importance for an economical realistic approach of the aimed NMR research. In the context of these studies, attention was also paid to the site specific N-glycosylation in phCG produced in P. pastoris GS115. In contrast to the rather simple N-glycosylation pattern of phCG expressed in the GS115 strain, phCG and phFSH expressed in the X-33 strain revealed, besides neutral high-mannose-type N-glycans, also high concentrations of neutral hypermannose-type N-glycans (Manup-to-30GlcNAc2). The latter finding made the X-33 strain not very suitable for generating 15N-labeled material. Therefore, 15N-phCG was expressed in the GS115 strain using the new optimized protocol. The 15N-enrichment was evaluated by 15N-HSQC NMR spectroscopy and GLC-EI/MS. Circular dichroism studies indicated that 15N-phCG/GS115 had the same folding as urinary hCG. Furthermore, 15N-phCG/GS115 was found to be similar to the unlabeled protein in every respect as judged by radioimmunoassay, radioreceptor assays, and in vitro bioassays.

Metal-ion-mediated tuning of duplex DNA binding by bis(2-(2-pyridyl)-1H-benzimidazole)

Studies of double-stranded-DNA binding have been performed with three isomeric bis)2-(n-pyridyl)-1H-benzimidazole)s (n = 2, 3, 4). Like the well-known Hoechst 33258, which is a bisbenzimidazole compound, these three isomers bind to the minor groove of duplex DNA. DNA binding by the three isomers was investigated in the presence of the divalent metal ions Mg2+, Co2+, Ni2+, Cu2+, and Zn2+. Ligand-DNA interactions were probed with fluorscence and circular dichroism spectroscopy. These studies revealed that the binding of the 2-pyridyl derivative to DNA is dramatically reduced in the presence of Co2+, Ni2+, and Cu2+ ions and is abolished completely at a ligand/metal-cation ratio of 1:1. Control experiments done with the isomeric 3- and 4-pyridyl derivatives showed that their binding to DNA is unaffected by the aforementioned transition-metal ions. The ability of 2-(2-pyridyl)benzimidazole changes of the ligand associated with ion chelation probably ledto such unusual binding results for the ortho isomer. The addition of ethylenediaminetetraacetic acid (EDTA) reversed the effects completely.

Negative nuclear Overhauser effects as probes of macromolecular structure

Nuclear Overhauser effects (NOE) and circular dichroism (CD) techniques have been used to probe @-turn conformations in acyclic and cyclic peptides containingPro-Xsequences. The model peptides studied are of the type Piv-Pro-X-NHMe (X = Aib, D-Ala, Gly, Val, and Leu) and Boc-Cys-Pro-X-C s NHMe (X = Aib, L-Ala, D-Ala, Gly, and Leu). In the acyclic series, observation of NOES between Pro C"H and X-NH, together with solvent and temperature dependence of NH chemical shifts, establishes a 4 - 1 hydrogen bond stabilized type I1 @-turn in the Gly, D-Ala, and Aib peptides, in CDC13 and (CD3)2S0. A positive n-r* CD band at -225-230 nm appears to be characteristic of this structure. For the acyclic Pro-Leu peptide the observation of NOE's for both Pro and Leu C"H resonances on saturation of Leu NH is compatible with a type V bend or consecutive y-turn conformation. In the cyclic disulfide series the Pro-Aib and Pro-D-Ala peptides favor type I1 @-turns, whereas all other peptides adopt type I (111) conformations. All the cyclic disulfides exhibit an intense negative CD band at -228-230 nm. The results suggest thatgeneralcorrelations between CD spectral type and specific 0-turn conformations may not be obtained. Evidence for solvent-dependent structural changes in the Pro-Aib sequence in both cyclic and acyclic peptides is presented.

Unfolding of Plasmodium falciparum Triosephosphate Isomerase in Urea and Guanidinium Chloride: Evidence for a Novel Disulfide Exchange Reaction in a Covalently Cross-Linked Mutant†

The conformational stability of Plasmodium falciparum triosephosphate isomerase (TIMWT) enzyme has been investigated in urea and guanidinium chloride (GdmCl) solutions using circular dichroism, fluorescence, and size-exclusion chromatography. The dimeric enzyme is remarkably stable in urea solutions. It retains considerable secondary, tertiary, and quaternary structure even in 8 M urea. In contrast, the unfolding transition is complete by 2.4 M GdmCl. Although the secondary as well as the tertiary interactions melt before the perturbation of the quaternary structure, these studies imply that the dissociation of the dimer into monomers ultimately leads to the collapse of the structure, suggesting that the interfacial interactions play a major role in determining multimeric protein stability. The C-m(urea)/C-m(GdmCl) ratio (where C-m is the concentration of the denaturant required at the transition midpoint) is unusually high for triosephosphate isomerase as compared to other monomeric and dimeric proteins. A disulfide crosslinked mutant protein (Y74C) engineered to form two disulfide cross-links across the interface (13-74') and (13'-74) is dramatically destablized in urea. The unfolding transition is complete by 6 M urea and involves a novel mechanism of dimer dissociation through intramolecular thiol-disulfide exchange.

Hybrid peptide hairpins containing alpha- and omega-amino acids: Conformational analysis of decapeptides with unsubstituted beta-, gamma-, and delta-residues at positions 3 and 8

The effects of inserting unsubstituted omega-amino acids into the strand segments of model beta-hairpin peptides was investigated by using four synthetic decapeptides, Boc-Lcu-Val-Xxx-Val-D-Pro-Gly-Leu-Xxx-Val-Val- OMe: pepticle 1 (Xxx=Gly), pepticle 2 (Xxx=beta Gly=beta hGly=homoglycine, beta-glycine), pepticle 3 (Xxx=gamma Abu=gamma-aminobutyric acid), pepticle 4 (Xxx= delta Ava=delta-aminovaleric acid). H-1 NMR studies (500 MHz, methanol) reveal several critical cross-strand NOEs, providing evidence for P-hairpin conformations in peptides 2-4. In peptide 3, the NMR results support the formation of the nucleating turn, however, evidence for cross-strand registry is not detected. Single-crystal X-ray diffraction studies of peptide 3 reveal a beta-hairpin conformation for both molecules in the crystallographic asymmetric unit, stabilized by four cross-strand hydrogen bonds, with the gamma Abu residues accommodated within the strands. The D-Pro-Gly segment in both molecules (A,B) adopts a type II' beta-turn conformation. The circular dichroism spectrum for peptide 3 is characterized by a negative CD band at 229 rim, whereas for peptides 2 and 4, the negative band is centered at 225 nm, suggesting a correlation between the orientation of the amide units in the strand segments and the observed CD pattern.