A viral expression factor behaves as a prion

Prions are proteins that can fold into multiple conformations some of which are self-propagating. Such prion-forming proteins have been found in animal, plant, fungal and bacterial species, but have not yet been identified in viruses. Here we report that LEF-10, a baculovirus-encoded protein, behaves as a prion. Full-length LEF-10 or its candidate prion-forming domain (cPrD) can functionally replace the PrD of Sup35, a widely studied prion-forming protein from yeast, displaying a [PSI+]-like phenotype. Furthermore, we observe that high multiplicity of infection can induce the conversion of LEF-10 into an aggregated state in virus-infected cells, resulting in the inhibition of viral late gene expression. Our findings extend the knowledge of current prion proteins from cellular organisms to non-cellular life forms and provide evidence to support the hypothesis that prion-forming proteins are a widespread phenomenon in nature

The molecular lifecycle of amyloid – Mechanism of assembly, mesoscopic organisation, polymorphism, suprastructures, and biological consequences

The formation of a diverse range of amyloid structures from normally soluble proteins and peptides is a hallmark of devastating human disorders as well as biological functions. The current molecular understanding of the amyloid lifecycle reveals four processes central to their growth and propagation: primary nucleation, elongation, secondary nucleation and division. However, these processes result in a wide range of cross-β packing and filament arrangements, including diverse assemblies formed from identical monomeric precursors with the same amino acid sequences. Here, we review current structural and mechanistic understanding of amyloid self-assembly, and discuss how mesoscopic, i.e. micrometre to nanometre, organisation of amyloid give rise to suprastructural features that may be the key link between the polymorphic amyloid structures and the biological response they elicit. A greater understanding of the mechanisms governing suprastructure formation will guide future strategies to combat amyloid associated disorders and to use and control the amyloid quaternary structure in synthetic biology and materials applications

Intercalative interactions of ethidium dyes with triplex structures

The binding of phenanthridine dyes to tripler poly(dT)*poly(dA). poly(dT) and its precursor duplex poly(dA). poly(dT) is characterized using linear dichroism and circular dichroism spectroscopy, and thermal denaturation. The two monomeric dyes ethidium bromide and propidium iodide are shown to behave similarly to each other in intercalating into and stabilizing both the duplex and the tripler structures. However, contrary to expectations, the extra cationic side-chain of propidium iodide provides no significant extra stabilization of tripler compared with ethidium bromide, although propidium does stabilize the duplex more than ethidium. The monomeric dyes appear to have somewhat different binding geometries with the duplex and tripler polymers. The dimeric dye ethidium homodimer is found to bis-intercalate in the tripler as well as the duplex but, in contrast to the monomers, no variation in geometry between duplex and tripler is observed. However, although dimer stabilizes the duplex, it has no effect on the thermal stability of the tripler. This lack of binding preferentiality of the dimer for tripler compared with the monomeric dyes indicates greater constraints on the accommodation of a bis-intercalator in the tripler structure than in the duplex

Linear and circular dichroism characterization of thionine binding mode with DNA polynucleotides

The binding mode of thionine (3,7-diamino-5-phenothiazinium) with alternating and non-alternating DNA poly nucleotides at low binding ratios was conclusively determined using linear and circular dichroism spectroscopies. The binding to [poly(dG-dC)](2) and poly(dG).poly(dC) was purely intercalative and was insensitive to ionic strength. Intercalative binding to [poly(dA-dT)](2) is observed at low ionic strength, but a shift of some dye to an non-intercalative mode is observed as the background salt concentration increases. With poly(dA).poly(dT), intercalative binding is unfavourable, although some dye molecules may intercalate at low ionic strength, and groove binding is strongly promoted with increasing concentration of background salt. However, stacking with bases is observed with single-stranded poly(dA) and with triplex poly(dT).poly(dA).poly(dT) which suggests that the unusual structure of poly(dA). poly(dT) precludes intercalation. Thionine behaves similarly to the related dye methylene blue, and small differences may be attributed either to the ability of thionine to form H-bonds that stabilize intercalation or to its improved stacking interactions in the basepair pocket on steric grounds

Methylene blue intercalates with triplex poly(dT)*poly(dA)\ub7poly(dT) but not duplex poly(dA)\ub7poly(dT)

Methylene blue intercalates with tripler poly(dT*dA.dT) even though it binds to the precursor duplex poly(dA.dT) in the major groove; the switch may be related to conformational modifications of the polynucleotide structure upon binding of the third strand

SEQUENCE-SPECIFIC INTERACTIONS OF METHYLENE-BLUE WITH POLYNUCLEOTIDES AND DNA - A SPECTROSCOPIC STUDY

The modes of binding of the phenothiazinium dye methylene blue (1) to alternating and nonalternating polynucleotides and to calf thymus (CT) DNA have been characterized using linear dichroism (LD) and circular dichroism (CD) spectroscopy. With the polynucleotide [poly(dG-dC)](2) the interaction at low binding ratios is shown to be purely intercalative and the binding mode is insensitive to changes in ionic strength. The observed CD spectrum is bisignate, which may be due to intercalation at the different base-pair steps (5\u27G-C3\u27 and 5\u27C-G3\u27), giving rise to CD signals of different sign and shape, By contrast, a single intercalative binding mode with the alternating AT polynucleotide [poly(dA-dT)](2) is likely only at very low ionic strength; at high ionic strength (200 mM phosphate, pH 6.9), a second binding mode is also manifest which is attributed to groove binding of the dye. The absorption and linear dichroism spectroscopic features of the methylene blue/CT-DNA (42% GC) complex reflect those of complexes of the dye with both [poly(dA-dT)](2) and [poly(dG-dC)](2); the circular dichroism spectrum of the methylene blue/CT-DNA complex and its variation with ionic strength reflect the complexity of even this simple system where numerous possible binding sites exist. Comparative binding to the nonalternating polynucleotides poly(dA).poly(dT) and poly(da).poly(dC), which each possess only one base-pair step, was also examined. On the basis of the combined LD and CD evidence, it is proposed that the dye is loosely bound with poly(dA).poly(dT), probably in the major groove, and intercalated with poly(dG).poly(dC)

Structural Heterogeneity in Polynucleotide-Facilitated Assembly of Phenothiazine Dyes

The assembly of stacked dyes on DNA is of interest for electron transfer, light harvesting, sensing, and catalysis applications. A combination of UV/vis absorption, linear dichroism (LD), and circular dichroism (CD) was applied to characterize thoroughly the aggregation with DNA of the phenothiazine dyes methylene blue, azure B, and thionine. Aggregates of each dye with [poly(dG-dC)] 2 , [poly(dA-dT)] 2 , and calf thymus DNA were explored at high dye:DNA binding ratios, where excess dye groove-binds after all intercalation sites are filled. The organization of the aggregates (dimers, trimers, and multimers) with polydeoxynucleotides displays a structural diversity that depends on DNA sequence, extent of methylation of dye exocyclic amine groups, and ionic strength. The dyes typically form right-handed H-aggregates having negative LD, consistent with stepped stacking along the minor groove. However, aggregates in some dye:DNA aggregates show left-handed chirality or positive LD, indicating unusual modes of aggregation such as formation of adventitious dimers between intercalated and minor groove bound dye. In terms of sequence-dependence, methylene blue shows more extensive aggregation with [poly(dA-dT)] 2 , while thionine aggregates more with [poly(dG-dC)] 2 . Azure B has distinctive behavior that is unlike either other dyes. Thus, although these phenothiazine dyes possess a common tricyclic framework, the organization of their polynucleotide-facilitated aggregates depends sensitively on the extent of methylation of the exocyclic amines

Environmental effects on the photophysics of transition metal complexes with dipyrido 2,3-a:3 ',2 '-c phenazine (dppz) and related ligands

This review primarily covers studies of ruthenium(II) complexes with the dppz (dipyrido[2,3-a:3',2'-c]phenazine) ligand; in solution, in polymers and surfactant/lipid media, and when bound to DNA. Related studies with other transition metals, and with extended ligands that can form bimetallic as well as monometallic complexes are discussed. The review focuses on photophysics of these complexes with particular attention devoted to the nature of the excited states that give rise to emission, their dependence on solvent environment, and the behavior of the complexes as luminescent probes for DNA