Raman spectroscopic study on the copper(II) binding mode of prion octapeptide and its pH dependence

Copper and the prion protein: methods, structures, function, and disease

The transmissible spongiform encephalopathies (TSEs) arise from conversion of the membrane-bound prion protein from PrP(C) to PrP(Sc). Examples of the TSEs include mad cow disease, chronic wasting disease in deer and elk, scrapie in goats and sheep, and kuru and Creutzfeldt-Jakob disease in humans. Although the precise function of PrP(C) in healthy tissues is not known, recent research demonstrates that it binds Cu(II) in an unusual and highly conserved region of the protein termed the octarepeat domain. This review describes recent connections between copper and PrP(C), with an emphasis on the electron paramagnetic resonance elucidation of the specific copper-binding sites, insights into PrP(C) function, and emerging connections between copper and prion disease.

Immunoreactivity enhancement with chelators for increasing the detection sensitivity of human PrPSc by Western blotting

Prion diseases are neurodegenerative disorders affecting humans as Creutzfeldt-Jakob disease. The host-encoded prion protein (PrP(C)) will be converted into a structurally altered isoform (PrP(Sc)). PrP(Sc) differ in sizes and glycoform patterns and can be identified using molecular typing with Western blotting. The electrophoretic mobility of PrP(Sc) changes on treatment with metal ions or chelators prior to digestion with proteases. The effects of chelators applied to PrP(Sc) after protease digestion had not been examined in detail, we investigated these effects in this study. Application of EDTA, NTA and DTPA, and to a lesser extent EGTA, significantly enhanced PrP(Sc) signals in immunoblots. PrP(Sc) intensities increased two- to three-fold compared with untreated PrP(Sc). Since the immunoblot method is highly specific, sensitivity is the limiting factor. Enhancing sensitivity might be important in the determination of PrP(Sc) at levels close to or just below the limits of detection. It is to be expected that application of chelators to digested protein samples will increase the sensitivity of PrP(Sc) detection using the Western blot technique.

Multiple forms of copper (II) co-ordination occur throughout the disordered N-terminal region of the prion protein at pH 7.4

Although the physiological function of the prion protein remains unknown, in vitro experiments suggest that the protein may bind copper (II) ions and play a role in copper transport or homoeostasis in vivo. The unstructured N-terminal region of the prion protein has been shown to bind up to six copper (II) ions, with each of these ions co-ordinated by a single histidine imidazole and nearby backbone amide nitrogen atoms. Individually, these sites have micromolar affinities, which is weaker than would be expected of a true cuproprotein. In the present study, we show that with subsaturating levels of copper, different forms of co-ordination will occur, which have higher affinity. We have investigated the copper-binding properties of two peptides representing the known copper-binding regions of the prion protein: residues 57-91, which contains four tandem repeats of the octapeptide GGGWGQPH, and residues 91-115. Using equilibrium dialysis and spectroscopic methods, we unambiguously demonstrate that the mode of copper co-ordination in both of these peptides depends on the number of copper ions bound and that, at low copper occupancy, copper ions are co-ordinated with sub-micromolar affinity by multiple histidine imidazole groups. At pH 7.4, three different modes of copper co-ordination are accessible within the octapeptide repeats and two within the peptide comprising residues 91-115. The highest affinity copper (II)-binding modes cause self-association of both peptides, suggesting a role for copper (II) in controlling prion protein self-association in vivo.

A reassessment of copper(II) binding in the full-length prion protein

It has been shown previously that the unfolded N-terminal domain of the prion protein can bind up to six Cu2+ ions in vitro. This domain contains four tandem repeats of the octapeptide sequence PHGGGWGQ, which, alongside the two histidine residues at positions 96 and 111, contribute to its Cu2+ binding properties. At the maximum metal-ion occupancy each Cu2+ is co-ordinated by a single imidazole and deprotonated backbone amide groups. However two recent studies of peptides representing the octapeptide repeat region of the protein have shown, that at low Cu2+ availability, an alternative mode of co-ordination occurs where the metal ion is bound by multiple histidine imidazole groups. Both modes of binding are readily populated at pH 7.4, while mild acidification to pH 5.5 selects in favour of the low occupancy, multiple imidazole binding mode. We have used NMR to resolve how Cu2+ binds to the full-length prion protein under mildly acidic conditions where multiple histidine co-ordination is dominant. We show that at pH 5.5 the protein binds two Cu2+ ions, and that all six histidine residues of the unfolded N-terminal domain and the N-terminal amine act as ligands. These two sites are of sufficient affinity to be maintained in the presence of millimolar concentrations of competing exogenous histidine. A previously unknown interaction between the N-terminal domain and a site on the C-terminal domain becomes apparent when the protein is loaded with Cu2+. Furthermore, the data reveal that sub-stoichiometric quantities of Cu2+ will cause self-association of the prion protein in vitro, suggesting that Cu2+ may play a role in controlling oligomerization in vivo.

Computational Comparison of Cation Coordination to Human Prion Peptide Models

The coordination of the cations Cu(II), Co(II), Rh(III), Ir(III), Ni(II), Pd(II), Pt(II), and Zn(II) to the copper-binding octapeptide region in the human prion protein has been compared through structural optimization. The initial coordination mode used in the calculations is a five-coordinated mode obtained from previously published crystallographic data for Cu(II). The computational results show that, among these cations, the coordinations of Co(II) and Rh(III) are the most similar to that of Cu(II). The cations Ni(II), Pd(II), and Pt(II) prefer a four-coordinate square-planar coordination by the peptide ligand. The paramagnetic Co(II) ion with its large quadrupole moment is not a good substitute for Cu(II) to be used in NMR spectroscopic studies of the coordinated peptide region. Rh(III) has more attractive NMR spectroscopic characteristics than Cu(II) and Co(II) and may represent a suitable substitute for Cu(II) in these types of studies. Some preliminary experimental studies using NMR spectroscopic methods indicate that Rh(III) coordinates the copper-binding octapeptide region of the human prion protein, although further studies are required to determine the mode of interaction in detail.

Copper induces conformational changes in the N-terminal part of cell-surface PrPC

Prion diseases are caused by misfolding of the cellular prion protein, PrPC. In vitro studies have shown that PrP binds copper via the octarepeat region lying within the unstructured N-terminal segment of the protein, but the significance of copper in PrP metabolism remains unclear. Here, six specific antibodies recognizing different epitope regions of PrP were used to measure the effect of copper on the conformation of the molecule at the cell surface. Binding of an antibody, E149, to an epitope within the octarepeat domain of PrP is halved in the presence of copper, whereas binding of antibodies recognizing epitope motifs C-terminal to residue 90 of PrP remain relatively unaltered under equivalent conditions. These experiments strongly suggest that copper induces localized conformational change within the N-terminal portion of cell-surface PrPC.

A spectroscopic and voltammetric study of the pH-dependent Cu(II) coordination to the peptide GGGTH: relevance to the fifth Cu(II) site in the prion protein

The GGGTH sequence has been proposed to be the minimal sequence involved in the binding of a fifth Cu(II) ion in addition to the octarepeat region of the prion protein (PrP) which binds four Cu(II) ions. Coordination of Cu(II) by the N- and C-protected Ac-GGGTH-NH(2) pentapeptide (P(5)) was investigated by using potentiometric titration, electrospray ionization mass spectrometry, UV-vis spectroscopy, electron paramagnetic resonance (EPR) spectroscopy and cyclic voltammetry experiments. Four different Cu(II) complexes were identified and characterized as a function of pH. The Cu(II) binding mode switches from NO(3) to N(4) for pH values ranging from 6.0 to 10.0. Quasi-reversible reduction of the [Cu(II)(P(5))H(-2)] complex formed at pH 6.7 occurs at E (1/2)=0.04 V versus Ag/AgCl, whereas reversible oxidation of the [Cu(II)(P(5))H(-3)](-) complex formed at pH 10.0 occurs at E (1/2)=0.66 V versus Ag/AgCl. Comparison of our EPR data with those of the rSHaPrP(90-231) (Burns et al. in Biochemistry 42:6794-6803, 2003) strongly suggests an N(3)O binding mode at physiological pH for the fifth Cu(II) site in the protein.

Modification of blood cell PrP epitope exposure during prion disease

PrPC [normal cellular PrP (prion-related protein)] is a glycosylphosphatidylinositol-linked cell-surface glycoprotein that is expressed primarily by cells of the central and peripheral nervous system and the lymphoreticular system. During prion disease, PrPC undergoes structural modification to PrPSc (abnormal disease-specific conformation of PrP). The appearance of prion infectivity and PrPSc within different peripheral lymphoid tissue sites during natural scrapie infection in sheep is suggestive of haematogenic dissemination. For this to occur, blood cells may harbour or carry disease-associated PrP and in doing so present altered conformations of PrP on their cell-surface. In the present study, we show that changes in PrP epitope expression, or accessibility, can be detected on peripheral blood mononuclear cells during the course of experimental scrapie in susceptible sheep. Peripheral blood mononuclear cells isolated from VRQ homozygous lambs inoculated orally with scrapie were probed with either N- or C-terminal-specific anti-PrP monoclonal antibodies and analysed by flow cytometry. During the progression of scrapie, significant alterations were seen in the exposure of particular cell-surface PrP epitopes. These modifications included increased accessibility to N-terminal regions of the PrP molecule, to the region between beta-strand-2 and residue 171, and to the C-terminal region of helix-3. Increased accessibility in the globular C-terminal domain of PrP occurred in the vicinity of tyrosine dimers, which are believed to have increased solvent exposure in disease-associated PrP. We suggest that the alterations in anti-PrP monoclonal antibody recognition of cell-surface PrP on blood cells from scrapie-infected sheep are indicative of structural changes within this molecule that may be relevant to prion disease.

Synthesis, characterization and biological activity of ternary copper(II) complexes containing polypyridyl ligands

Ternary copper(II) complexes involving polypyridyl ligands in the coordination sphere of composition [Cu(tpy)(phen)](ClO4)2 (1), [Cu(tpy)(bipy)](ClO4)2 (2), [Cu(tptz)(phen)](ClO4)2 (3) and [Cu(tptz)(bipy)](BF4)2 (4) where tpy = 2,2':6',2''-terpyridine, tptz = 2,4,6-tri(2-pyridyl)-1,3,5-triazine, phen = 1,10-phenanthroline and bipy = 2,2'-bipyridine have been synthesized and characterized by elemental analysis, magnetic susceptibility, X-band e.p.r. spectroscopy and electronic spectroscopy. Single crystal X-ray of (1) has revealed the presence of a distorted square pyramidal geometry in the complex. Magnetic susceptibility measurements at room temperature were in the range of 1.77-1.81 BM. SOD and antimicrobial activities of these complexes were also measured. Crystal data of (1): P-1, a = 9.3010(7) A, b = 9.7900(6) A, c = 16.4620(6) A, Vc = 1342.73(14) A3, Z = 4. The bond distance of CuN in square base is 2+/-0.04 A.

Oxidative DNA cleavage mediated by a new copper (II) terpyridine complex: Crystal structure and DNA binding studies

The copper (II) complex [Cu(Itpy)(2)](ClO(4))(2) (1), (Itpy=imidazole terpyridine) has been synthesized and structurally characterized. Crystal structure of the complex shows the complex to be a monomeric copper (II) species with two Itpy ligands coordinated to the metal ion to give a six coordinate complex. The complex has a distorted octahedral geometry with axial elongation. Variable temperature crystal structure data shows dynamic nature of the Jahn-Teller distortion. The complex is an avid DNA binder with a binding constant of 4.26+/-0.20x10(3)M(-1). Observed changes in the viscosity and circular dichroic spectrum of calf thymus DNA solution in the presence of complex 1 suggests intercalative binding of complex 1 to DNA. The complex cleaves supercoiled pBR322 DNA oxidatively in the presence of hydrogen peroxide.

Chemical Exchange Reaction of Glycinatocopper(II) Complex in Water: A Theoretical Study

The axial water exchange on glycinatocopper(II) complexes was theoretically investigated by using density functional theory (DFT) calculations. Glycinatocopper(II) complexes are well-known by the diffusion controlled exchange of axial ligands. Calculations using explicitly coordinating water molecules and solvent models showed that bis-glycinatocopper(II) complexes have a four-coordinate planar structure, in which waters are excluded from the axial positions of Cu(II) due to the Jahn-Teller effect. This may be because coordinating axial waters induce the discrepancy in the most stable ligand field splittings of inner 3d and outer 4d orbitals of the Cu(II) cation. To estimate the reactivity of the axial water exchange, we calculated the rate constant by calculating Gibbs free energies for the activation. As a result, we obtained the rate constant as k = 3.61 x 10(10) s(-1) in aqueous solution at T = 298.15 K. This rate constant is slightly larger than that of the diffusion controlled exchange of axial waters, which is experimentally observed in the order of 10(9) s(-1). Finally, we determined the structures of tris-glycinatocopper(II) complexes. It was consequently found that the third glycine is coordinated to Cu with the amino groups as experimentally observed.

Reactive Oxygen Species-mediated β-Cleavage of the Prion Protein in the Cellular Response to Oxidative Stress

The cellular prion protein (PrP(C)) is critical for the development of prion diseases. However, the physiological role of PrP(C) is less clear, although a role in the cellular resistance to oxidative stress has been proposed. PrP(C) is cleaved at the end of the copper-binding octapeptide repeats through the action of reactive oxygen species (ROS), a process termed beta-cleavage. Here we show that ROS-mediated beta-cleavage of cell surface PrP(C) occurs within minutes and was inhibited by the hydroxyl radical quencher dimethyl sulfoxide and by an antibody against the octapeptide repeats. A construct of PrP lacking the octapeptide repeats, PrPDeltaoct, failed to undergo ROS-mediated beta-cleavage, as did two mutant forms of PrP, PG14 and A116V, associated with human prion diseases. As compared with cells expressing wild type PrP, when challenged with H2O2 and Cu2+, cells expressing PrPdeltaoct, PG14, or A116V had reduced viability and glutathione peroxidase activity and increased intracellular free radicals. Thus, lack of ROS-mediated beta-cleavage of PrP correlated with the sensitivity of the cells to oxidative stress. These data indicate that the beta-cleavage of PrP(C) is an early and critical event in the mechanism by which PrP protects cells against oxidative stress.

The octarepeat domain of the prion protein binds Cu(II) with three distinct coordination modes at pH 7.4

The prion protein (PrP) binds Cu2+ in its N-terminal octarepeat domain. This unusual domain is comprised of four or more tandem repeats of the fundamental sequence PHGGGWGQ. Previous work from our laboratories demonstrates that at full copper occupancy, each HGGGW segment binds a single Cu2+. However, several recent studies suggest that low copper occupancy favors different coordination modes, possibly involving imidazoles from histidines in adjacent octapeptide segments. This is investigated here using a combination of X-band EPR, S-band EPR, and ESEEM, along with a library of modified peptides designed to favor different coordination interactions. At pH 7.4, three distinct coordination modes are identified. Each mode is fully characterized to reveal a series of copper-dependent octarepeat domain structures. Multiple His coordination is clearly identified at low copper stoichiometry. In addition, EPR detected copper-copper interactions at full occupancy suggest that the octarepeat domain partially collapses, perhaps stabilizing this specific binding mode and facilitating cooperative copper uptake. This work provides the first complete characterization of all dominant copper coordination modes at pH 7.4.

Fragmentation and dimerization of copper-loaded prion protein by copper-catalysed oxidation

Prion protein consists of an N-terminal domain containing a series of octapeptide repeats with the consensus sequence PHGGGWGQ and a C-terminal domain composed of three alpha-helices and two short beta-strands. Several studies have shown that the N-terminal domain binds five Cu2+ ions. In the present study, we have investigated copper-catalysed oxidation of a recombinant mouse prion protein, PrP23-231. The copper-loaded PrP23-231 was found to be carbonylated by incubation with dopamine. Besides the formation of carbonyls, a cross-linked species with the dimeric size and C-terminally truncated species were generated. These reactions were retarded in the presence of Cu+- and Cu2+-specific copper chelators, catalase, and SOD (superoxide dismutase), but not in the presence of various bivalent metal ions. Together, these results indicate that the copper bound to prion protein undergoes catalytic cycling in the presence of catecholamines and causes the oxidation of the protein.

Cell-autonomous PrP–Doppel interaction regulates apoptosis in PrP gene-deficient neuronal cells

The Prnd-encoded prion protein (PrP)-like protein, Doppel (Dpl), is a homologue of Prnp-encoded PrP, and is N-glycosylated protein with glycosylphosphatidylinositol anchor like PrP. Recently, ectopic expressions of Prnp/Prnd chimeric mRNAs have been identified as the cause of late-onset ataxia observed in several lines of Prnp-knockout mice such as ZrchII, Ngsk, Rcm0, and Rikn mice. However, it remains unclear whether the toxic effect of Dpl expression is a cell-autonomous mechanism but rather reflect a systemic process of heterogeneous cell population in the brain. In this study, the cell-autonomous role of Dpl was estimated by investigating PrP-deficient cells (HpL3-4)-the SV40 large T-antigen immortalized and Rikn Prnp(-/-) mice-derived neuronal cell line expressing Prnp/Prnd chimeric mRNAs. The reverse transcription polymerase chain reaction revealed that serum deprivation did not increase Prnp/Prnd chimeric mRNAs, which in fact was translated into a small amount of Dpl in HpL3-4 cells, whereas serum deprivation induced apoptotic cell death of HpL3-4 cells. Dpl overexpression enhanced apoptotic cell death, whereas the toxic effect of Dpl on apoptotic cell death was neutralized by PrP expression. These results indicate that Dpl elicited dose-dependently toxic effects on PrP-deficient cells without affecting on PrP-expressing cells, suggesting that the PrP-Dpl interaction can regulate cell death in a cell-autonomous manner.

Copper binding to prion octarepeat peptides, a combined metal chelate affinity and immunochemical approaches

Based on the hypothetical proposal of Sulkowski [E. Sulkowski, FEBS Lett. 307 (2) (1992) 129] for the implication of transition metal ions in the structural changes/oligomerisation of normal cellular prion protein (PrPc) resulting in the pathological isoform (PrPsc), we focused our study on the octarepat domain of this protein which has been supposed to be the metal binding site. We have studied the copper binding to synthetic prion octarepeat peptides (PHGGGWGQ)n (n=1, 3, 6) using metal chelate and size-exclusion modes of chromatographies. This copper binding induces oligomerisation resulting in multiple aggregates. Moreover, heterogeneity of metal bound octarepeat oligomers by ESI-MS has been demonstrated. In addition, anti prion antibodies specific to the octarepeat region were used to discriminate between metal free and copper, nickel and zinc bound hexamer octarepeat peptide. Differential recognition of Cu(II) and Zn(II) bound complexes has been observed which signify differences in exposed epitopes of aggregated peptides.

[Prion protein and copper: a mysterious relationship]

Prion diseases form a group of fatal neurodegenerative disorders including Creutzfeldt-Jakob disease in humans and bovine spongiform encephalopathy in animals. All of which are characterized by the accumulation of abnormally folded isoform of the cellular prion protein (PrP(C)), denoted PrP(Sc), which is the major component of infectious prion diseases. The function of PrP(C) remains elusive. Its amino-terminal region contains a repeated five octapeptide domain that binds copper. The protein is believed to display a superoxide dismutase like activity, and hence a possible protective function against oxidative stress. In this review, relationship between PrP, copper and oxidative stress was analysed. Thus, metal ions and oxidative stress would play an essential role in the pathogenesis of prion diseases and represent important targets for future therapeutic targets or a novel diagnostic marker.

Copper-metalated peptide palindrome derived from prion octarepeat: synthesis, aggregation, and oxidative transformations

We report construction of a bis-pentapeptide conjugate 4 containing truncated pentapeptide sequence from prion octarepeats. Its copper-laden derivative 5 demonstrated a propensity for aggregate formation, which was studied by atomic force microscopy. Oxidative biochemical transformations catalyzed by 5 were evaluated by nucleic acid cleavage and by neurotransmitter oxidation, in the presence of external co-oxidants. Preliminary mechanistic analysis provides a hint for the involvement of reactive oxygen species in such transformations.

Comparative Analysis of the Human and Chicken Prion Protein Copper Binding Regions at pH 6.5

Recent experimental evidence supports the hypothesis that prion proteins (PrPs) are involved in the Cu(II) metabolism. Moreover, the copper binding region has been implicated in transmissible spongiform encephalopathies, which are caused by the infectious isoform of prion proteins (PrP(Sc)). In contrast to mammalian PrP, avian prion proteins have a considerably different N-terminal copper binding region and, most interestingly, are not able to undergo the conversion process into an infectious isoform. Therefore, we applied x-ray absorption spectroscopy to analyze in detail the Cu(II) geometry of selected synthetic human PrP Cu(II) octapeptide complexes in comparison with the corresponding chicken PrP hexapeptide complexes at pH 6.5, which mimics the conditions in the endocytic compartments of neuronal cells. Our results revealed that structure and coordination of the human PrP copper binding sites are highly conserved in the pH 6.5-7.4 range, indicating that the reported pH dependence of copper binding to PrP becomes significant at lower pH values. Furthermore, the different chicken PrP hexarepeat motifs display homologous Cu(II) coordination at sub-stoichiometric copper concentrations. Regarding the fully cation-saturated prion proteins, however, a reduced copper coordination capability is supposed for the chicken prion protein based on the observation that chicken PrP is not able to form an intra-repeat Cu(II) binding site. These results provide new insights into the prion protein structure-function relationship and the conversion process of PrP.

Conformation of prion protein repeat peptides probed by FRET measurements and molecular dynamics simulations

We report the combined use of steady-state fluorescence resonance energy transfer (FRET) experiments and molecular dynamics (MD) simulations to investigate conformational distributions of the prion protein (PrP) repeat system. FRET was used for the first time to probe the distance, as a function of temperature and pH, between a donor Trp residue and an acceptor dansyl group attached to the N-terminus in seven model peptides containing one to three repeats of the second decarepeat of PrP from marsupial possum (PHPGGSNWGQ)nG, and one and two human PrP consensus octarepeats (PHGGGWGQ)nG. In multirepeat peptides, single-Trp mutants were made by replacing other Trp(s) with Phe. As previous work has shown PrP repeats do not adopt a single preferred stable conformation, the FRET values are averages reflecting heterogeneity in the donor-acceptor distances. The T-dependence of the conformational distributions, and derived average dansyl-Trp distances, were obtained directly from MD simulation of the marsupial dansyl-PHPGGSNWGQG peptide. The results show excellent agreement between the FRET and MD T-dependent distances, and demonstrate the remarkable sensitivity and reproducibility of the FRET method in this first-time use for a set of disordered peptides. Based on the results, we propose a model involving cation-pi or pi-pi His-Trp interactions to explain the T- (5-85 degrees C) and pH- (6.0, 7.2) dependencies on distance, with HW i, i + 4 or WH i, i + 4 separations in sequence being more stable than HW i, i + 6 or WH i, i + 6 separations. The model has peptides adopting loosely folded conformations, with dansyl-Trp distances very much less than estimates for fully extended conformations, for example, approximately 16 vs. 33, approximately 21 vs. 69, and approximately 22 vs. 106 A for 1-3 decarepeats, and approximately 14 vs. 25 and approximately 19 vs. 54 A for 1-2 octarepeats, respectively. The study demonstrates the usefulness of combining FRET with MD, a combination reported only once previously. Initial "mapping" of the conformational distribution of flexible peptides by simulation can assist in designing and interpreting experiments using steady-state intensity methods, and indicating how time-resolved or anisotropy methods might be used.

Long Distance Charge Redistribution Upon Cu,Zn-Superoxide Dismutase Reduction

Cu,Zn-superoxide dismutase (Cu,Zn-SOD) is a ubiquitous enzyme with an essential role in antioxidant defense. To better understand structural factors at the origin of the highly efficient superoxide dismutation mechanism, we analyzed the consequence of copper reduction on the electronic properties of the backbone and individual amino acids by using electrochemistry coupled to Fourier transform infrared spectroscopy. Comparison of data recorded with bovine erythrocyte and recombinant chloroplastic Cu,Zn-SOD from Lycopersicon esculentum, expressed as a functional tetramer in Escherichia coli and (14)N- or fully (15)N-labeled, demonstrated that the infrared changes were dominated by reorganizations of peptide bonds and histidine copper ligands. Two main infrared modes of histidine side chain, markers of metal coordination, were identified by using Cu- and Zn-methylimidazole models: the nu(C(4)C(5))at 1605-1594 cm(-1) or approximately 1586 cm(-1) for Ntau or Npi coordination, and the nu(C(5)Ntau) at approximately 1113-1088 cm(-1). These modes, also identified in Cu,Zn-SOD by using (15)N labeling, showed that the electronic properties of the histidine Ntau ligands of copper are mostly affected upon copper reduction. A striking conclusion of this work is that peptide groups from loops and beta-sheet largely participate in charge redistribution upon copper reduction, and in contrast, electronic properties of polar and charged amino acids of the superoxide access channel remain unaffected. This is notably shown for the strictly conserved Arg-143 by site-directed mutagenesis on chloroplastic Cu,Zn-SOD. Charge compensation by the peptide backbone and preserved electronic properties of the superoxide access channel and docking site upon copper reduction may be the determinant factors for the high reaction kinetics of superoxide with both reduced and oxidized Cu,Zn-SOD.

Inhibition of the Escherichia coli RecA protein: zinc(II), copper(II) and mercury(II) trap RecA as inactive aggregates

In bacteria, the RecA protein plays important roles in a number of DNA recombination and repair processes, including homologous recombination, SOS induction and recombinational DNA repair. We have explored the idea that the Escherichia coli RecA protein's functions could be controlled by small molecules. We investigated the 2:1 complex of zinc(II) with 1,4-dithio-l-threitol (l-DTT) that inhibits the E. coli rho transcription terminator, which is a hexameric ATP motor protein and is structurally homologous to RecA. We found that both the complex and ZnCl(2) inhibit the single-stranded DNA-dependent ATPase activity of RecA at sub-millimolar concentrations. Investigation of a variety of metal dications (0.4 mM final concentration) determined that zinc(II), copper(II) and mercury(II) all induce the precipitation of RecA, while the dichloride salts of calcium, manganese, barium, cobalt, and nickel do not. The inhibition of RecA activity by Zn(II), Cu(II) and Hg(II) results from the metal-dependent initiation of RecA aggregation. These observations may have implications for the design of biophysical experiments requiring solid-phase RecA protein, for a more complete understanding of metal toxicities, and for the design of metal-chelate inhibitors of prokaryotic DNA repair.

Copper induces increased beta-sheet content in the scrapie-susceptible ovine prion protein PrPVRQ compared with the resistant allelic variant PrPARR

Prion diseases are characterized by conformational change in the copper-binding protein PrP (prion protein). Polymorphisms in ovine PrP at amino acid residues 136, 154 and 171 are associated with variation in susceptibility to scrapie. PrPVRQ [PrP(Val136/Arg154/Gln171)] or PrPARQ [PrP(Ala136/Arg154/Gln171)] animals show susceptibility to scrapie, whereas those that express Ala136/Arg154/Arg171 (PrPARR) show resistance. Results are presented here that show PrPVRQ and PrPARR display different conformational responses to metal-ion interaction. At 37 degrees C copper induced different levels of b-sheet content in the allelic variants of ovine full-length prion protein (amino acid 25-232). PrPVRQ showed a significant increase in b-sheet content when exposed to copper at 37 degrees C, whereas PrPARR remained relatively unchanged. The conversion of a-helical PrPVRQ to b-sheet form was shown by CD spectroscopy and the decreased binding of C-terminal specific monoclonal anti-PrP antibodies. This conversion to an increased b-sheet form did not occur with truncated PrPVRQ (amino acids 89-233), which demonstrates that additional metal-binding sites outside of the N-terminus may not overtly influence the overall structure of ovine PrP. Despite the difference in b-sheet content, both the scrapie-susceptible and -resistant allelic forms of ovine PrP acquired resistance to proteinase K digestion following exposure to copper at 37 degrees C, suggesting the potential for disease-associated PrPARR to accumulate in vivo. Our present study demonstrates that allelic variants of ovine PrP differ in their structure and response to the interaction with copper. These observations will contribute to a better understanding of the mechanism of susceptibility and resistance to prion disease.

A new method to determine the structure of the metal environment in metalloproteins: investigation of the prion protein octapeptide repeat Cu2+ complex

Since high-intensity synchrotron radiation is available, "extended X-ray absorption fine structure" spectroscopy (EXAFS) is used for detailed structural analysis of metal ion environments in proteins. However, the information acquired is often insufficient to obtain an unambiguous picture. ENDOR spectroscopy allows the determination of hydrogen positions around a metal ion. However, again the structural information is limited. In the present study, a method is proposed which combines computations with spectroscopic data from EXAFS, EPR, electron nuclear double resonance (ENDOR) and electron spin echo envelope modulation (ESEEM). From EXAFS a first picture of the nearest coordination shell is derived which has to be compatible with EPR data. Computations are used to select sterically possible structures, from which in turn structures with correct H and N positions are selected by ENDOR and ESEEM measurements. Finally, EXAFS spectra are re-calculated and compared with the experimental data. This procedure was successfully applied for structure determination of the Cu(2+) complex of the octapeptide repeat of the human prion protein. The structure of this octarepeat complex is rather similar to a pentapeptide complex which was determined by X-ray structure analysis. However, the tryptophan residue has a different orientation: the axial water is on the other side of the Cu.

Ionic strength and transition metals control PrPSc protease resistance and conversion-inducing activity

The essential component of infectious prions is a misfolded protein termed PrPSc, which is produced by conformational change of a normal host protein, PrPC. It is currently unknown whether PrPSc molecules exist in a unique conformation or whether they are able to undergo additional conformational changes. Under commonly used experimental conditions, PrPSc molecules are characteristically protease-resistant and capable of inducing the conversion of PrPC molecules into new PrPSc molecules. We describe the effects of ionic strength, copper, and zinc on the conformation-dependent protease resistance and conversion-inducing activity of PrPSc molecules in scrapie-infected hamster brains. In the absence of divalent cations, PrPSc molecules were > 20-fold more sensitive to proteinase K digestion in low ionic strength buffers than in high ionic strength buffers. Addition of micromolar concentrations of copper or zinc ions restored the protease resistance of PrPSc molecules under conditions of low ionic strength. These transition metals also controlled the conformation of purified truncated PrP-(27-30) molecules at low ionic strength, confirming that the N-terminal octapeptide repeat region of PrPSc is not required for binding to copper or zinc ions. The protease-sensitive and protease-resistant conformations of PrPSc were reversibly interchangeable, and only the protease-resistant conformation of PrPSc induced by high ionic strength was able to induce the formation of new protease-resistant PrP (PrPres) molecules in vitro. These findings show that PrPSc molecules are structurally interconvertible and that only a subset of PrPSc conformations are able to induce the conversion of other PrP molecules.

Copper selectively triggers beta-sheet assembly of an N-terminally truncated amyloid beta-peptide beginning with Glu3

Metal ions have been suggested to induce aggregation of amyloid beta-peptide (Abeta), which is a key event in Alzheimer's disease. However, direct evidence that specific metal-peptide interactions are responsible for the amyloid formation has not previously been provided. Here we present the first example of the metal-induced amyloid formation by an Abeta fragment, which exhibits a clear-cut dependence on the amino acid sequence. A heptapeptide, EFRHDSG, corresponding to the amino acid residues 3-9 of Abeta (Abeta(3-9)) undergoes a conformational transition from irregular to beta-sheet and self-associates into insoluble aggregates upon Cu(II) binding. A Raman spectrum analysis of the Cu(II)-Abeta(3-9) complex and aggregation assays of mutated Abeta(3-9) peptides demonstrated that a concerted Cu(II) coordination of the imidazole side chain of His6, the carboxyl groups of Glu3 and Asp7, and the amino group at the N-terminus is essential for the amyloid formation. Although Abeta(1-9) and Abeta(2-9) also contain the metal binding sites, neither of these peptides forms amyloid depositions in the presence of Cu(II). The results of this study may not only provide new insight into the mechanism of amyloid formation, but also be important as a step toward the construction of proteinaceous materials with a specific function under the control of Cu(II).

Cu2+ binding triggers alphaBoPrP assembly into insoluble laminar polymers

Cu(2+) binding is so far the best characterized property of the prion protein. This interaction has been mapped to the N-terminal domain of the prion protein where multiple His residues occur largely embedded within the repetitive PHGGGWGQ sequence known as octarepeats. When Cu(2+) interaction is studied using a solution of full-length bovine prion protein containing six octarepeats at protein concentrations above 25 microM, a drastic increase in solution turbidity is observed due to the formation of insoluble cation-protein complexes that appear as bidimensional polymer meshes. These bidimensional meshes consist of a single layer of protein molecules crosslinked by Cu(2+) cations. Polymer formation is a cooperative process that proceeds by nucleation of protein molecules with a Cu(2+) site occupancy of above 2. These results support the hypothesis that the N-terminal domain of prion protein is a ligand binding module that promotes crosslinked assembly, and suggest the existence of inter-repeat Cu(2+) sites.

Prion protein suppresses perturbation of cellular copper homeostasis under oxidative conditions

Prion protein (PrP) binds copper and exhibits superoxide dismutase-like activity, while the roles of PrP in copper homeostasis remain controversial. Using Zeeman graphite furnace atomic absorption spectroscopy, we quantified copper levels in immortalized PrP gene (Prnp)-deficient neuronal cells transfected with Prnp and/or Prnd, which encodes PrP-like protein (PrPLP/Dpl), in the presence or absence of oxidative stress induced by serum deprivation. In the presence of serum, copper levels were not significantly affected by the expression of PrP and/or PrPLP/Dpl, whereas serum deprivation induced a decrease in copper levels that was inhibited by PrP but not by PrPLP/Dpl. The inhibitory effect of PrP on the decrease of copper levels was prevented by overexpression of PrPLP/Dpl. These findings indicate that PrP specifically stabilizes copper homeostasis, which is perturbed under oxidative conditions, while PrPLP/Dpl overexpression prevents PrP function in copper homeostasis, suggesting an interaction of PrP and PrPLP/Dpl and distinct functions between PrP and PrPLP/Dpl on metal homeostasis. Taken together, these results strongly suggest that PrP, in addition to its antioxidant properties, plays a role in stabilizing cellular copper homeostasis under oxidative conditions.

Inter- and Intra-octarepeat Cu(II) Site Geometries in the Prion Protein

Cu(II) binding to the alpha prion protein (alphaPrP) can be both intramolecular and intermolecular. X-ray absorption spectroscopy at the copper K-edge has been used to explore the site geometry under each binding mode using both insoluble polymeric Cu(II).alphaBoPrP-(24-242) (bovine PrP) complexes and soluble Cu(II) complexes of peptides containing one, two, and four copies of the octarepeat. Analysis of the extended region of the spectra using a multiple scattering approach revealed two types of sites differing in the number of His residues in the first coordination shell of Cu(II). Peptides containing one and two-octarepeat copies in sub-stoichiometric Cu(II) complexes showed the direct binding of a single His in accord with crystallographic intra-repeat geometry. Alternatively, the polymeric Cu(II).alphaBoPrP-(24-242) complex and Cu(II) in its soluble complex with a four-octarepeat peptide at half-site-occupancy showed Cu(II) directly bound to two His residues, consistent with an inter-repeat binding mode. Increasing the Cu(II) site occupancy from 0.5 to 0.75 in the peptide containing four octarepeats resulted in spectral features that are intermediate to those of the inter- and intra-repeat modes. The transition from His-Cu-His (inter-repeat) to Cu-His (intra-repeat) on increasing Cu(II) saturation offers a structural basis for the positive cooperativity of the cation binding process and explains the capacity of alphaPrP to participate in Cu(II)-mediated intermolecular interactions.

Quinoline derivatives are therapeutic candidates for transmissible spongiform encephalopathies

We previously reported that quinacrine inhibited the formation of an abnormal prion protein (PrPres), a key molecule in the pathogenesis of transmissible spongiform encephalopathy, or prion disease, in scrapie-infected neuroblastoma cells. To elucidate the structural aspects of its inhibiting action, various chemicals with a quinoline ring were screened in the present study. Assays of the scrapie-infected neuroblastoma cells revealed that chemicals with a side chain containing a quinuclidine ring at the 4 position of a quinoline ring (represented by quinine) inhibited the PrPres formation at a 50% inhibitory dose ranging from 10(-1) to 10(1) micro M. On the other hand, chemicals with a side chain at the 2 position of a quinoline ring (represented by 2,2'-biquinoline) more effectively inhibited the PrPres formation at a 50% inhibitory dose ranging from 10(-3) to 10(-1) micro M. A metabolic labeling study revealed that the action of quinine or biquinoline was not due to any alteration in the biosynthesis or turnover of normal prion protein, whereas surface plasmon resonance analysis showed a strong binding affinity of biquinoline with a recombinant prion protein. In vivo studies revealed that 4-week intraventricular infusion of quinine or biquinoline was effective in prolonging the incubation period in experimental mouse models of intracerebral infection. The findings suggest that quinoline derivatives with a nitrogen-containing side chain have the potential of both inhibiting PrPres formation in vitro and prolonging the incubation period of infected animals. These chemicals are new candidates for therapeutic drugs for use in the treatment of transmissible spongiform encephalopathies.

Copper binding in the prion protein

A conformational change of the prion protein is responsible for a class of neurodegenerative diseases called the transmissible spongiform encephalopathies that include mad cow disease and the human afflictions kuru and Creutzfeldt-Jakob disease. Despite the attention given to these diseases, the normal function of the prion protein in healthy tissue is unknown. Research over the past few years, however, demonstrates that the prion protein is a copper binding protein with high selectivity for Cu(2+). The structural features of the Cu(2+) binding sites have now been characterized and are providing important clues about the normal function of the prion protein and perhaps how metals or loss of protein function play a role in disease. The link between prion protein and copper may provide insight into the general, and recently appreciated, role of metals in neurodegenerative disease.

Effects of different experimental conditions on the PrPSc core generated by protease digestion: implications for strain typing and molecular classification of CJD

The discovery of molecular subtypes of the pathological prion protein PrPSc has provided the basis for a novel classification of human transmissible spongiform encephalopathies (TSEs) and a potentially powerful method for strain typing. However, there is still a significant disparity regarding the understanding and nomenclature of PrPSc types. In addition, it is still unknown whether a specific PrPSc type is associated with each TSE phenotypic variant. In sporadic Creutzfeldt-Jakob disease (sCJD), five disease phenotypes are known, but only two major types of PrPSc, types 1 and 2, have been consistently reproduced. We further analyzed PrPSc properties in sCJD and variant CJD using a high resolution gel electrophoresis system and varying experimental conditions. We found that pH varies among CJD brain homogenates in standard buffers, thereby influencing the characteristics of protease-treated PrPSc. We also show that PrPSc type 1 and type 2 are heterogeneous species which can be further distinguished into five molecular subtypes that fit the current histopathological classification of sCJD variants. Our results shed light on previous disparities in PrPSc typing, provide a refined classification of human PrPSc types, and support the notion that the pathological TSE phenotype is related to PrPSc structure.

PrPc on the road: trafficking of the cellular prion protein

The glycosylphosphatidylinositol (GPI)-anchored cellular prion protein (PrPc) has a fundamental role in prion diseases. Intracellular trafficking of PrPc is important in the generation of protease resistant PrP species but little is known of how endocytosis affects PrPc function. Here, we discuss recent experiments that have illuminated how PrPc is internalized and what are the possible destinations taken by the protein. Contrary to what would be expected for a GPI-anchored protein there is increasing evidence that clathrin-mediated endocytosis and classical endocytic organelles participate in PrPc trafficking. Moreover, the N-terminal domain of PrPc may be involved in sorting events that can direct the protein during its intracellular journey. Indeed, the concept that the GPI-anchor determines PrPc trafficking has been challenged. Cellular signaling can be triggered or be regulated by PrPc and we suggest that endocytosis of PrPc may influence signaling in several ways. Definition of the processes that participate in PrPc endocytosis and intracellular trafficking can have a major impact on our understanding of the mechanisms involved in PrPc function and conversion to protease resistant conformations.

Micellar environments induce structuring of the N-terminal tail of the prion protein

In the physiological form, the prion protein is a glycoprotein tethered to the cell surface via a C-terminal glycosylphosphatidylinositol anchor, consisting of a largely alpha-helical globular C-terminal domain and an unstructured N-terminal portion. This unstructured part of the protein contains four successive octapeptide repeats, which were shown to bind up to four Cu(2+) ions in a cooperative manner. To mimic the location of the protein on the cell membrane and to analyze possible structuring effects of the lipid/water interface, the conformational preferences of a single octapeptide repeat and its tetrameric form, as well of the fragment 92-113, proposed as an additional copper binding site, were comparatively analyzed in aqueous and dodecylphosphocholine micellar solution as a membrane mimetic. While for the downstream fragment 92-113 no conformational effects were detectable in the presence of DPC micelles by CD and NMR, both the single octapeptide repeat and, in an even more pronounced manner, its tetrameric form are restricted into well-defined conformations. Because of the repetitive character of the rigid structural subdomain in the tetrarepeat molecule, the spatial arrangement of these identical motifs could not be resolved by NMR analysis. However, the polyvalent nature of the repetitive subunits leads to a remarkably enhanced interaction with the micelles, which is not detectably affected by copper complexation. These results strongly suggest interactions of the cellular form of PrP (PrP(c)) N-terminal tail with the cell membrane surface at least in the octapeptide repeat region with preorganization of these sequence portions for copper complexation. There are sufficient experimental facts known that support a physiological role of copper complexation by the octapeptide repeat region of PrP(c) such as a copper-buffering role of the PrP(c) protein on the extracellular surface.

Cellular prion protein acquires resistance to proteolytic degradation following copper ion binding

The conversion of cellular prion protein (PrP(C)) into its pathological isoform (PrP(Sc)) conveys an increase in hydrophobicity and induces a partial resistance to proteinase K (PK). Interestingly, co-incubation with high copper ion concentrations also modifies the solubility of PrP(c) and induces a partial PK resistance which was reminiscent of PrP(Sc). However, concerns were raised whether this effect was not due to a copper-induced inhibition of the PK itself. We have therefore analyzed the kinetics of the formation of PK-resistant PrP(C) and excluded possible interference effects by removing unbound copper ions prior to the addition of PK by methanol precipitation or immobilization of PrP(C) followed by washing steps. We found that preincubation of PrPc with copper ions at concentrations as low as 50 microM indeed rendered these proteins completely PK resistant, while control substrates were proteolyzed. No other divalent cations induced a similar effect. However, in addition to this specific stabilizing effect on PrP(C), higher copper ion concentrations in solution (>200 microM) directly blocked the enzymatic activity of PK, possibly by replacing the Ca2+ ions in the active center of the enzyme. Therefore, as a result of this inhibition the proteolytic degradation of PrP(C) as well as PrP(Sc) molecules was suppressed.

Raman and IR study on copper binding of histamine

A comparative Raman and FTIR study of histamine (Hm), a small hormone present in a wide selection of living organisms, and its complexes with copper(II) at different pH values was carried out. Both the Raman and IR spectra present some marker bands useful for the identification of the structure of the species predominating in the Cu(II) aqueous and alcoholic systems. In particular, Raman spectroscopy appears to be a useful tool for analyzing the tautomeric equilibrium of the imidazole ring of Hm, because some bands (i.e., nuC(4)dbond;C(5)) appear at different wavenumbers, depending on whether the imidazole moiety is in the N(tau)-H (tautomer I) or N(pi)-H (tautomer II) protonated form. In aqueous solutions the manner in which Hm binds to Cu(II) depends on the pH. At basic pH the most relevant species formed are a dimer, [Cu(2)L(2)H(-2)](2+), and a monomeric complex, [CuL](2-) or [CuL(2)](+). On the contrary, by decreasing the pH, Hm acts as a mono- or bidentate ligand, giving rise to two types of monomeric complexes, [CuLH](2-) and [CuL](2-) or [CuL(2)](+). With respect to the Cu(II)-Hm alcoholic system, both the aminic group and the imidazole ring (tautomer I) take part in the Cu(II) coordination, leading to the formation of the [CuL](2-) or [CuL(2)](+) monomeric complex.

Cu nanocrystal growth on peptide nanotubes by biomineralization: size control of Cu nanocrystals by tuning peptide conformation

With recent interest in seeking new biologically inspired device-fabrication methods in nanotechnology, a new biological approach was examined to fabricate Cu nanotubes by using sequenced histidine-rich peptide nanotubes as templates. The sequenced histidine-rich peptide molecules were assembled as nanotubes, and the biological recognition of the specific sequence toward Cu lead to efficient Cu coating on the nanotubes. Cu nanocrystals were uniformly coated on the histidine-incorporated nanotubes with high packing density. In addition, the diameter of Cu nanocrystal was controlled between 10 and 30 nm on the nanotube by controlling the conformation of histidine-rich peptide by means of pH changes. Those nanotubes showed significant change in electronic structure by varying the nanocrystal diameter; therefore, this system may be developed to a conductivity-tunable building block for microelectronics and biological sensors. This simple biomineralization method can be applied to fabricate various metallic and semiconductor nanotubes with peptides whose sequences are known to mineralize specific ions.

Raman structural markers of tryptophan and histidine side chains in proteins

The Raman spectrum of a protein contains a wealth of information on the structure and interaction of the protein. To extract the structural information from the Raman spectrum, it is necessary to identify and interpret the marker bands that reflect the structure and interaction in the protein. Recently, new Raman structural markers have been proposed for the tryptophan and histidine side chains by examining the spectra-structure correlations of model compounds. Raman structural markers are now available for the conformation, hydrogen bonding, hydrophobic interaction, and cation-pi interaction of the indole ring of Trp. For His, protonation, tautomerism, and metal coordination of the imidazole ring can be studied by using Raman markers. The high-resolution X-ray crystal structures of proteins provide the basis for testing and modifying the Raman structural markers of Trp and His. The structures derived from Raman spectra are generally consistent with the X-ray crystal structures, giving support for the applicability of most Raman structural makers. Possible modifications and limitations to some marker bands are also discussed.

Copper Chelation Delays the Onset of Prion Disease

The prion protein (PrP) binds copper and under some conditions copper can facilitate its folding into a more protease resistant form. Hence, copper levels may influence the infectivity of the scrapie form of prion protein (PrPSc). To determine the feasibility of copper-targeted therapy for prion disease, we treated mice with a copper chelator, D-(-)-penicillamine (D-PEN), starting immediately following intraperitoneal scrapie inoculation. D-PEN delayed the onset of prion disease in the mice by about 11 days (p = 0.002), and reduced copper levels in brain by 29% (p < 0.01) and in blood by 22% (p = 0.03) compared with control animals. Levels of other metals were not significantly altered in the blood or brain. Modest correlation was observed between incubation period and levels of copper in brain (p = 0.08) or blood (p = 0.04), indicating that copper levels are only one of many factors that influence the rate of progression of prion disease. In vitro, copper dose-dependently enhanced the proteinase K resistance of the prion protein, and this effect was counteracted in a dose-dependent manner by co-incubation with D-PEN. Overall, these findings indicate that copper levels can influence the conformational state of PrP, thereby enhancing its infectivity, and this effect can be attenuated by chelator-based therapy.

Copper in the silk formation process ofBombyx morisilkworm

Evidence is presented here that cupric ions play a part in the natural spinning of Bombyx mori silk. Proton induced X-ray emission studies revealed that the copper content increased from the posterior part to the anterior part of silk gland, and then further increased in the silk fiber. Spectrophotometric analysis demonstrated that cupric ions formed coordination complexes with silk fibroin chains while Raman spectroscopy indicated that they induced a conformation transition from random coil/helix to beta-sheet. Taken together these findings indicate that copper could play a role in the natural spinning process in silkworms.

Characterization of 2'-fluoro-RNA aptamers that bind preferentially to disease-associated conformations of prion protein and inhibit conversion

We have isolated artificial ligands or aptamers for infectious prions in order to investigate conformational aspects of prion pathogenesis. The aptamers are 2'-fluoro-modified RNA produced by in vitro selection from a large, randomized library. One of these ligands (aptamer SAF-93) had more than 10-fold higher affinity for PrPSc than for recombinant PrPC and inhibited the accumulation of PrPres in near physiological cell-free conversion assay. To understand the molecular basis of these properties and to distinguish specific from non-specific aptamer-PrP interactions, we studied deletion mutants of bovine PrP in denatured, alpha-helix-rich and beta-sheet-rich forms. We provide evidence that, like scrapie-associated fibrils (SAF), the beta-oligomer of PrP bound to SAF-93 with at least 10-fold higher affinity than did the alpha-form. This differential affinity could be explained by the existence of two binding sites within the PrP molecule. Site 1 lies within residues 23-110 in the unstructured N terminus and is a nonspecific RNA binding site found in all forms of PrP. The region between residue 90 and 110 forms a hinge region that is occluded in the alpha-rich form of PrP but becomes exposed in the denatured form of PrP. Site 2 lies in the region C-terminal of residue 110. This site is beta-sheet conformation-specific and is not recognized by control RNAs. Taken together, these data provide for the first time a specific ligand for a disease conformation-associated site in a region of PrP critical for conformational conversion. This aptamer could provide tools for the further analysis of the processes of PrP misfolding during prion disease and leads for the development of diagnostic and therapeutic approaches to TSEs.

Copper and zinc cause delivery of the prion protein from the plasma membrane to a subset of early endosomes and the Golgi

The cellular isoform of prion protein (PrPC) is a plasma membrane glycoprotein whose conformational conversion into PrPSc is the central molecular event in the propagation of infectious prions. However, the physiological function of PrPC has remained uncertain. The finding that PrPC binds copper ions with low micromolar affinity, coupled with several other observations, has led to the proposal that the protein plays a role in copper homeostasis. Using biochemical techniques, we had shown previously that copper ions rapidly and reversibly stimulate endocytosis of PrPC from the cell surface. In this report, we employ immunofluorescence microscopy to further investigate the specificity and kinetics of metal effects on PrPC trafficking and to identify the intracellular compartments to which internalized PrPC is delivered in response to copper and zinc. We find that both of these metals stimulate redistribution of surface PrPC to a subset of transferrin-containing early endosomes as well as to Golgi compartments. These results are consistent with models in which PrPC plays a role in the cellular uptake or efflux of transition metals.

Cellular prion protein function in copper homeostasis and redox signalling at the synapse

The fundamental physiological function of native cellular prion (PrPC) remains unknown. Herein, the most salient observations as regards prion physiology are critically evaluated. These include: (i) the role of PrPC in copper homeostasis, particularly at the pre-synaptic membrane; (ii) involvement of PrPC in neuronal calcium disturbances; and (iii) the neuroprotective properties of PrPC in response to copper and oxidative stress. Ultimately, a tentative hypothesis of basic prion function is derived, namely that PrPC acts as a sensor for copper and/or free radical stimuli, thereby triggering intracellular calcium signals that finally translate into modulation of synaptic transmission and maintenance of neuronal integrity.

Synthesis, structure and nuclease properties of several ternary copper(II) peptide complexes with 1,10-phenanthroline

Three new ternary peptide-Cu(II)-1,10-phenanthroline (phen) complexes, [Cu(L-ala-gly)(phen)].3.5H(2)O 1, [Cu(L-val-gly)(phen)] 2 and [Cu(gly-L-trp)(phen)].2H(2)O 3, have been prepared and structurally characterised. These compounds exist as distorted square pyramidal complexes with the five co-ordination sites occupied by the tridentate peptide dianion and the two heterocyclic nitrogens of the phenanthroline ligand. The bulk of the lateral chain in the peptide moiety determines the relative disposition of the phen ligand. Thus, in [Cu(L-val-gly)(phen)] 2, the phenanthroline plane is deviated towards the opposite side of the isopropyl group of the L-valine moiety. On the other hand, in [Cu(gly-L-trp)(phen)].2H(2)O 3 the absence of stacking interactions between phen and indole rings and the presence of an intramolecular CH...pi interaction should be pointed out. These complexes exhibit significant differences in their nuclease activity which depends on the nature of the peptidic moiety, the complex [Cu(gly-L-trp) (phen)].2H(2)O 3 being the most active.