Copper and zinc promote interactions between membrane-anchored peptides of the metal binding domain of the prion protein

A voltammetric peptide biosensor for Cu2+ metal ion quantification in coffee seeds

A prion-derived copper(II)-binding peptide was assembled onto a gold electrode for the building of a voltammetric biosensor for measuring the Cu2+ metal ion in biological samples. The chosen sequence was H-CVNITKQHTVTTTT-NH2, with an appended cysteine residue for binding to the gold surface as a self-assembled monolayer and a histidine residue as the anchorage point for copper(II) complexation. The biosensor showed a linear range of 10-7 to 10-6 M with an 8.0 × 10-8 M detection limit and a 1.0 × 10-7 M quantification limit, with good precision, trueness, and absence of matrix effect. The quantification of Cu2+ was performed in the presence of other transition metal ions, such as Zn2+, Cd2+, Fe2+, or Ni2+, which indicates the excellent selectivity of the biosensor. When the modified electrode was applied for measuring copper(II) in calcined coffee seeds, a difference in copper amount was observed between two Coffea arabica cultivars that were submitted to a treatment with a copper-based antifungal, showing the applicability of the biosensor in the agricultural field.

Formation of Supplementary Metal-Binding Centers in Proteins under Stress Conditions

In many proteins, supplementary metal-binding centers appear under stress conditions. They are known as aberrant or atypical sites. Physico-chemical properties of proteins are significantly changed after such metal binding, and very stable protein aggregates are formed, in which metals act as "cross-linking" agents. Supplementary metal-binding centers in proteins often arise as a result of posttranslational modifications caused by reactive oxygen and nitrogen species and reactive carbonyl compounds. New chemical groups formed as a result of these modifications can act as ligands for binding metal ions. Special attention is paid to the role of cysteine SH-groups in the formation of supplementary metal-binding centers, since these groups are the main target for the action of reactive species. Supplementary metal binding centers may also appear due to unmasking of amino acid residues when protein conformation changing. Appearance of such centers is usually considered as a pathological process. Such unilateral approach does not allow to obtain an integral view of the phenomenon, ignoring cases when formation of metal complexes with altered proteins is a way to adjust protein properties, activity, and stability under the changed redox conditions. The role of metals in protein aggregation is being studied actively, since it leads to formation of non-membranous organelles, liquid condensates, and solid conglomerates. Some proteins found in such aggregates are typical for various diseases, such as Alzheimer's and Huntington's diseases, amyotrophic lateral sclerosis, and some types of cancer.

Second Sphere Interactions in Amyloidogenic Diseases

Amyloids are protein aggregates bearing a highly ordered cross β structural motif, which may be functional but are mostly pathogenic. Their formation, deposition in tissues and consequent organ dysfunction is the central event in amyloidogenic diseases. Such protein aggregation may be brought about by conformational changes, and much attention has been directed toward factors like metal binding, post-translational modifications, mutations of protein etc., which eventually affect the reactivity and cytotoxicity of the associated proteins. Over the past decade, a global effort from different groups working on these misfolded/unfolded proteins/peptides has revealed that the amino acid residues in the second coordination sphere of the active sites of amyloidogenic proteins/peptides cause changes in H-bonding pattern or protein-protein interactions, which dramatically alter the structure and reactivity of these proteins/peptides. These second sphere effects not only determine the binding of transition metals and cofactors, which define the pathology of some of these diseases, but also change the mechanism of redox reactions catalyzed by these proteins/peptides and form the basis of oxidative damage associated with these amyloidogenic diseases. The present review seeks to discuss such second sphere modifications and their ramifications in the etiopathology of some representative amyloidogenic diseases like Alzheimer's disease (AD), type 2 diabetes mellitus (T2Dm), Parkinson's disease (PD), Huntington's disease (HD), and prion diseases.

Zn(II) binding causes interdomain changes in the structure and flexibility of the human prion protein

The cellular prion protein (PrPC) is a mainly α-helical 208-residue protein located in the pre- and postsynaptic membranes. For unknown reasons, PrPC can undergo a structural transition into a toxic, β-sheet rich scrapie isoform (PrPSc) that is responsible for transmissible spongiform encephalopathies (TSEs). Metal ions seem to play an important role in the structural conversion. PrPC binds Zn(II) ions and may be involved in metal ion transport and zinc homeostasis. Here, we use multiple biophysical techniques including optical and NMR spectroscopy, molecular dynamics simulations, and small angle X-ray scattering to characterize interactions between human PrPC and Zn(II) ions. Binding of a single Zn(II) ion to the PrPC N-terminal domain via four His residues from the octarepeat region induces a structural transition in the C-terminal α-helices 2 and 3, promotes interaction between the N-terminal and C-terminal domains, reduces the folded protein size, and modifies the internal structural dynamics. As our results suggest that PrPC can bind Zn(II) under physiological conditions, these effects could be important for the physiological function of PrPC.

Zinc-dysprosium functionalized amyloid fibrils

The heterometallic Zn2Dy2 entity bearing partially saturated metal centres covalently decorates a highly ordered amyloid fibril core and the functionalised assembly exhibits catalytic Lewis acid behaviour.

Serum zinc levels in acute psychiatric patients: A case series

Zinc dysregulation is linked to neuropsychiatric disorders and a beneficial response to zinc supplementation has been demonstrated for depression. In this case series, we examined serum zinc levels with respect to clinical factors among 20 acutely ill psychiatric cases admitted to a large urban public hospital. The results showed frank clinical zinc insufficiency in a quarter of the subjects. Group-wise analyses showed a significant association between reduced serum zinc and diagnosis of depression, and reduced serum zinc in those with aggressive, assaultive, or violent behaviors. By contrast, relatively elevated zinc levels were observed in a subset of psychotic cases on antipsychotics and mood stabilizers who had no mood symptoms. In summary, clinical zinc insufficiency was common in these acutely admitted psychiatric cases. Zinc supplementation may ameliorate symptoms in certain cases and should be considered in treatment planning. A separate patient group had elevated zinc levels, which could conceivably be pathogenic. Larger studies are needed to confirm and extend this pilot data.

Pyrene conjugation and spectroscopic analysis of hydroxypropyl methylcellulose compounds successfully demonstrated a local dielectric difference associated with in vivo anti-prion activity

Our previous study on prion-infected rodents revealed that hydroxypropyl methylcellulose compounds (HPMCs) with different molecular weights but similar composition and degree of substitution have different levels of long-lasting anti-prion activity. In this study, we searched these HPMCs for a parameter specifically associated with in vivo anti-prion activity by analyzing in vitro chemical properties and in vivo tissue distributions. Infrared spectroscopic and thermal analyses revealed no differences among HPMCs, whereas pyrene conjugation and spectroscopic analysis revealed that the fluorescence intensity ratio of peak III/peak I correlated with anti-prion activity. This correlation was more clearly demonstrated in the anti-prion activity of the 1-year pre-infection treatment than that of the immediate post-infection treatment. In addition, the intensity ratio of peak III/peak I negatively correlated with the macrophage uptake level of HPMCs in our previous study. However, the in vivo distribution pattern was apparently not associated with anti-prion activity and was different in the representative tissues. These findings suggest that pyrene conjugation and spectroscopic analysis are powerful methods to successfully demonstrate local dielectric differences in HPMCs and provide a feasible parameter denoting the long-lasting anti-prion activity of HPMCs in vivo.

Metal Dyshomeostasis and Their Pathological Role in Prion and Prion-Like Diseases: The Basis for a Nutritional Approach

Metal ions are key elements in organisms' life acting like cofactors of many enzymes but they can also be potentially dangerous for the cell participating in redox reactions that lead to the formation of reactive oxygen species (ROS). Any factor inducing or limiting a metal dyshomeostasis, ROS production and cell injury may contribute to the onset of neurodegenerative diseases or play a neuroprotective action. Transmissible spongiform encephalopathies (TSEs), also known as prion diseases, are a group of fatal neurodegenerative disorders affecting the central nervous system (CNS) of human and other mammalian species. The causative agent of TSEs is believed to be the scrapie prion protein PrP^Sc, the β sheet-rich pathogenic isoform produced by the conformational conversion of the α-helix-rich physiological isoform PrP^C. The peculiarity of PrP^Sc is its ability to self-propagate in exponential fashion in cells and its tendency to precipitate in insoluble and protease-resistance amyloid aggregates leading to neuronal cell death. The expression “prion-like diseases” refers to a group of neurodegenerative diseases that share some neuropathological features with prion diseases such as the involvement of proteins (α-synuclein, amyloid β, and tau) able to precipitate producing amyloid deposits following conformational change. High social impact diseases such as Alzheimer's and Parkinson's belong to prion-like diseases. Accumulating evidence suggests that the exposure to environmental metals is a risk factor for the development of prion and prion-like diseases and that metal ions can directly bind to prion and prion-like proteins affecting the amount of amyloid aggregates. The diet, source of metal ions but also of natural antioxidant and chelating agents such as polyphenols, is an aspect to take into account in addressing the issue of neurodegeneration. Epidemiological data suggest that the Mediterranean diet, based on the abundant consumption of fresh vegetables and on low intake of meat, could play a preventive or delaying role in prion and prion-like neurodegenerative diseases. In this review, metal role in the onset of prion and prion-like diseases is dealt with from a nutritional, cellular, and molecular point of view.

Copper and Zinc Interactions with Cellular Prion Proteins Change Solubility of Full-Length Glycosylated Isoforms and Induce the Occurrence of Heterogeneous Phenotypes

Prion diseases are characterized biochemically by protein aggregation of infectious prion isoforms (PrP^Sc), which result from the conformational conversion of physiological prion proteins (PrP^C). PrP^C are variable post-translationally modified glycoproteins, which exist as full length and as aminoterminally truncated glycosylated proteins and which exhibit differential detergent solubility. This implicates the presence of heterogeneous phenotypes, which overlap as protein complexes at the same molecular masses. Although the biological function of PrP^C is still enigmatic, evidence reveals that PrP^C exhibits metal-binding properties, which result in structural changes and decreased solubility. In this study, we analyzed the yield of PrP^C metal binding affiliated with low solubility and changes in protein banding patterns. By implementing a high-speed centrifugation step, the interaction of zinc ions with PrP^C was shown to generate large quantities of proteins with low solubility, consisting mainly of full-length glycosylated PrP^C; whereas unglycosylated PrP^C remained in the supernatants as well as truncated glycosylated proteins which lack of octarepeat sequence necessary for metal binding. This effect was considerably lower when PrP^C interacted with copper ions; the presence of other metals tested exhibited no effect under these conditions. The binding of zinc and copper to PrP^C demonstrated differentially soluble protein yields within distinct PrP^C subtypes. PrP^C–Zn²⁺-interaction may provide a means to differentiate glycosylated and unglycosylated subtypes and offers detailed analysis of metal-bound and metal-free protein conversion assays.

Zinc: indications in brain disorders

Zinc is the authoritative metal which is present in our body, and reactive zinc metal is crucial for neuronal signaling and is largely distributed within presynaptic vesicles. Zinc also plays an important role in synaptic function. At cellular level, zinc is a modulator of synaptic activity and neuronal plasticity in both development and adulthood. Different importers and transporters are involved in zinc homeostasis. ZnT-3 is a main transporter involved in zinc homeostasis in the brain. It has been found that alterations in brain zinc status have been implicated in a wide range of neurological disorders including impaired brain development and many neurodegenerative disorders such as Alzheimer's disease, and mood disorders including depression, Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis, and prion disease. Furthermore, zinc has also been implicated in neuronal damage associated with traumatic brain injury, stroke, and seizure. Understanding the mechanisms that control brain zinc homeostasis is thus critical to the development of preventive and treatment strategies for these and other neurological disorders.

Evolutionary implications of metal binding features in different species' prion protein: an inorganic point of view

Prion disorders are a group of fatal neurodegenerative conditions of mammals. The key molecular event in the pathogenesis of such diseases is the conformational conversion of prion protein, PrPC, into a misfolded form rich in β-sheet structure, PrPSc, but the detailed mechanistic aspects of prion protein conversion remain enigmatic. There is uncertainty on the precise physiological function of PrPC in healthy individuals. Several evidences support the notion of its role in copper homeostasis. PrPC binds Cu2+ mainly through a domain composed by four to five repeats of eight amino acids. In addition to mammals, PrP homologues have also been identified in birds, reptiles, amphibians and fish. The globular domain of protein is retained in the different species, suggesting that the protein carries out an essential common function. However, the comparison of amino acid sequences indicates that prion protein has evolved differently in each vertebrate class. The primary sequences are strongly conserved in each group, but these exhibit a low similarity with those of mammals. The N-terminal domain of different prions shows tandem amino acid repeats with an increasing amount of histidine residues going from amphibians to mammals. The difference in the sequence affects the number of copper binding sites, the affinity and the coordination environment of metal ions, suggesting that the involvement of prion in metal homeostasis may be a specific characteristic of mammalian prion protein. In this review, we describe the similarities and the differences in the metal binding of different species' prion protein, as revealed by studies carried out on the entire protein and related peptide fragments.

Neuronal zinc regulation and the prion protein

Zinc, the most abundant trace metal in the brain, has numerous functions in health and disease. It is released into the synaptic cleft alongside glutamate and this connection between zinc and glutamatergic neurotransmission allows the ion to modulate overall excitability of the brain and influence synaptic plasticity. To maintain healthy synapses, extracellular zinc levels need to be tightly regulated. We recently reported that the cellular prion protein (PrP (C) ) can directly influence neuronal zinc concentrations by promoting zinc uptake via AMPA receptors. The octapeptide repeat region of PrP (C) is involved in zinc sensing or scavenging and the AMPA receptor provides the channel for transport of the metal across the membrane, facilitated by a direct interaction between the N-terminal polybasic region of PrP (C) and AMPA receptors. PrP (C) has been evolutionarily linked to the Zrt/Irt-like protein (ZIP) metal ion transport family with the C-terminus of PrP (C) sharing sequence similarities with the N-terminal extracellular domains of ZIP 5, 6 and 10. By incorporating the properties of ZIP transporters (both zinc sensing and zinc transport) into two existing neuronal proteins, (PrP (C) as zinc sensor, AMPA receptor as zinc transporter), neuronal cells are enhancing their biological efficiency and functionality.

Prion protein facilitates uptake of zinc into neuronal cells

Zinc is released into the synaptic cleft upon exocytotic stimuli, although the mechanism for its reuptake into neurons is unresolved. Here we show that the cellular prion protein enhances the uptake of zinc into neuronal cells. This prion-protein-mediated zinc influx requires the octapeptide repeats and amino-terminal polybasic region in the prion protein, but not its endocytosis. Selective antagonists of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors block the prion protein-mediated zinc uptake, and the prion protein co-immunoprecipitates with both GluA1 and GluA2 AMPA receptor subunits. Zinc-sensitive intracellular tyrosine phosphatase activity is decreased in cells expressing prion protein and increased in the brains of prion-protein-null mice, providing evidence of a physiological consequence of this process. Prion protein-mediated zinc uptake is ablated in cells expressing familial associated mutants of the protein and in prion-infected cells. These data suggest that alterations in the cellular prion protein-mediated zinc uptake may contribute to neurodegeneration in prion and other neurodegenerative diseases.

Role of metal ions in aggregation of intrinsically disordered proteins in neurodegenerative diseases

Neurodegenerative diseases constitute a set of pathological conditions originating from the slow, irreversible, and systematic cell loss within the various regions of the brain and/or the spinal cord. Depending on the affected region, the outcomes of the neurodegeneration are very broad and diverse, ranging from the problems with movements to dementia. Some neurodegenerative diseases are associated with protein misfolding and aggregation. Many proteins that misfold in human neurodegenerative diseases are intrinsically disordered; i.e., they lack a stable tertiary and/or secondary structure under physiological conditions in vitro. These intrinsically disordered proteins (IDPs) functionally complement ordered proteins, being typically involved in regulation and signaling. There is accumulating evidence that altered metal homeostasis may be related to the progression of neurodegenerative diseases. This review examines the effects of metal ion binding on the aggregation pathways of IDPs found in neurodegenerative diseases.

Physicochemical characterisation of four peptide sequences related to thrombospondin-1B

Thrombospondin-1 (TSP-1) is a protein involved in angiogenesis and tumor metastasis. In a previous study, a tridecapeptide sequence of TSP-1B [KRFKQDGGWSHWG] was synthesized and its biological activity was determined as well as the activity of three related sequences TSPB-(E), TSPB-(S), and TSPB-(Abu)(6). These peptides were tested for activity on the cell growth of three human carcinoma cells lines and only TSPB-(Abu)(6) increased proliferation of MCF7 and HT-29. The main aim of this study was to perform physicochemical measurements, in a comparative way, to determine if the differences in activity could be related to physicochemical properties. Peptides were characterised by HPLC capacity factors, UV, fluorescence, and CD spectra (either in buffer solution or in the presence of lipid vesicles), surface activity, and aggregation. Moreover, the interaction of these peptides with phospholipids was determined through their penetration in monolayers of DPPC, PG, or PS as well as their miscibility in mixed monolayers. Besides, using liposomes as model membranes, the affinity of these peptides for phosphatidylcholine was measured with vesicles labeled with fluorescent markers (TMA-DPH, laurdan, pyrene). Results show that these molecules are highly hydrophilic and their surface activity is low. Mixed monolayers indicate that there is almost no miscibility. Besides, its presence does not modify noticeably the microviscosity of bilayers. Moreover, UV and fluorescence spectra of peptides were not affected by the presence of lipids in the media but CD spectra recorded in TFE/water (1/1) resulted in small changes for TSPB, TSPB-(E), and TSPB-(S) peptides. On the contrary CD spectra of TSPB-(Abu)(6) derivatives were clearly much more sensitive to the polarity of the environment. According to these data the biological activity of peptide with a cyclic aspartimide moiety at position 6 could be related to a specific conformational change in the peptide chain promoted by a hydrophobic membrane-like environment.

Aspergillus fumigatus survival in alkaline and extreme zinc-limiting environments relies on the induction of a zinc homeostasis system encoded by the zrfC and aspf2 genes

Aspergillus fumigatus has three zinc transporter-encoding genes whose expression is regulated by both pH and the environmental concentration of zinc. We have previously reported that the zrfA and zrfB genes of A. fumigatus are transcribed at higher levels and are required for fungal growth under acidic zinc-limiting conditions whereas they are dispensable for growth in neutral or alkaline zinc-limiting media. Here we report that the transporter of the zinc uptake system that functions in A. fumigatus growing in neutral or alkaline environments is encoded by zrfC. The transcription of zrfC occurs divergently with respect to the adjacent aspf2 gene, which encodes an immunodominant antigen secreted by A. fumigatus. The two genes-zrfC and aspf2-are required to different extents for fungal growth in alkaline and extreme zinc-limiting media. Indeed, these environmental conditions induce the simultaneous transcription of both genes mediated by the transcriptional regulators ZafA and PacC. ZafA upregulates the expression of zrfC and aspf2 under zinc-limiting conditions regardless of the ambient pH, whereas PacC represses the expression of these genes under acidic growth conditions. Interestingly, the mode of action of PacC for zrfC-aspf2 transcription contrasts with the more widely accepted model for PacC function, according to which under alkaline growth conditions PacC would activate the transcription of alkaline-expressed genes but would repress the transcription of acid-expressed genes. In sum, this report provides a good framework for investigating several important aspects of the biology of species of Aspergillus, including the repression of alkaline genes by PacC at acidic pH and the interrelationship that must exist between tissue pH, metal availability in the host tissue, and fungal virulence.

Metal ion physiopathology in neurodegenerative disorders

Metal dyshomeostasis in the brain (BMD) has often been proposed as a possible cause for several neurodegenerative disorders (NDs). Nevertheless, the precise nature of the biochemical mechanisms of metal involvement in NDs is still largely unknown. Mounting evidence suggests that normal aging itself is characterized by, among other features, a significant degree of metal ion dysmetabolism in the brain. This is probably the result of a progressive deterioration of the metal regulatory systems and, at least in some cases, of life-long metal exposure and brain accumulation. Although alterations of metal metabolism do occur to some extent in normal aging, they appear to be highly enhanced under various neuropathological conditions, causing increased oxidative stress and favoring abnormal metal-protein interactions. Intriguingly, despite the fact that most common NDs have a distinct etiological basis, they share striking similarities as they are all characterized by a documented brain metal impairment. This review will primarily focus on the alterations of metal homeostasis that are observed in normal aging and in Alzheimer's disease. We also present a brief survey on BMD in other NDs (Amyotrophic Lateral Sclerosis, Parkinson's, and Prion Protein disease) in order to highlight what represents the most reliable evidence supporting a crucial involvement of metals in neurodegeneration. The opportunities for metal-targeted pharmacological strategies in the major NDs are briefly outlined as well.

NMR studies of zinc binding in a multi-histidinic peptide fragment

A multi-histidinic peptide and its minimal models have been investigated for Zn(ii) binding. We have used NMR spectroscopy to probe the binding of zinc to the three repeats (T(1)R(2)S(3)R(4)S(5)H(6)T(7)S(8)E(9)G(10))(3) and to its mono-histidinic minimal models, the 9- and 10-aminoacid fragment. (1)H-(1)H TOCSY, (1)H-(13)C HSQC, (1)H-(1)H NOESY and (1)H-(1)H ROESY multidimensional NMR techniques were performed to understand the details of metal binding sites and the conformational behaviour of the peptides at different pH values and at different ligand to metal molar ratios. Zinc coordination involves imidazole N(delta) of His6 and carboxyl gamma-O of Glu9 residues; interaction with peptide oxygens of the His6-Thr7 or Thr7-Ser8 bonds in a tetrahedral arrangement with the minimal model peptides, cannot be excluded. Zinc coordination involves, at physiologic pH, all the three imidazole N(delta) donors of His6, His16 and His26 as well as carboxyl gamma-O of Glu residues in a tetra, penta or octahedral arrangement with the three repeats, the 30-aminoacid fragment. Zinc complexation induces important structural changes with the C-terminal portion of the ligand, constraining it to leave its disordered conformation. Our results give rise to a model of the induced structure of the peptides when bound to zinc. At high pH, amide deprotonation does not take place and hydroxo or high molecular weight polymeric species may be formed.

Structure of the flexible amino-terminal domain of prion protein bound to a sulfated glycan

The intrinsically disordered amino-proximal domain of hamster prion protein (PrP) contains four copies of a highly conserved octapeptide sequence, PHGGGWGQ, that is flanked by two polycationic residue clusters. This N-terminal domain mediates the binding of sulfated glycans, which can profoundly influence the conversion of PrP to pathological forms and the progression of prion disease. To investigate the structural consequences of sulfated glycan binding, we performed multidimensional heteronuclear ((1)H, (13)C, (15)N) NMR (nuclear magnetic resonance), circular dichroism (CD), and fluorescence studies on hamster PrP residues 23-106 (PrP 23-106) and fragments thereof when bound to pentosan polysulfate (PPS). While the majority of PrP 23-106 remain disordered upon PPS binding, the octarepeat region adopts a repeating loop-turn structure that we have determined by NMR. The beta-like turns within the repeats are corroborated by CD data demonstrating that these turns are also present, although less pronounced, without PPS. Binding to PPS exposes a hydrophobic surface composed of aligned tryptophan side chains, the spacing and orientation of which are consistent with a self-association or ligand binding site. The unique tryptophan motif was probed by intrinsic tryptophan fluorescence, which displayed enhanced fluorescence of PrP 23-106 when bound to PPS, consistent with the alignment of tryptophan side chains. Chemical-shift mapping identified binding sites on PrP 23-106 for PPS, which include the octarepeat histidine and an N-terminal basic cluster previously linked to sulfated glycan binding. These data may in part explain how sulfated glycans modulate PrP conformational conversions and oligomerizations.

Evolutionary descent of prion genes from the ZIP family of metal ion transporters

In the more than twenty years since its discovery, both the phylogenetic origin and cellular function of the prion protein (PrP) have remained enigmatic. Insights into a possible function of PrP may be obtained through the characterization of its molecular neighborhood in cells. Quantitative interactome data demonstrated the spatial proximity of two metal ion transporters of the ZIP family, ZIP6 and ZIP10, to mammalian prion proteins in vivo. A subsequent bioinformatic analysis revealed the unexpected presence of a PrP-like amino acid sequence within the N-terminal, extracellular domain of a distinct sub-branch of the ZIP protein family that includes ZIP5, ZIP6 and ZIP10. Additional structural threading and orthologous sequence alignment analyses argued that the prion gene family is phylogenetically derived from a ZIP-like ancestral molecule. The level of sequence homology and the presence of prion protein genes in most chordate species place the split from the ZIP-like ancestor gene at the base of the chordate lineage. This relationship explains structural and functional features found within mammalian prion proteins as elements of an ancient involvement in the transmembrane transport of divalent cations. The phylogenetic and spatial connection to ZIP proteins is expected to open new avenues of research to elucidate the biology of the prion protein in health and disease.

Structural features of the Zn(2+) complex with the single repeat region of "prion related protein" (PrP-rel-2) of zebrafish zPrP63-70 fragment

The interaction between Zn(2+) and the single repeat of PrP-rel-2 of zebrafish at physiological pH was investigated by NMR spectroscopy; the chemical shift mapping and the proton-proton distances were used to obtain the structural model of the Zn(2+) complex.

Metal binding sheds light on mechanisms of amyloid assembly

Beta-2 microglobulin (beta2m) is the protein responsible for amyloid deposition in Dialysis-Related Amyloidosis (DRA). Aggregation can be induced by various solution conditions including exposure to divalent metal, incubation at acidic pH, and limited proteolysis. Using Cu(2+) as a trigger, we have trapped, isolated, and crystallized a stable oligomer of beta2m that is populated under amyloidogenic solution conditions (Calabrese et al. Nat Struct Mol Biol 2008; 15:965-71). This structure reveals that Cu(2+)-binding is associated with dramatic conformational rearrangements. This has allowed us to postulate a set of structural changes common to all beta2m aggregation pathways. Cu(2+) serves as a potential trigger in other aggregation systems such as Abeta, alpha-synuclein, and mammalian Prion (PrP). A comparison of Cu(2+) binding to beta2m and PrP reveals common features. Therefore, in addition to providing insight into DRA, induction of structure by Cu(2+) binding appears to be a recurring structural motif for pathological changes in conformation.

Identification of the copper(II) coordinating residues in the prion protein by metal-catalyzed oxidation mass spectrometry: evidence for multiple isomers at low copper(II) loadings

While the Cu(II) binding sites of the prion protein have been well studied under Cu-saturation conditions, the identity of the residues involved in coordinating Cu(II) at low stoichiometries and the order in which the binding sites load with Cu(II) remain unresolved. In this study, we have used two mass spectrometry based methods to gather insight into Cu(II)-prion binding under different stoichiometric loadings of Cu(II). The first method uses metal-catalyzed oxidation reactions to site specifically modify the residues bound to Cu(II) in solution, and the second method determines Cu binding sites based on the protection of His from modification by diethyl pyrocarbonate when this residue binds Cu(II) in solution. For both methods, the residues that are labeled by these reactions can then be unambiguously identified using tandem mass spectrometry. Upon applying these two complementary methods to a construct of the prion protein that contains residues 23-28 and 57-98, several noteworthy observations are made. Coordination of Cu(II) by multiple His imidazoles is found at 1:1 and 1:2 PrP:Cu(II) ratios. Notably, there appear to be four to seven isomers of this multiple histidine coordination mode in the 1:1 complex. Furthermore, our data clearly show that His96 is the dominant Cu(II) binding ligand, as in every isomer His96 is bound to Cu(II). The individual octarepeat binding sites begin to fill at ratios of 1:3 PrP:Cu(II) with no clear preference for the order in which they load with Cu(II), although the His77 octarepeat appears to saturate last. The existence of several "degenerate" Cu binding modes at low PrP:Cu(II) ratios may allow it to more readily accept additional Cu(II) ions, thus allowing PrP to transition from a singly Cu(II) bound state to a multiply Cu(II) bound state as a function of cellular Cu(II) concentration.

Expansion of the octarepeat domain alters the misfolding pathway but not the folding pathway of the prion protein

A misfolded conformation of the prion protein (PrP), PrP (Sc), is the essential component of prions, the infectious agents that cause transmissible neurodegenerative diseases. Insertional mutations that lead to an increase in the number of octarepeats (ORs) in PrP are linked to familial human prion disease. In this study, we investigated how expansion of the OR domain causes PrP to favor a prion-like conformation. Therefore, we compared the conformational and aggregation modulating properties of wild-type versus expanded OR domains, either as a fusion construct with the protein G B1 domain (GB1-OR) or as an integral part of full-length mouse PrP (MoPrP). Using circular dichroism spectroscopy, we first demonstrated that ORs are not unfolded but exist as an ensemble of three distinct conformers: polyproline helix-like, beta-turn, and "Trp-related". Domain expansion had little effect on the conformation of GB1-OR fusion proteins. When part of MoPrP however, OR domain expansion changed PrP's folding landscape, not by hampering the production of native alpha-helical monomers but by greatly reducing the propensity to form amyloid and by altering the assembly of misfolded, beta-rich aggregates. These features may relate to subtle pH-dependent conformational differences between wild-type and mutant monomers. In conclusion, we propose that PrP insertional mutations are pathogenic because they enhance specific misfolding pathways of PrP rather than by undermining native folding. This idea was supported by a trial bioassay in transgenic mice overexpressing wild-type MoPrP, where intracerebral injection of recombinant MoPrP with an expanded OR domain but not wild-type MoPrP caused prion disease.

The prion protein is a combined zinc and copper binding protein: Zn2+ alters the distribution of Cu2+ coordination modes

PrP binds copper in the highly conserved, unstructured N-terminal half of the protein. The octarepeat region consists of 4 tandem repeats of PHGGGWGQ and binds four equivalents of copper at full occupancy. Adjacent to the octarepeats are two additional histidines that may also bind copper. We recently showed that when the octarepeat region is titrated with Cu2+, the copper binding mode depends on the number of equivalents of copper bound. In addition to copper, other metals have been associated with PrP, however zinc is the only metal other than copper that induces PrP endocytosis, inhibits fibril formation and promotes inter-molecular interactions. In this work we show that even large excesses of zinc (> 1mM) are unable to displace copper from either the octarepeat region or the full-length protein. However, EPR reveals that physiologically relevant levels of zinc significantly alter the distribution of copper among the available binding modes. Diethyl pyrocarbonate (DEPC) modification and Mass Spectrometry is used to identify the octarepeat region as the zinc binding site and to confirm that the affinity of PrP for zinc is ~200 μM. PrP can simultaneously bind both copper and zinc by shifting to binding modes that minimize the ratio of histidines to copper.