High-level expression and characterization of a glycosylated covalently linked dimer of the prion protein

Production, purification and oxidative folding of the mouse recombinant prion protein

The method leading to overexpression of the full-length mouse recombinant prion protein (mrPrP 23-231) in the cytoplasm of E. coli as a his-PrP fusion protein and its effective purification using affinity chromatography is described. A typical yield of the method was 8-10 mg his-mrPrP per L of the bacterial culture. The purity of purified protein was > 95 %. The purified his-mrPrP was converted to a soluble form and its folding to alpha-helical and beta-sheet conformations was studied. The properties of differently folded mrPrP were determined by measuring their circular dichroism spectra, partial resistance to cleavage by proteinase K and by centrifugation in sucrose gradient.

Overexpression of the plasma membrane-localized NDR1 protein results in enhanced bacterial disease resistance in Arabidopsis thaliana

Previous studies have established that mutations in the NDR1 gene in Arabidopsis thaliana suppress the resistance response of three resistance proteins, RPS2, RPM1, and RPS5, to Pseudomonas syringae pv. tomato (Pst) strain DC3000 containing the cognate effector genes, avrRpt2, avrRpm1, and avrpPhB, respectively. NDR1 is a plasma membrane (PM)-localized protein, and undergoes several post-translational modifications including carboxy-terminal processing and N-linked glycosylation. Expression of NDR1 under the NDR1 native promoter complements the ndr1-1 mutation, while overexpression of NDR1 results in enhanced resistance to virulent Pst. Sequence analysis and mass spectrometry suggest that NDR1 is localized to the PM via a C-terminal glycosylphosphatidyl-inositol (GPI) anchor. GPI modification would potentially place NDR1 on the outer surface of the PM, perhaps allowing NDR1 to act as a transducer of pathogen signals and/or interact directly with the pathogen.

The prion-like protein Doppel fails to interact with itself, the prion protein and the 37 kDa/67 kDa laminin receptor in the yeast two-hybrid system

The prion-like protein termed Doppel (Dpl) shows approx. 25% sequence identity with all known prion proteins (PrP). We recently showed that the cellular PrP is dimeric under native conditions, a finding which was confirmed by the investigation of its crystal structure. Human PrP further interacts with its cellular receptor, the 37 kDa/67 kDa laminin receptor (LRP/LR). Here we report that human Doppel fails to interact with (i). itself, (ii). the human 37 kDa/67 kDa LRP/LR, and (iii). the human cellular prion protein (huPrP) in the yeast two-hybrid system. Our findings suggest that Dpl and PrP are not related or are only marginally related with respect to their ligand binding behaviour.

Cell Surface Expression of the Prion Protein in Yeast Does Not Alter Copper Utilization Phenotypes

Prion diseases are fatal neurodegenerative disorders that result from conversion of a normal, cell surface glycoprotein (PrP(C)) into a conformationally altered isoform (PrP(Sc)) that is thought to be infectious. Although a great deal is known about the role of PrP(Sc) in the disease process, the physiological function of PrP(C) has remained enigmatic. In this report, we have used the yeast Saccharomyces cerevisiae to test one hypothesized function of PrP(C), as a receptor for the uptake or efflux of copper ions. We first modified the PrP signal peptide by replacing its hydrophobic core with the signal sequence from the yeast protein dipeptidyl aminopeptidase B, so that the resulting protein was targeted cotranslationally to the secretory pathway when synthesized in yeast. PrP molecules with the modified signal peptide were efficiently glycosylated, glycolipid-anchored, and localized to the plasma membrane. We then tested whether PrP expression altered the growth deficiency phenotypes of yeast strains harboring deletions in genes that encode key components of copper utilization pathways, including transporters, chaperones, pumps, reductases, and cuproenzymes. We found that PrP did not rescue any of these mutant phenotypes, arguing against a direct role for the protein in copper utilization. Our results provide further clarification of the physiological function of PrP(C), and lay the groundwork for using PrP-expressing yeast to study other aspects of prion biology.

Ageing and exposure to oxidative stress in vivo differentially affect cellular levels of PrP in mouse cerebral microvessels and brain parenchyma

The biological function of cellular prion protein PrPc has not been established, despite in vitro studies suggesting antioxidant activity or link to signal transduction pathways. In this study, mice were exposed to hyperoxia to establish whether oxidative stress affected prion expression in vivo. C57Bl/6J mice aged 6, 18, and 24 months, maintained under normoxic conditions, exhibited age-related increases in PrPc in both cerebral microvessels and in microvessel-depleted brain homogenate. We demonstrate that PrPc is differentially affected by exposure to hyperoxia in vivo for 1 (24 h) or 2 (48 h) days, or for 1 day hyperoxia, followed by 1 day normoxia. Brain parenchymal cells from 6-month-old mice exposed to 1 day hyperoxia showed elevation of a glycosylated approximately 36 kDa form, whereas in 24-month-old mice cellular prion level was substantially reduced. Extending hyperoxia from 1 to 2 days resulted in significantly reduced PrPc level, regardless of age. Parenchymal PrPc is substantially elevated in 6-month-old mice, but declines in 18- and 24-month-old animals following 1 day hyperoxia. By contrast, PrPc content in cerebral microvessels from 6-month-old mice declined after a 2 day exposure to hyperoxia, while microvessels from 24-month-old brains showed elevated prion levels 24 h after hyperoxia. Moreover, unglycosylated 25-30 kDa PrPc, and a previously undescribed 50-64 kDa band containing at least some glycosylated protein, predominated in microvessels with lesser content of the glycosylated approximately 36 kDa form. Cellular content of these unglycosylated forms was correlated with age, while the response to hyperoxia was evident in both unglycosylated and glycosylated forms of the protein following 1 and 2 day exposures. The observed elevation of the 25-30 and 50-64 kDa bands of microvessel PrPc is not sustainable following 1 day hyperoxia, but returns to near normoxic levels within 24 h after hyperoxia. We also show in a knockout mouse for methionine sulfoxide reductase (MsrA), the enzyme responsible for reducing methionine sulfoxide back to methionine, and a regulator of cellular antioxidant defence, that following hyperoxia brain PrPc in the null mutant is elevated relative to PrPc content in the parent strain. Our results show up-regulated PrPc expression or reduced turnover in response to age-related, and hyperoxia-induced oxidative stress.

Molecular dynamics simulation of dimeric and monomeric forms of human prion protein: insight into dynamics and properties

A central theme in prion protein research is the detection of the process that underlies the conformational transition from the normal cellular prion form (PrP(C)) to its pathogenic isoform (PrP(Sc)). Although the three-dimensional structures of monomeric and dimeric human prion protein (HuPrP) have been revealed by NMR spectroscopy and x-ray crystallography, the process underlying the conformational change from PrP(C) to PrP(Sc) and the dynamics and functions of PrP(C) remain unknown. The dimeric form is thought to play an important role in the conformational transition. In this study, we performed molecular dynamics (MD) simulations on monomeric and dimeric HuPrP at 300 K and 500 K for 10 ns to investigate the differences in the properties of the monomer and the dimer from the perspective of dynamic and structural behaviors. Simulations were also undertaken with Asp178Asn and acidic pH, which is known as a disease-associated factor. Our results indicate that the dynamics of the dimer and monomer were similar (e.g., denaturation of helices and elongation of the beta-sheet). However, additional secondary structure elements formed in the dimer might result in showing the differences in dynamics and properties between the monomer and dimer (e.g., the greater retention of dimeric than monomeric tertiary structure).

Intra- and interspecies interactions between prion proteins and effects of mutations and polymorphisms

Recently, crystallization of the prion protein in a dimeric form was reported. Here we show that native soluble homogeneous FLAG-tagged prion proteins from hamster, man and cattle expressed in the baculovirus system are predominantly dimeric. The PrP/PrP interaction was confirmed in Semliki Forest virus-RNA transfected BHK cells co-expressing FLAG- and oligohistidine-tagged human PrP. The yeast two-hybrid system identified the octarepeat region and the C-terminal structured domain (aa90-aa230) of PrP as PrP/PrP interaction domains. Additional octarepeats identified in patients suffering from fCJD reduced (wtPrP versus PrP + 9OR) and completely abolished (PrP + 9OR versus PrP + 9OR) the PrP/PrP interaction in the yeast two-hybrid system. In contrast, the Met/Val polymorphism (aa129), the GSS mutation Pro102Leu and the FFI mutation Asp178Asn did not affect PrP/PrP interactions. Proof of interactions between human or sheep and bovine PrP, and sheep and human PrP, as well as lack of interactions between human or bovine PrP and hamster PrP suggest that interspecies PrP interaction studies in the yeast two-hybrid system may serve as a rapid pre-assay to investigate species barriers in prion diseases.