Biomedical subjects

J Stöckel

Publications and source records attributed to J Stöckel.

7 recordsLinked to original sources

PubMed2004

Characterization of a T-DNA insertion mutant for the protein import receptor atToc33 from chloroplasts.

In Arabidopsis thaliana, the Toc34 receptor component of the chloroplast import machinery is encoded by two independent but highly homologous genes, atToc33 and atToc34. We have isolated a T-DNA insertion mutant of atToc33 which is characterized by a pale phenotype, due to reductions in the levels of photosynthetic pigments, and alterations in protein composition. The latter involve not only chloroplast proteins but also some cytosolic polypeptides, including 14-3-3 proteins which, among other functions, have been proposed to be cytosolic targeting factors for nucleus-encoded chloroplast proteins. Within the chloroplast, many, though not all, proteins of the photosynthetic apparatus, as well as proteins not directly involved in photosynthesis, are found in significantly reduced amounts in the mutant. However, the accumulation of other chloroplast proteins is unaffected. This suggests that the atToc33 receptor is responsible for the import of a specific subset of nucleus-encoded chloroplast proteins. Supporting evidence for this conclusion was obtained by antisense repression of the atToc34 gene in the atToc33 mutant, which results in an exacerbation of the phenotype.

Arabidopsis↗

PubMed2001

Chaperonin-mediated de novo generation of prion protein aggregates.

The infectious prion protein, PrP(Sc), a predominantly beta-sheet aggregate, is derived from PrP(C), the largely alpha-helical cellular isoform of PrP. Conformational conversion of PrP(C) into PrP(Sc) has been suggested to involve a chaperone-like factor. Here we report that the bacterial chaperonin GroEL, a close homolog of eukaryotic Hsp60, can catalyze the aggregation of chemically denatured and of folded, recombinant PrP in a model reaction in vitro. Aggregates form upon ATP-dependent release of PrP from chaperonin and have certain properties of PrP(Sc), including a high beta-sheet content, the ability to bind the dye Congo red, detergent-insolubility and increased protease-resistance. A conserved sequence segment of PrP (residues 90-121), critical for PrP(Sc) generation in vivo, is also required for chaperonin-mediated aggregate formation in vitro. Initial binding of refolded, alpha-helical PrP to chaperonin is mediated by the unstructured N-terminal segment of PrP (residues 23-121) and is followed by a rearrangement of the globular PrP core-domain. These results show that chaperonins of the Hsp60 class can, in principle, mediate PrP aggregation de novo, i.e. independently of a pre-existent PrP(Sc) template.

Adenosine Triphosphate↗

PubMed2001

Multiple effects of chromate on the photosynthetic apparatus of Spirodela polyrhiza as probed by OJIP chlorophyll a fluorescence measurements.

Chromate (Cr) decreases the growth of Spirodela polyrhiza. The fronds lost their pigments. The O2 evolution was also decreased. The Cr effect was found to be dose dependent. The toxic effects of Cr have further been studied on the photosynthetic activity of Spirodela polyrhiza by means of the chlorophyll a (Chl a) fluorescence transient O-J-I-P. The Chl a fluorescence transients were recorded in vivo with high time resolution and analyzed according to the JIP-test which can quantify the photosystem II behavior. Cr treated plants show a decrease in yield for primary photochemistry, phi Po. The performance index of PSII, PIABS, which is the combination of the indexes of three independent parameters, (1) the total number of active reaction centers per absorption (RC/ABS), (2) yield of primary photochemistry (phi Po) and (3) efficiency with which a trapped exciton can move an electron into the electron transport chain (psi 0), decreased due to Cr treatment. Chromate sensitivity varies within plant populations. In summary Cr affects several targets of PSII. More specifically, the main targets of Cr, according to the JIP-test, can be listed as a decrease in the number of active reaction centers and damage to the oxygen-evolving complex.

Araceae↗

PubMed1998

Prion protein selectively binds copper(II) ions.

The infectious isoform of the prion protein (PrPSc) is derived from cellular PrP (PrPC) in a conversion reaction involving a dramatic reorganization of secondary and tertiary structure. While our understanding of the pathogenic role of PrPSc has grown, the normal physiologic function of PrPC still remains unclear. Using recombinant Syrian hamster prion protein [SHaPrP(29-231)], we investigated metal ions as possible ligands of PrP. Near-UV circular dichroism spectroscopy (CD) indicates that the conformation of SHaPrP(29-231) resembles PrPC purified from hamster brain. Here we demonstrate by CD and tryptophan (Trp) fluorescence spectroscopy that copper induces changes to the tertiary structure of SHaPrP(29-231). Binding of copper quenches the Trp fluorescence emission significantly, shifts the emission spectrum to shorter wavelengths, and also induces changes in the near-UV CD spectrum of SHaPrP(29-231). The binding sites are highly specific for Cu2+, as indicated by the lack of a change in Trp fluorescence emission with Ca2+, Co2+, Mg2+, Mn2+, Ni2+, and Zn2+. Binding of Cu2+ also promotes the conformational shift from a predominantly alpha-helical to a beta-sheet structure. Equilibrium dialysis experiments indicate a binding stoichiometry of approximately 2 copper molecules per PrP molecule at physiologically relevant concentrations, and pH titration of Cu2+ binding suggests a role for histidine as a chelating ligand. NMR spectroscopy has recently demonstrated that the octarepeats (PHGGGWGQ) in SHaPrP(29-231) lack secondary or tertiary structure in the absence of Cu2+. Our results suggest that each Cu2+ binds to a structure defined by two octarepeats (PHGGGWGQ) with one histidine and perhaps one glycine carbonyl chelating the ion. We propose that the binding of two copper ions to four octarepeats induces a more defined structure to this region.

Animals↗

PubMed1997

Pathway of detergent-mediated and peptide ligand-mediated refolding of heterodimeric class II major histocompatibility complex (MHC) molecules.

We investigated the mechanism of refolding and reassembly of recombinant alpha and beta chains of the class II major histocompatibility molecules (MHC-II) HLA-DRB5*0101. Both chains were expressed in the cytosol of Escherichia coli, purified in urea and SDS, and reassembled to functional heterodimers by replacement of SDS by mild detergents, incubation in a redox-shuffling buffer and finally by oxidation and removal of detergent. Refolding was mediated by mild detergents and by peptide ligands. Early stages of structure formation were characterized by circular dichroism, fluorescence, and time-resolved fluorescence anisotropy decay (FAD) spectroscopies. We found that formation of secondary structure was detectable after replacement of SDS by mild detergents. At that stage the alpha and beta chains were still monomeric, the buffer was strongly reducing, and the folding intermediates did not yet interact with peptide ligands. Formation of folding intermediates capable of interacting with peptide ligands was detected after adjusting the redox potential with oxidized glutathione and incubation in mild detergents. We conclude that at that stage a tertiary structure close to the native structure is formed at least locally. The nature and concentration of detergent critically determined the refolding efficiency. We compared detergents with different carbohydrate headgroups, and with aliphatic chains ranging from C6 to C14 in length. For each of the detergents we observed a narrow concentration range for mediating refolding. Surprisingly, detergents with long aliphatic chains had to be used at higher concentrations than short-chain detergents, indicating that increasing the solubility of folding intermediates is not the only function of detergents during a refolding reaction. We discuss structure formation and interactions of detergents with stable folding intermediates. Understanding such interactions will help to develop rational strategies for refolding hydrophobic or oligomeric proteins.

Circular Dichroism↗

PubMed1996

High-level expression and characterization of a purified 142-residue polypeptide of the prion protein.

The major, and possible only, component of the infectious prion is the scrapie prion protein (PrPSc); the protease resistant core of PrPSc is PrP 27-30, a protein of approximately 142 amino acids. PrPSc is derived from the cellular PrP isoform (PrPC) by a post-transliatonal process in which a profound conformational change occurs. Syrian hamster (SHa) PrP genes of varying length ranging from the N- and C- terminally truncated 90-228 up to the full-length mature protein 23-231 were inserted into various secretion and intracellular expression vectors that were transformed into Escherichia coli deficient for proteases. Maximum expression was obtained for a truncated SHaPrP containing residues 90-231, which correspond to the sequence of PrP 27-30; disruption of the bacteria using a microfluidizer produced the highest yields of this protein designated rPrP. After solubilization of rPrP in 8 M GdnHC1, it was purified by size exclusion chromatography and reversed phase chromatography. During purification the recovery was approximately 50%, and from each liter of E. coli culture, approximately 50 mg of purified rPrP was obtained. Expression of the longer species containing the basic N-terminal region was less successful and was not pursued further. The primary structure of rPrP was verified by Edman sequencing and mass spectrometry, and secondary structure determined by circular dichroism and Fourier transform infrared spectroscopy. When rPrP was purified under reducing conditions, it had a high beta-sheet content and relatively low solubility similar to PrPSc, particularly at pH values > 7. Refolding of rPrP by oxidation to form a disulfide bond between the two Cys residues of this polypeptide produced a soluble protein with a high alpha-helical content similar to PrPC. These multiple conformations of rPrP are reminiscent of the structural plurality that characterizes the naturally occurring PrP isoforms. The high levels of purified rPrP which can now be obtained should facilitate determination of the multiple tertiary structures that Prp can adopt.

Amino Acid Sequence↗

PubMed1994

Refolding of human class II major histocompatibility complex molecules isolated from Escherichia coli. Assembly of peptide-free heterodimers and increased refolding-yield in the presence of antigenic peptide.

The alpha- and beta-chains of the heterodimeric major histocompatibility complex molecules HLA-DRB5*0101 and DRB1*0101 were expressed separately in Escherichia coli. The cytoplasmic and membrane-spanning domains of both chains were replaced by oligohistidine tags to allow purification by metal chelate chromatography. The recombinant proteins were refolded to peptide-free, water-soluble heterodimers by removal of major amounts of detergents and concomitant reoxidiation of disulfide bonds. Correct conformation was documented by three criteria: (a) affinity binding experiments using the antibody L243, which is known to recognize a conformational epitope formed only by correctly associated heterodimers; (b) specific binding of peptides to the refolded molecules; (c) recognition of peptides bound to refolded HLA-DR molecules by T-cells as reflected by Ca2+ influx into T-cells and production of interferon-gamma. The refolding reaction did not absolutely depend on the presence of peptides. The yield of peptide-free heterodimers was 3.0%. However, the yield of refolded heterodimer was increased to 10% if refolding was performed in the presence of antigenic peptides.

Amino Acid Sequence↗