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Biomedical subjects

J Walz

Publications and source records attributed to J Walz.

31 records · Page 2Linked to original sources
PubMed1997

Perspectives of molecular and cellular electron tomography.

After a general introduction to three-dimensional electron microscopy and particularly to electron tomography (ET), the perspectives of applying ET to native (frozen-hydrated) cellular structures are discussed. In ET, a set of 2-D images of an object is recorded at different viewing directions and is then used for calculating a 3-D image. ET at a resolution of 2-5 nm would allow the 3-D organization of structural cellular components to be studied and would provide important information about spatial relationships and interactions. The question of whether it is a realistic long-term goal to visualize or--by sophisticated pattern recognition methods--identify macromolecules in cells frozen in toto or in frozen sections of cells is addressed. Because of the radiation sensitivity of biological specimens, a prerequisite of application of ET is the automation of the imaging process. Technical aspects of automated ET as realized in Martinsried and experiences are presented, and limitations of the technique are identified, both theoretically and experimentally. Possible improvements of instrumentation to overcome at least part of the limitations are discussed in some detail. Those means include increasing the accelerating voltage into the intermediate voltage range (300 to 500 kV), energy filtering, the use of a field emission gun, and a liquid-helium-cooled specimen stage. Two additional sections deal with ET of isolated macromolecules and of macromolecular structures in situ, and one section is devoted to possible methods for the detection of structures in volume data.

Actins↗
PubMed1997

Electron Tomography of Single Ice-Embedded Macromolecules: Three-Dimensional Alignment and Classification

From 3-D reconstructions of automatically recorded tilt series of ice-embedded macromolecules, several hundred 3-D images of single particles can be extracted. Here we describe correlation-based techniques to align the particles with respect to translation and orientation in 3-D and the calculation of an averaged reconstruction after application of the correct weighting function to the particle projections. Multivariate statistical analysis and classification are applied to the set of three-dimensionally reconstructed particles to investigate interimage variations on the 3-D level. Copyright 1997 Academic Press. Copyright 1997 Academic Press

Journal Article↗
PubMed1997

Tricorn protease exists as an icosahedral supermolecule in vivo.

Tricorn protease is the core enzyme of a recently discovered modular proteolytic system. We present evidence that tricorn protease exists in vivo in the form of a higher-order assembly, namely as an icosahedral capsid. Its size exceeds that of many virus particles and represents by far the largest known homooligomeric enzyme complex. Each capsid is built from 20 copies of the tricorn hexameric toroid and thus has a molecular weight of 14.6 MDa. Three-dimensional reconstructions of ice-embedded capsids from electron micrographs show that it is hollow and has large void volumes in its wall. We suggest that the tricorn capsid, in addition to its intrinsic proteolytic activity, serves as the organizing center of a multienzyme complex.

Archaeal Proteins↗
PubMed1995

Structural features of archaebacterial and eukaryotic proteasomes.

The 26S proteasome is the central protease of the ubiquitin-dependent pathway of protein degradation. The molecule has a molecular mass of approximately 2000 kD and has a highly conserved structure in eukaryotes. The 26S proteasome is formed by a barrel-shaped 20S core complex and two polar 19S complexes. The 20S complex has C2 symmetry and is formed by four seven-membered rings of which the outer rings (alpha-type subunits) are rotated by 25.7 degrees relative to the inner rings while the inner rings (beta-type subunits) are in register. From a comparison of the activity and regulation of the 26S and 20S particles it can be deduced that the 20S particle contains the protease activity while the 19S complex contains isopeptidase, ATPase and protein unfolding activities. In this article we describe the structures of various proteasome complexes as determined by electron microscopy and discuss structural implications of their subunit sequences.

Animals↗
PubMed1995

Three-dimensional structure of lipid vesicles embedded in vitreous ice and investigated by automated electron tomography.

Automated electron tomography is shown to be a suitable means to visualize the shape of phospholipid vesicles embedded in vitrified ice. With a slow-scan charge-coupled device camera as a recording device, the cumulative electron dose needed to record a data set of 60 projections at a magnification of 20,000X can be kept as low as 15 e-/A2 (or 1500 electrons/nm2). The membrane of the three-dimensionally reconstructed vesicles is clearly visible in two-dimensional sections through the three-dimensionally reconstructed volume. Some edges indicating a polygonal shape of the vesicles, frozen from the gel phase, are also clearly recognized. Because of the presently limited tilt angle range (+/- 60 degrees), the upper and lower "caps" of the vesicles (representing about 35% of the surface of the ellipsoidal particles) remain invisible in the three-dimensional reconstruction.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed1994

Predicted secondary structure of the 20 S proteasome and model structure of the putative peptide channel.

Secondary structure prediction has made great progress in recent years due to the incorporation of evolutionary information, and may be close to a point where (in combination with biochemical and low-resolution structural data) it can guide the modelling of tertiary structure in cases where no model building is possible by homology. Towards this goal it is important to gather information on the performance of prediction methods in advance of the publication of new structures. In anticipation of the soon-to-be-released structure of the 20 S proteasome from Thermoplasma acidophilum, we have applied several widely used secondary structure prediction methods to proteasome sequences and have attempted to model the putative channel in the outer proteasome rings (alpha-rings) based on the obtained predictions.

Amino Acid Sequence↗
PubMed1993

Differential regulation of human leukocyte antigen class I genes by interferon in vivo and in vitro.

Modulation of human leukocyte antigen (HLA) class I antigens on peripheral blood lymphocytes, monocytes, and hematopoietic precursors during interferon-alpha (IFN-alpha) therapy was investigated in 18 patients with myeloproliferative syndrome. After 1 month of IFN-alpha therapy, an increased number of monocytes and hematopoietic precursor cells but not of lymphocytes expressed HLA-DQ antigens. In addition, a strong induction of HLA class I antigens was found on both hematopoietic progenitors and normal peripheral blood mononuclear cells. By daily injections of IFN in the first month of therapy, stimulation continuously increased, suggesting a major effect of IFN alpha on hematopoietic progenitors with sustained enhanced expression of HLA class I antigens during differentiation of myelomonocytic cells. HLA class I antigen expression was consistently augmented by IFN alpha in all patients irrespective of their hematologic response. Differential in vivo regulation of HLA class I antigens by IFN was confirmed by comparison of HLA-A2 with HLA-B antigen expression. In vitro expression of the HLA-B7 and -Bw64 genes had been shown earlier to be significantly more inducible by IFN than the genes coding for several other HLA class I antigens after transfection into mouse L cells. Modification of the 5' ends of the HLA-B7 and HLA-B27 genes before transfection in mouse L cells showed the presence of enhancer sequences responding to IFN treatment in the 5' untranslated region of the HLA-B7 but not of the HLA-B27 gene and suggested the presence of independently acting regulatory mechanisms independent of these enhancers.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1991

In vivo induction of HLA molecules in patients with myeloproliferative syndrome during IFN alpha treatment.

Eighteen patients with myeloproliferative syndrome (14 with chronic myeloid leukemia, four with essential thrombocytosis) were investigated for modulation of HLA antigens on peripheral blood lymphocytes, monocytes, and hematopoietic precursors during IFN alpha therapy as a sign of potentially increased immune recognition of malignant cells. After 1 month of IFN alpha therapy, an increased number of monocytes and hematopoietic precursor cells, but not of lymphocytes, expressed HLA-DQ antigens. In addition, a strong induction of HLA class-I antigens was found on both hematopoietic progenitors and normal peripheral blood mononuclear cells. With daily injections of IFN in the first month of therapy stimulation continuously increased, suggested a major effect of IFN alpha on hematopoietic progenitors with sustained enhanced expression of HLA class-I antigens during differentiation of myelomonocytic cells. HLA class-I antigen expression was consistently augmented by IFN alpha in all patients, irrespective of their hematological response.

Blood Cell Count↗