Quasi-elastic light scattering of Artemia ribosomes sedimented in a CsCl density gradient

It is impossible to measure the diffusion coefficient of macromolecules directly and accurately by quasi—elastic light scattering, when aggregates cannot be eliminated from the solutions to be investigated. Nevertheless, a simple method can be applied to overcome this problem in many cases. Aggregates are separated from the monomeric macromolecules by rate-zonal sedimentation in a CsCl density gradient in a transparent centrifugation tube; the monomers are then located by laser light scattering intensity measurements; photon correlation spectroscopy of the scattered light finally yields their diffusion coefficient. The viscosity of aqueous CsCl solutions at different temperatures and concentrations allows a good separation by centrifugation and a low uncertainty in the reduction of the measured diffusion coefficient to standard conditions.The application of the method to eukaryotic large ribosomal subunits is described as an example.     

The molecular weight of Artemia ribosomes, as determined from their refractive-index increment and light-scattering intensity

Cytoplasmic ribosomes were isolated from the cryptobiotic embryos of the brine shrimp Artemia salina. Measurements of their refractive-index increments and light-scattering intensities give a value for their molecular weight of (3.4±0.2)×10⁶.     

Genetic determination of the mitochondrial adenine nucleotide translocation system and ITS role in the eukaryotic cell

On integrating experimental data published previously, the following picture of the mitochondrial adenine nucleotide (AdN) translocation system is being presented: 1. The AdN translocation system serves not only to transport ATP synthesized within mitochondria into the cytosol but also to transport cytosolic ATP into the mitochondria when oxidative phosphorylation is not functioning. 2. The AdN translocator is coded for by nuclear genes and the mitochondrial protein synthesis is not involved in its formation. 3. The AdN translocation system must be preserved and functioning even in cells which could dispense with oxidative phosphorylation. It assures appropriate concentrations of intramitochondrial ATP. 4. The intramitochondrial ATP is required for normal replication of mitochondrial DNA. Tis supports the view that the mitochondrion is a self-replicating semi-autonomous organelle. 5. The appropriate concentration of ATP must be present in mitochondria to make possible cell growth or multiplication. This points to a direct or indirect role of mitochondria in the control of cell proliferation.     

Valine-sensitive acetohydroxy acid synthases in Escherichia coli K-12: unique regulation modulated by multiple genetic sites.

Summary Spontaneous mutants (146) of Escherichia coli K-12 were selected that were resistant to inhibition of growth by 1.2 mM L-valine (Val^r). The Val^r isolates, containing acetohydroxy acid synthase resistant to feedback inhibition by L-valine (AHAS^r), were classed according to cotransduction of the mutation with leu. Several mutations resulting in an AHAS^r phenotype were found to be cotransducible with glyA. However, no mutations causing a Val^r phenotype were linked to ilv. AHAS activity was more closely examined in representatives of three classes of mutants with Val^r linked to leu, labeled ilv-660, ilv-661, and ilv-662. The ilvE503 allele in E. coli K-12, known to cause a two- to three-fold derepression of AHAS, was found to affect regulation of synthesis of both valine-sensitive AHAS (AHAS^s) and AHAS^r in the mutants containing ilv-660 and ilv-661, whereas it affected repression of AHAS^s, only, in the mutant containing ilv-662. Further, both AHAS^s and AHAS^r in the ilv-661 mutant were repressed by valine, whereas valine did not repress AHAS^r synthesis in the strain carrying ilv-660 and only partially repressed AHAS^r in the strain carrying ilv-662. Unexpectedly, AHAS^r synthesis in strains carrying ilv-660 or ilv-662 was repressible by leucine. The ilv-660 locus appears to be similar in position to ilvH and encodes a product that confers valine-sensitivity upon AHAS activity in the wild-type E. coli K-12. The ilv-660 and ilv-662 loci may normally encode products that influence both the feedback sensitivity of AHAS and control of AHAS biosynthesis.     

Multimeric structure of the membrane erythropoietin receptor of murine erythroleukemia cells (Friend cells). Cross-linking of erythropoietin with the spleen focus-forming virus envelope protein.

In erythroleukemia cells infected with the polycythemia strain of the Friend virus complex, erythropoietin could be cross-linked mainly to a protein of 63 kDa when using disuccinimidyl suberate. In contrast, erythropoietin in other erythroleukemia cells cross-linked to two proteins of 85 and 100 kDa. When native erythropoietin receptor complexes were immunoprecipitated, the 63-kDa erythropoietin-cross-linked protein could be precipitated both by antibodies directed against the intracellular part of the cloned chain of the erythropoietin receptor and by antibodies directed against the envelope proteins of the Friend virus. However, after denaturation of the complexes, the 63-kDa protein was only precipitated by antibodies directed against the envelope proteins of the Friend virus. Enzymatic deglycosylation confirmed that erythropoietin was cross-linked with the envelope protein of the defective virus and bidimensional diagonal gel electrophoresis analyses showed that some of the erythropoietin cross-linked envelope proteins were dimerized by disulfide bonds. Thus, the main erythropoietin-receptor complex in the plasma membrane of these cells consisted of a molecule of the cloned chain of the erythropoietin receptor noncovalently associated with one or two disulfide-bonded molecule(s) of the envelope protein of the defective virus. Moreover, our results also showed that the viral envelope protein associated with the cloned chain of the erythropoietin receptor at a site distinct from the erythropoietin binding site.     

A single amino acid substitution in an MHC class I molecule allows heteroclitic recognition by lymphocytic choriomeningitis virus-specific cytotoxic T lymphocytes

Class I molecules of the MHC bind foreign and endogenous peptides allowing recognition by the TCR on CTL. The recognition and killing of cells infected with lymphocytic choriomeningitis virus (LCMV) depends on the recognition of LCMV peptides bound to class I MHC. Mutations in class I MHC molecules have enabled the delineation of regions in the class I molecule important for binding peptides and for interaction with the TCR. We have constructed a library of class I mutants using saturation mutagenesis and report a phenotypic change resulting from a single amino acid substitution that results in the heteroclitic (increased) killing of LCMV-infected cells. This amino acid change, asparagine to serine at position 30, is in a conserved region of the class I molecule contacting the alpha 3 domain. This mutation does not result in increased expression of the class I molecule on the cell surface, does not affect the binding of CD8, and does not affect allogeneic recognition. Cold target experiments show that this heteroclitic killing is due to increased recognition by CTL. These data point toward a critical function for this region of the class I molecule in the binding of peptides or their presentation to CTL.     

Conservation of binding site specificity of three yeast DNA binding proteins.

Sequence specific binding of protein extracts from 13 different yeast species to three oligonucleotide probes and two points mutants derived from Saccharomyces cerevisiae DNA binding proteins were tested using mobility shift assays. The probes were high affinity binding sites for GRF1/RAP1/ABF1 and CP1/CPF1. Most yeasts in the genus Saccharomyces showed specific binding to all three probes and also displayed similar sequence requirements when challenged by molar excesses of mutant probes. The affinities for the probes varied amongst the other yeasts tested, but in general, CPF1 binding activity was the most widespread, while the other two were more limited.     

Erythropoietin induces the tyrosine phosphorylation of its own receptor in human erythropoietin-responsive cells.

Using the human erythropoietin-responsive hematopoietic cell line UT-7, we showed that erythropoietin (Epo) rapidly and specifically induced the tyrosine phosphorylation of its own receptor (M(r) 75,000) and increased the tyrosine phosphorylation of other proteins of M(r) 140,000, 120,000, 95,000, 60,000, 57,000, and 42,000. Neither granulocyte-macrophage colony-stimulating factor, interleukin 3, interleukin 6, nor the kit ligand induced the phosphorylation of the M(r) 75,000 receptor protein, although these growth factors induced the phosphorylation of other proteins. Cross-linking experiments using 125I-Epo indicated that the UT-7 cells expressed three Epo receptor subunits, of M(r) 100,000, 85,000, and 75,000, among which only the M(r) 75,000 subunit was tyrosine-phosphorylated following activation with Epo.