Exit from mitosis in Drosophila syncytial embryos requires proteolysis and cyclin degradation, and is associated with localized dephosphorylation

The cyclin proteolysis that accompanies the exit from mitosis in diverse systems appears to be essential for restoration of interphase. The early syncytial divisions of Drosophila embryos, however, occur without detectable oscillations in the total cyclin level or Cdk1 activity. Nonetheless, we found that injection of an established inhibitor of cyclin proteolysis, a cyclin B amino-terminal peptide, prevents exit from mitosis in syncytial embryos. Similarly, injection of a version of Drosophila cyclin B that is refractory to proteolysis results in mitotic arrest. We infer that proteolysis of cyclins is required for exit from syncytial mitoses. This inference can be reconciled with the failure to observe oscillations in total cyclin levels if only a small pool of cyclins is destroyed in each cycle. We find that antibody detection of histone H3 phosphorylation (PH3) acts as a reporter for Cdk1 activity. A gradient of PH3 along anaphase chromosomes suggests local Cdk1 inactivation near the spindle poles in syncytial embryos. This pattern of Cdk1 inactivation would be consistent with local cyclin destruction at centrosomes or kinetochores. The local loss of PH3 during anaphase is specific to the syncytial divisions and is not observed after cellularization. We suggest that exit from mitosis in syncytial cycles is modified to allow nuclear autonomy within a common cytoplasm.     

Analysis of Residual Stress in 6H-SiC Particles within Al2O3/SiC Composites through Raman Spectroscopy

We measured the Raman spectrum associated with the E₂-TO_quasi and LO_quasi modes of 6H-SiC particles as a function of hydrostatic pressure using a diamond anvil cell. The results of this calibration experiment were used to analyze the residual stress in 6H-SiC particles within Al₂O₃/SiC composites with 12%, 20%, and 30% SiC by volume. The Raman spectra show that residual stress in the SiC near the surface of the composites is −2040 ± 120, −1841 ± 110, and −1615 ± 100 MPa for the 12%, 20%, and 30% SiC composites, respectively. The measured decrease in stress with increasing packing fraction is consistent with theoretical predictions based on micromechanics.     

The interaction of myristylated peptides with the catalytic domain of protein kinase C revealed by their sequence palindromy and the identification of a myristyl binding site

Using a model of the enzyme structure and the results from a series of free and myristylated peptides, we provide evidence that peptides corresponding to the pseudosubstrate sequence of protein kinase C bind to the enzyme substrate binding site in an essentially extended conformation. This and the nearly symmetrical location of positive charges around the substrate phosphoritable site allow the peptide to bind to the enzyme in either an N-to-C orientation or its C-to-N opposite orientation. The latter is favoured by a change in residue chirality or when the peptide bears a myristoyl chain at its N- terminus. A myristyl binding site was also identified in the enzyme structure and its location in a region proximal to the C-terminal residue of pseudosubstrate bound in the N-to-C direction suggested that C-myristylation of peptide substrates should be more effective than N- myristoylation in antagonizing the enzyme. A peptide (H-RFARKGALRQKN- CONH-Myr) which contains the myristyl chain covalently linked to the C- terminal residue of the pseudosubstrate was thus made and shown to be a potent inhibitor of the histone kinase reaction of protein kinase C and the phosphorylation of p47 in intact cells.      

The interaction of myristylated peptides with the catalytic domain of protein kinase C revealed by their sequence palindromy and the identification of a myristyl binding site

The nature of products of contamination intake were investigated in cattle dosed with [14C]di-n-butylphthalate (DBP). Radio-labelled metabolites were extracted from bile, faeces, plasma and urine onto solid-phase media, fractionated by ion-exchange chromatography, separated by reverse phase HPLC and analysed by negative ion atmospheric pressure chemical ionization mass spectrometry(n) (LCQ, Finnigan). All matrices contained a common major metabolite [deprotonated molecular ion (M-H)- m/z 221] which coeluted with and had an identical daughter ion spectrum to reference monobutylphthalate (MBP). MBP was metabolised to a beta-glucuronidase sensitive compound (M-H)- m/z 397 whose spectrum contained daughter ions (m/z 175 and 221) consistent with the parent glucuronide. A further three beta-glucuronidase resistant radio-labelled metabolites were also produced (M-H- m/z 165, 193 and 237); comparison of daughter ion spectra with those of reference MBP and phthalic acid indicated identity with phthalic acid, monoethylphthalate (MEP) and monohydroxybutylphthalate (MHBP) respectively. The presence of a benzoate daughter ion (m/z 121) in all spectra was indicative of side chain biotransformation. Both MBP and MEP contained a phthalate daughter ion (m/z 165) indicating loss of a butyl and ethyl side chain respectively. A daughter ion of m/z 89 derived from the side chain provided evidence that the third metabolite was MHBP. Incubation of DBP with isolated bovine hepatocytes produced the same metabolites and provided relatively clean samples for LC/MSn analysis. Detection of these DBP metabolites in meat or dairy food products will provide evidence for environmental exposure and biotransformation in vivo, whereas the presence of the parent compound would suggest contamination during food processing and packaging.