Showing papers by "Simon C. Watkins published in 1989"

Cytotoxicity of gelonin and its conjugates with antibodies is determined by the extent of their endocytosis.

Abstract: Conjugates of the single-chain ribosome-inactivating protein gelonin with ligands that bind to cell surface molecules vary greatly in their cytotoxicity. Conjugates that are not endocytosed after binding to cells exhibit low cytotoxicity similar to that of free gelonin, while conjugates that are endocytosed demonstrate enhanced cytotoxicity relative to free gelonin. However, the number of internalized gelonin molecules needed to intoxicate cells to the same degree has been found to be similar for all conjugates and for free gelonin. The intracellular concentration of gelonin has to be between 2,000-10,000 molecules/cells to achieve a surviving fraction of 0.37. Our studies revealed the presence of three distinct categories of cell surface molecules, those that are efficient in mediating endocytosis of immunotoxins, those that are only moderately efficient, and those that seem not to cause internalization of bound immunotoxins.

Dystrophin distribution in heterozygote MDX mice

Abstract: The distribution of dystrophin in myofibers from normal, mdx hemizygous, and mdx heterozygous mice was studied at various times in development. While normal mice exhibit dystrophin immunostaining around the entire fiber periphery regardless of age, mdx hemizygous mice exhibit no staining (0-35 days). In contrast, young (10 day) heterozygous mdx mice showed neighboring dystrophin-negative and dystrophin-positive fibers as well as fibers with a discontinuous or patchy dystrophin labelling. Older heterozygotes displayed very few negative fibers, with most fibers exhibiting apparently complete dystrophin immunostaining. This, coupled with the absence of muscle fiber degeneration at any age point, and the apparently normal levels of dystrophin in older heterozygous mice, indicates that myonuclei containing the dystrophin gene can compensate for myonuclei which do not contain the dystrophin gene within the same myofiber.

Detection of a specific isoform of alpha-actinin with antisera directed against dystrophin.

Abstract: We have characterized a protein immunologically related to dystrophin, the protein product of the Duchenne muscular dystrophy gene. We identify this related protein as a fast-twitch glycolytic isoform (mouse extensor digitorum longus-specific) of myofibrillar alpha-actinin. This specific isoform of alpha-actinin exhibits a more restricted pattern of expression in skeletal muscle than fast-twitch-specific isoforms of both myosin and Ca2+-ATPase. Our results provide evidence that dystrophin and myofibrillar alpha-actinin are related proteins, reinforcing the previous data concerning the sequence homologies noted between nonmuscle cytoskeletal alpha-actinin and dystrophin. In addition, we describe the first antisera directed against a specific myofibrillar skeletal muscle isoform of alpha-actinin.

Formation of new quasi-crystalline ordered aggregates by gizzard myosin.

Abstract: Turkey gizzard myosin was found to self-assemble into new polymorphic forms as detected by thin-section electron microscopy. In high ionic strength buffers (0.3 ihm KCl, pH 6.0), aggregates of sidepolar filaments were produced. Electron microscopy of thin sections revealed individual filaments with a 13.5 nm axial repeat. Under a number of conditions, with varying ionic strength, pH, MgCl2 and ATP, the filaments assembled through the head region with the tail portion projecting out radially from the aggregate. The regions corresponding to heads and tails within the aggregates were established by immunoelectron microscopy using anti-Si and anti-LMM antibodies coupled to gold. These filaments often interacted to produce bilayer sheets, which, when cut perpendicular to the plane of the sheet, appeared as ladders. A hitherto unreported structure was obtained at 0.2m KCl (pH 8.0): myosin aggregated to generate a three-dimensional quasi-crystalline lattice with a 270 nm period. In these aggregates, myosin was arranged in an antiparallel fashion, stacked on one another, producing ribbon-like strips stabilized through non-covalent interactions between heads, thereby producing a crystalline lattice. Neither Mg2+ nor ATP were required for this form. Phosphorylation of the regulatory light chains or the cleavage of the heavy chains at a single site in the head region prevented myosin from assembling in the 3-D lattice form. Generally, unphosphorylated myosin produced periodic paracrystals at low ionic strength in the presence of 10 him MgCl2, but as the ionic strength was increased the regular 3-D lattice became the predominant form. Some paracrystalline forms could be obtained at high ionic strength without magnesium with phosphorylated myosin.