Localisation of light chain and actin binding sites on myosin

A gel overlay technique has been used to identify a region of the myosin S-1 heavy chain that binds myosin light chains (regulatory and essential) and actin. The 125I-labelled myosin light chains and actin bound to intact vertebrate skeletal or smooth muscle myosin, S-1 prepared from these myosins and the C-terminal tryptic fragments from them (i.e. the 20-kDa or 24-kDa fragments of skeletal muscle myosin chymotryptic or Mg2+/papain S-1 respectively). MgATP abolished actin binding to myosin and to S-1 but had no effect on binding to the C-terminal tryptic fragments of S-1. The light chains and actin appeared to bind to specific and distinct regions on the S-1 heavy chain, as there was no marked competition in gel overlay experiments in the presence of 50-100 molar excess of unlabelled competing protein. The skeletal muscle C-terminal 24-kDa fragment was isolated from a tryptic digest of Mg2+/papain S-1 by CM-cellulose chromatography, in the presence of 8 M urea. This fragment was characterised by retention of the specific label (1,5-I-AEDANS) on the SH1 thiol residue, by its amino acid composition, and by N-terminal and C-terminal sequence analyses. Electron microscopical examination of this S-1 C-terminal fragment revealed that: it had a strong tendency to form aggregates with itself, appearing as small 'segment-like' structures that formed larger aggregates, and it bound actin, apparently bundling and severing actin filaments. Further digestion of this 24-kDa fragment with Staphylococcus aureus V-8 protease produced a 10-12-kDa peptide, which retained the ability to bind light chains and actin in gel overlay experiments. This 10-12-kDa peptide was derived from the region between the SH1 thiol residue and the C-terminus of S-1. It was further shown that the C-terminal portion, but not the N-terminal portion, of the DTNB regulatory light chain bound this heavy chain region. Although at present nothing can be said about the three-dimensional arrangement of the binding sites for the two kinds of light chain (regulatory and essential) and actin in S-1, it appears that these sites are all located within a length of the S-1 heavy chain of about 100 amino acid residues.     

Separation of phosphoinositides and other phospholipids by two-dimensional thin-layer chromatography

A simple, rapid, two-dimensional TLC system is presented which resolves the four phosphoinositide cycle phospholipids as well as all commonly encountered major and minor phospholipids. Ca2+-free lipid samples are loaded onto silica gel HL plates and developed first in 48:40:7:5 chloroform:methanol:water:concentrated ammonia, and then in 55:25:5 chloroform:methanol:formic acid. The method was applied successfully to human erythrocytes, human platelets, and BL/VL3 murine lymphoma cells.     

Effect of gamma radiation on resting B lymphocytes. I. Oxygen-dependent damage to the plasma membrane results in increased permeability and cell enlargement

Although the susceptibility of resting B lymphocytes to radiation-induced interphase death is well known, the mechanism by which this occurs is not understood. In this report, we use three measures of plasma membrane integrity (increase in cell volume, uptake of trypan blue, and release of 51Cr) to assess the effect of radiation on the resting B cell plasma membrane. The delivery of 500 to 1000 rad caused the majority of resting B cells to enlarge slightly, whereas 3000 rad caused virtually all of the cells to approximately double in size within 3 to 4 hr. Measurement of the release of 51Cr from resting B cells revealed a similar relationship between the dose of radiation and the loss of radioactive label. Trypan blue exclusion was also found to diminish as a function of radiation dose. An analysis of a variety of lymphoid cells suggested that sensitivity to the membrane damaging effects of gamma radiation was in the order of resting B cells greater than resting T cells greater than a long-term L3T4+ T cell clone greater than a B cell lymphoma. LPS-induced B cell blasts treated with 3000 rad were equivalent to 1000 rad-treated resting B cells. The effects of the gamma radiation could be ameliorated by excluding oxygen (a diradical molecule that can potentially enhance the generation and propagation of highly reactive free radicals) at the time of irradiation, or by adding the free radical scavenging agent cysteamine. These data are compatible with the hypothesis that gamma radiation results in damage to the plasma membrane of resting lymphocytes via the generation of highly reactive free radical species. This damage is reflected in a rapid increase in plasma membrane permeability and swelling of the cells, and may play a major role in causing interphase death.     

Roles of H1 domains in determining higher order chromatin structure and H1 location

Peptides derived from calf thymus H1 and rat liver H1, comprising only the globular and COOH-terminal domains of the intact molecule and therefore lacking NH2-terminal domains, have been shown by reconstitution to be as effective as the complete H1 molecule in inducing higher-order-chromatin structure. As the globular domain of H1 alone cannot induce chromatin folding, our results demonstrate that this function is primarily controlled by the COOH-terminal domain of the molecule. Surprisingly, these peptides do not locate correctly with respect to the nucleosome. This is demonstrated by their failure to confer upon reconstitutes the ability to protect DNA fragments of chromatosome length when digested with micrococcal nuclease. The precise placement of the H1 molecule (globular domain) with respect to the nucleosome is shown to be influenced by the "tail" domains of both H1 and the core histones.     

The relative cytotoxicity and mutagenicity of cyclobutane pyrimidine dimers and (6-4) photoproducts in Escherichia coli cells

In order to calculate the relative cytotoxicity and mutagenicity of (5-6) cyclobutane pyrimidine dimers and (6-4) photoproducts, we have measured survival and mutation induction in UV-irradiated excision-deficient E. coli uvrA cells, with or without complete photoreactivation of the (5-6) dimers. Radioimmunoassays with specificity for (5-6) dimers or (6-4) photoproducts have shown that maximum photoreactivation eliminates all of the (5-6) dimers produced up to 10 Jm-2 254-nm light, while it has no effect on (6-4) photoproducts. These results were confirmed by measuring the frequency of T4 endonuclease V-sensitive sites. Based on the best fit equations for survival and mutation induction, we have found that the calculated cytotoxicity of (6-4) photoproducts is similar to that of (5-6) dimers; however, the former is much more mutagenic than the latter.     

Molecular cloning and expression of flagellar radial spoke and dynein genes of Chlamydomonas

Several flagellar dynein ATPase and radial spokehead genes have been isolated from a Chlamydomonas genomic expression library in lambda gt11. The library was probed with polyclonal and monoclonal antibodies raised against purified flagellar polypeptides, and recombinant phage giving positive signals were cloned. In vitro translation of mRNAs hybrid-selected by the cloned sequences from whole cell RNA provided confirmation of identity for three of the four clones. Evidence supporting the identification of the fourth, which encodes a dynein heavy chain, was provided by antibody selection; the fusion protein produced by this clone selected heavy chain-specific antibodies from a complex polyclonal antiserum recognizing many dynein determinants. One of the radial spoke sequences isolated here is of particular interest because it encodes the wild-type allele of a locus which was defined previously by temperature-sensitive paralyzed flagella mutation pf-26ts (Huang, B., G. Piperno, Z. Ramanis, and D. J. L. Luck, 1981, J. Cell Biol., 88:80-88). The cloned sequence was used to hybrid-select mRNA from mutant pf-26ts cells, and when translated in vitro, the selected mRNA produced a mutant spokehead polypeptide with an altered electrophoretic mobility. This confirms that the pf-26ts mutation alters the primary structure of a radial spokehead polypeptide. To quantify spokehead and dynein mRNAs during flagellar regeneration, all of the cloned sequences were used as hybridization probes in RNA dot experiments. Levels increased rapidly and coordinately after deflagellation, peaked 3-10-fold above nondeflagellated controls, and then returned to control values within 2 h. This accumulation pattern was similar to that of flagellar alpha-tubulin mRNA.     

Stimulator requirements for primed alloreactive T cells: macrophages and dendritic cells activate T cells across all genetic disparities

The cellular requirements for stimulating primed alloreactive T cells have been investigated. In vitro-primed secondary alloreactive cells, long-term lines, and Ly 1+2- noncytolytic clones which reacted with allo-H-2K, D, or Mls (M locus) antigens were tested. The data indicated that a specialized antigen-presenting cell such as a macrophage or a dendritic cell was required for stimulating primed alloreactive cells across all the genetic disparities tested. B and T lymphocytes were ineffective stimulators. The stimulator requirement for secondary and Ly 1+2- clone responses was heterogeneous, since both macrophages and dendritic cells were effective stimulators. Thus, the allostimulator requirement for inducing proliferation and mediator secretion by the primed T-cell populations closely paralleled the requirement for stimulating unprimed populations. The only exception found was the peritoneal washout population, which did not stimulate a primary response but did stimulate secondary responses. The failure of peritoneal macrophages to stimulate a primary response was shown to be due to an inhibitory pathway which did not occur when the responding population was alloantigen primed.     

Inhibition of proliferation without affecting the generation of cytotoxicity in the human mixed lymphocyte reaction

Phosphoramide mustard (PM) is considered to be the major tumoricidal metabolite of cyclophosphamide in vivo. The effects of this metabolite in vitro on several immune functions of human lymphocytes have been investigated. Very low concentrations (10(-7) to 10(-9) M) of PM added to lymphocyte cultures inhibited proliferation of the lymphocytes in response to mitogens and alloantigens. At these concentrations, inhibition of proliferation appeared to be due to a direct action of PM on the proliferative cells. Thus, concanavalin A-stimulated lymphocytes still acquired IL-2 receptors (Tac antigen) normally in the presence of PM (10(-6) to 10(-9) M). Only exceedingly high concentrations of PM (10(-5) M or greater) prevented the acquisition of Tac antigen. Similarly, the inhibition of proliferation was probably not related to endogenous IL-2 levels: addition of exogenous IL-2 to PM-containing cultures did not result in any restoration of proliferation. Further evidence that PM directly affected proliferative cells was that low concentrations of PM inhibited the proliferation of T cells continuously growing in IL-2. The exposure time to PM necessary for inhibition was essentially identical to those for lymphoproliferative responses to mitogens and alloantigens. Paradoxically, however, the generation of cytotoxic lymphocytes in mixed lymphocyte reactions (MLRs) and mixed lymphocyte tumor cell cultures (MLTCs) was very resistant to PM. In parallel MLRs and MLTCs the cytotoxic responses were resistant to approximately 1000-fold more PM than were the proliferative responses. Only at 10(-5) M PM were these inhibited. These data suggest that clonal expansion of cytotoxic lymphocytes or their precursors by proliferation is not an absolute requirement for the generation of cytolytic activity.