Dicty News Electronic Edition Volume 15, number 5 September 2, 2000 Please submit abstracts of your papers as soon as they have been accepted for publication by sending them to dicty@nwu.edu. Back issues of Dicty-News, the Dicty Reference database and other useful information is available at the Dictyostelium Web Page "http://dicty.cmb.nwu.edu/dicty" ============== Abstracts ============== Tyrosine phosphorylation in plant movement Kazuhisa Kameyama*†, Yoshiro Kishi*‡, Masanori Yoshimura†, Nobuyuki Kanzawa†, Masazumi Sameshima‡ & Takahide Tsuchiya† †Department of Chemistry, Faculty of Science and Technology, Sophia University, 7-1 Kioicho, Chiyoda-ku, Tokyo 102-8554, Japan ‡Department of Cell Biology, The Tokyo Metropolitan Institute of Medical Science, 3-18-22 Honkomagome, Bunkyo-ku, Tokyo 171-0021, Japan *These authors contributed equally to the work. Nature, in press The phosphorylation of the amino acid tyrosine in animal proteins acts as an on-off switch in numerous pathways that regulate growth, differentiation and oncogenesis. In the cellular slime Dictyostelium discoideum, tyrosine-phosphorylation of actin controls cell-shape changes and spore activity, but the significance of protein-tyrosine phosphorylation in plants is unknown. Here we show that actin in contact-sensitive plant Mimosa pudica L. is heavily tyrosine-phosphorylated and that changes in the extent of phosphorylation correlate with the degree of bending of the plant's petioles. We propose that tyrosine- phosphorylation of actin controls movements in plants. ---------------------------------------------------------------------------- Cell-fate choice in Dictyostelium: intrinsic biases modulate sensitivity to DIF signalling Christopher R. L. Thompson and Robert R. Kay MRC Laboratory of Molecular Biology, Hills Road, Cambridge, CB2 2QH England Developmental Biology, in press Cell fate in Dictyostelium development depends on intrinsic differences between cells, dating from their growth period, and on cell interactions occurring during development. We have sought for a mechanism linking these two influences on cell fate. First, we confirmed earlier work showing that the vegetative differences are biases, not commitments, since cells that are stalky-biased when developed with one partner are sporey with another. Then we tested the idea that these biases operate by modulating the sensitivity of cells to the signals controlling cell fate during development. Cells grown without glucose are stalky-biased when developed with cells grown with glucose. We find, using monolayer culture conditions, that they are more sensitive to each of the stalk- inducing signals, DIFs 1-3. Mixing experiments show that this bias is a cell-intrinsic property. Cells initiating development early in the cell cycle are stalky compared to those initiating development later in the cycle. Likewise, they are more sensitive to DIF-1. Assays of standard markers for prestalk and prespore cell differentiation reveal similar differences in DIF-1 sensitivity between biased cells; DIF-1 dechlorinase (an early prestalk cell marker enzyme) behaves in a consistent manner. We propose that cell-fate biases are manifest as differences in sensitivity to DIF. ---------------------------------------------------------------------------- A precise group size in Dictyostelium is generated by a cell-counting factor modulating cell-cell adhesion Celine Roisin-Bouffay1, Wonhee Jang2, David R. Caprette2, and Richard H. Gomer1,2 1Howard Hughes Medical Institute and 2Department of Biochemistry and Cell Biology MS-140, Rice University 6100 S. Main Street Houston, TX 77005-1892 Molecular Cell, in press. A remarkable aspect of Dictyostelium development is that cells form evenly sized groups of ~2 x 10exp4 cells. A secreted 450 kD protein complex called counting factor (CF) regulates the number of cells per group. We find that CF regulates group size by repressing cell-cell adhesion. In both experiments and computer simulations, high levels of CF (and thus low adhesion) result in aggregation streams breaking up into small groups, while no CF (and thus high adhesion) results in no stream breakup and large groups. These results suggest that in Dictyostelium and possibly other systems a secreted factor regulating cell-cell adhesion can regulate the size of a group of cells. ---------------------------------------------------------------------------- Related Ga subunits play opposing roles during Dictyostelium development Kanchana Natarajan, Casey A. Ashley and Jeffrey A. Hadwiger Department of Microbiology and Molecular Genetics Oklahoma State University, Stillwater, Oklahoma 74078-3020 Differentiation, In Press Of the several known Dictyostelium G protein subunits, the Ga4 and Ga5 subunits are the most closely related pair based on phylogenetic analysis and expression patterns but these subunits perform different roles during development. To investigate potential relationships between these subunits with respect to cell differentiation, chimeric organisms composed of strains lacking or overexpressing either subunit were created and examined for developmental morphogenesis and spore production. Chimeras of ga4 null and ga5 null strains or Ga4 and Ga5 overexpression strains displayed compensatory morphogenesis, implying the subunits promote complementary developmental processes. However, chimeras composed of ga4 null and Ga5 overexpression strains or ga5 null and Ga4 overexpression strains displayed distorted tip morphogenesis, suggesting the strains of each chimera share common developmental deficiencies. Cells lacking the Ga5 subunit localized to the prespore region of chimeras similar to the pattern observed for cells overexpressing the Ga4 subunit and cells overexpressing the Ga5 subunit displayed localization patterns similar to ga4 null mutants. A strain overexpressing both subunits displayed a partial suppression of morphology, gene expression and cell localization phenotypes associated with the overexpression of the individual Ga subunit genes suggesting each Ga subunits can inhibit signaling mediated by the other subunit. Overexpression of the Ga5 subunit inhibited chemotaxis and cGMP accumulation in response to folic acid, indicating the Ga5 subunit can inhibit early steps in the Ga4-mediated signal transduction pathway. The contrasting phenotypes of the Ga mutants suggest the Ga4 and Ga5 subunits provide opposing functions in cell differentiation, localization, and chemotactic responses to folic acid. ---------------------------------------------------------------------------- [End Dicty News, volume 15, number 5]