Dicty News Electronic Edition Volume 11, number 10 October 31, 1998 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/dicty.html" ============= Abstracts ============= Unusual centrosome cycle in Dictyostelium: correlation of dynamic behavior and structural changes Masahiro Ueda, Manfred Schliwa and Ursula Euteneuer Adolf Butenandt Institute, Cell Biology, University of Munich, 80336 Munich, Germany Mol.Biol. Cell, in press Abstract Centrosome duplication and separation are of central importance for cell division. Here we provide a detailed account of this dynamic process in Dictyostelium. Centrosome behavior was monitored in living cells using a g-tubulin-GFP fusion protein and correlated with morphological changes at the ultrastructural level. All aspects of the duplication and separation process of this centrosome are unusual when compared to, e.g., vertebrate cells. In interphase the Dictyostelium centrosome is a box-shaped structure comprised of three major layers, surrounded by an amorphous corona from which microtubules emerge. Structural duplication takes place during prophase, as opposed to G1/S in vertebrate cells. The three layers of the box-shaped core structure increase in size. The surrounding corona is lost, an event accompanied by a decrease in signal intensity of g-tub-GFP at the centrosome and the breakdown of the interphase microtubule system. At the prophase/prometaphase transition the separation into two mitotic centrosomes takes place via an intriguing lengthwise splitting process where the two outer layers of the prophase centrosome peel away from each other and become the mitotic centrosomes. Spindle microtubules are now nucleated from surfaces that previously were buried inside the interphase centrosome. Finally, at the end of telophase the mitotic centrosomes fold in such a way that the microtubule-nucleating surface remains on the outside of the organelle. Thus in each cell cycle the centrosome undergoes an apparent inside-out/outside-in reversal of its layered structure. ---------------------------------------------------------------------------- MODELING CHEMOTACTIC CELL SORTING DURING DICTYOSTELIUM DISCOIDEUM MOUND FORMATION Bakhtier Vasiev, Cornelis J.Weijer Department of Anatomy and Physiology, Wellcome Trust Building, University of Dundee, Dundee, DD1 4HN, UK Biophysical Journal, in press ABSTRACT Coordinated cell movement is a major mechanism of the multicellular development of most organisms. The multicellular morphogenesis of the slime mould Dictyostelium discoideum, from single cells into a multicellular fruiting body, results from differential chemotactic cell movement. During aggregation cells differentiate into prestalk and prespore cells which will form the stalk and spores in the fruiting body. These cell types arise in a salt and pepper pattern after what the prestalk cells chemotactically sort out to form a tip. The tip functions as an organizer since it directs the further development. It has been difficult to get a satisfactory formal description of the movement behavior of cells in tissues. Based on our experiments we consider the aggregate as a drop of a viscous fluid and show that this consideration is very well suited to mathematically describe the motion of cells in the tissue. We show that the transformation of a hemispherical mound into an elongated slug can result from the coordinated chemotactic cell movement in response to scroll waves of the chemoattractant cAMP. The model calculations furthermore show that cell sorting can result from differences in chemotactic cell movement and cAMP relay kinetics between the two cell types. During this process the faster moving and stronger signaling cells collect on the top of the mound to form a tip. The mound then extends into an elongated slug just as observed in experiments. The model is able to describe cell movement patterns in the complex multicellular morphogenesis of Dictyostelium rather well and we expect that this approach may be useful in the modeling of tissue transformations in other systems. ---------------------------------------------------------------------------- Filament structure as an essential factor for regulation of Dictyostelium myosin by regulatory light chain phosphorylation Xiong Liu1,2, Kohji Ito1,3, Sayuri Morimoto4, Atuko Hikkoshi-Iwane4, Toshio Yanagida4,5 and Taro Q. P. Uyeda1 1: Biomolecular Research Group, National Institute for Advanced Interdisciplinary Research, Higashi 1-1-4 Tsukuba, Ibaraki 305-8562, Japan 2: Present address: Laboratory of Cell Biology, National Heart, Lung, and Blood Institute, NIH, Bethesda, MD 20892, USA 3: Present address: Department of Biology, Faculty of Science, Chiba University, Inage, Chiba 263-8522, Japan 4: Yanagida Biomotron Project, ERATO, JST, 2-4-14 Senba-Higashi, Mino, Osaka 562, Japan 5: Department of Biophysical Engineering, Osaka University, Toyonaka, Osaka 560, Japan Proc. Natl. Acad. Sci. USA, in press. Abstract Phosphorylation of the regulatory light chain (RLC) activates the actin-dependent ATPase activity of Dictyostelium myosin II. To elucidate this regulatory mechanism, we characterized two mutant myosins, MyDC1225 and MyDC1528, which are truncated at Ala 1224 and Ser 1527, respectively. These mutant myosins do not contain the C-terminal assembly domain, and thus are unable to form filaments. Their activities were only weakly regulated by RLC phosphorylation, suggesting that, unlike smooth muscle myosin, efficient regulation of Dictyostelium myosin II requires filament assembly. Consistent with this hypothesis, wild type myosin progressively lost the regulation as its concentration in the assay mixture was decreased. Dephosphorylated RLC did not inhibit the activity when the concentration of myosin in the reaction mixture was very low. Furthermore, 3xAsp myosin, which does not assemble efficiently due to point mutations in the tail, was also less well-regulated than the wild type. We conclude that the activity in the monomer state is exempt from inhibition by the dephosphorylated RLC, and that the complete regulatory switch is formed only in the filament structure. Interestingly, a chimeric myosin composed of Dictyostelium heavy meromyosin fused to chicken skeletal light meromyosin was not well regulated by RLC phosphorylation. This suggests that, in addition to filament assembly, some specific feature of the filament structure is required for efficient regulation. ---------------------------------------------------------------------------- [End Dicty News, volume 11, number 10]