Dicty News Electronic Edition Volume 23, number 17 November 19, 2004 Please submit abstracts of your papers as soon as they have been accepted for publication by sending them to dicty@northwestern.edu or by using the form at http://dictybase.org/db/cgi-bin/dictyBase/abstract_submit. Back issues of Dicty-News, the Dicty Reference database and other useful information is available at dictyBase - http://dictybase.org. ============= Abstracts ============= Actin-activation of myosin heavy chain kinase A in Dictyostelium: A biochemical mechanism for the spatial regulation of myosin II filament disassembly Thomas T. Egelhoff, Daniel Croft, and Paul A. Steimle Department of Biology, University of North Carolina at Greensboro, Greensboro, NC 27402 Corresponding Author: Paul Steimle (p_steiml@uncg.edu) Journal of Biological Chemistry Published online ahead of print November 14, 2004 10.1074/jbc.M410803200 Studies in Dictyostelium discoideum have established that the cycle of myosin II bipolar filament assembly and disassembly controls the temporal and spatial localization of myosin II during critical cellular processes, such as cytokinesis and cell locomotion. Myosin heavy chain kinase A (MHCK A) is a key enzyme regulating myosin II filament disassembly through myosin heavy chain phosphorylation in Dictyostelium. Under various cellular conditions, MHCK A is recruited to actin-rich cortical sites and is preferentially enriched at sites of pseudopod formation; and thus is proposed to play a role in regulating localized disassembly of myosin II filaments in the cell. MHCK A possesses an aminoterminal coiled-coil domain that participates in the oligomerization, cellular localization, and actin binding activities of the kinase. In the current study, we show that the interaction between the coiled-coil domain of MHCK A and filamentous-actin leads to an approximately 40-fold increase in the initial rate of kinase catalytic activity. Actin-mediated activation of MHCK A involves increased rates of kinase autophosphorylation and requires the presence of the coiled-coil domain. Structure-function analyses revealed that the coiled-coil domain alone binds to actin filaments (apparent KD= 0.9 uM) thus mediates the direct interaction with F-actin required for MHCK A activation. Collectively, these results indicate that MHCK A recruitment to actin-rich sites could lead to localized activation of the kinase via direct interaction with actin filaments; and thus may represent an important mechanism by which the cell achieves localized disassembly of myosin II filaments required for specific changes in cell shape. Submitted by: Paul Steimle [p_steiml@uncg.edu] ----------------------------------------------------------------------------- A cost to chimerism in Dictyostelium discoideum on natural substrates David I. Castillo, Ghislaine T. Switz, Kevin R. Foster, David C. Queller, Joan E. Strassmann Evolutionary Ecology Research Most multicellular organisms go through a single cell bottleneck in development, a process that ensures subsequent clonality of the cells within the individual. Selection for clonality among cells could reduce costly intra-organismal conflicts that would occur in mixtures of unrelated cells (chimeras). In Dictyostelium discoideum, the usually solitary amoebae aggregate with nearby cells when starving to form a motile, multicellular slug that may be clonal or chimeric. This slug migrates to the soil surface and forms a ball of spores held aloft by a stalk of dead cells. Previous work on D. discoideum has shown that uniclonal slugs migrate further than chimeric slugs of the same size across agar, indicating a functional cost to chimerism. Here we test whether this cost to chimerism results in a fitness cost under more natural conditions. First, we examine migration of slugs across decaying leaves or soil. Second, we examine migration up through layers of these substrates, which most closely reflects the natural migration of D. discoideum slugs to the soil surface. In most trials, chimeras performed worse than single clones. Our results indicate that chimerism in D. discoideum has a real fitness cost in the wild, likely to be compensated only by the larger size chimeras can attain in nature. Submitted by: David I. Castillo [Navais@rice.edu] ----------------------------------------------------------------------------- A mitotic kinesin-like protein required for normal karyokinesis, myosin localization to the furrow, and cytokinesis in Dictyostelium Gandikota S. Lakshmikanth, Hans M. Warrick, and James A. Spudich * Department of Biochemistry, Stanford University School of Medicine, Stanford, CA 94305 Published online before print November 16, 2004 Proc. Natl. Acad. Sci. USA, 10.1073/pnas.0407304101 OPEN ACCESS ARTICLE Dictyostelium mitotic kinesin Kif12 is required for cytokinesis. Myosin II localization to the cleavage furrow is severely depressed in Kif12-null (dkif12) cells, which accounts in part for the cytokinesis failure. Myosin II-null cells, however, undergo mitosis-coupled cytokinesis when adhering to a surface, whereas the dkif12 cells cannot. During mitosis, the rate of change of internuclear separation in dkif12 cells is reduced compared with wild-type cells, indicating multiple roles of this molecular motor during mitosis and cytokinesis. GFP-Kif12, which rescues wild-type behavior when expressed in the dkif12 strain, is concentrated in the nucleus in interphase cells, translocates to the cytoplasm at the onset of mitosis, appears in the centrosomes and spindle, and later is concentrated in the spindle midbody. Given these results, we hypothesize a mechanism for myosin II translocation to the furrow to set up the contractile ring. Submitted by: Lakshmikanth gandikota [lakshmi@pmgm2.stanford.edu] ============================================================================== [End Dicty News, volume 23, number 17]