dictyNews Electronic Edition Volume 35, number 17 Dec 10, 2010 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 dictyNews, the Dicty Reference database and other useful information is available at dictyBase - http://dictybase.org. Follow dictyBase on twitter: http://twitter.com/dictybase ========= Abstracts ========= Molecular mechanism of Ena/VASP-mediated actin filament elongation 1 Dennis Breitsprecher, 1 Antje K. Kiesewetter, 1 Joern Linkner, 2 Marlene Vinzenz, ,4 Theresia E.B. Stradal, 2 J. Victor Small, 1 Ute Curth, 4 Richard B. Dickinson and 1 Jan Faix. 1) Institute for Biophysical Chemistry, Hannover Medical School, Germany. 2) Institute of Molecular Biotechnology, Austrian Academy of Sciences, Austria. 3) Institute for Molecular Cell Biology, University of Muenster, Germany. 4) Department of Chemical Engineering, University of Florida, USA. EMBO Journal, in press Ena/VASP proteins are implicated in a variety of fundamental cellular processes including axon guidance and cell migration. In vitro, they enhance elongation of actin filaments, but at rates differing in nearly an order of magnitude according to species, raising questions about the molecular determinants of rate control. Chimeras from fast and slow elongating VASP proteins were generated and their ability to promote actin polymerization and to bind G-actin was assessed. By in vitro TIRF microscopy as well as thermodynamic and kinetic analyses we show that the velocity of VASP-mediated filament elongation depends on G-actin recruitment by the WH2 motif. Comparison of the experimentally observed elongation rates with a quantitative mathematical model moreover revealed that Ena/VASP-mediated filament elongation displays a saturation dependence on the actin monomer concentration, implying that Ena/VASP proteins, independent of species, are fully saturated with actin in vivo and generally act as potent filament elongators. Moreover, our data showed that spontaneous addition of monomers does not occur during processive VASP-mediated filament elongation on surfaces, suggesting that most filament formation in cells is actively controlled. Submitted by Jan Faix [faix.jan@mh-hannover.de] -------------------------------------------------------------------------------- The Dictyostelium Model for Mitochondrial Disease Lisa M. Francione,1 Sarah J. Annesley1, Sergio Carilla-Latorre2, Ricardo Escalante2, Paul R. Fisher1* 1 Department of Microbiology, La Trobe University, VIC 3086, Australia. 2 Instituto de Investigaciones BiomŽdicas Alberto Sols, CSIC-UAM, Arturo Duperier 4, 28029-Madrid, Spain. Seminars in Cell and Developmental Biology, in press. Mitochondrial diseases are a diverse family of genetic disorders caused by mutations affecting mitochondrial proteins encoded in either the nuclear or the mitochondrial genome. By impairing mitochondrial oxidative phosphorylation, they compromise cellular energy production and the downstream consequences in humans are a bewilderingly complex array of signs and symptoms that can affect any of the major organ systems in unpredictable combinations. This complexity and unpredictability has limited our understanding of the cytopathological consequences of mitochondrial dysfunction. By contrast, in Dictyostelium the mitochondrial disease phenotypes are consistent, measurable ÒreadoutsÓ of dysregulated intracellular signalling pathways. When the underlying genetic defects would produce coordinate, generalized deficiencies in multiple mitochondrial respiratory complexes, the disease phenotypes are mediated by chronic activation of an energy-sensing protein kinase, AMPK (AMP-activated protein kinase). This chronic AMPK hyperactivity maintains mitochondrial mass and cellular ATP concentrations at normal levels, but chronically impairs growth, cell cycle progression, multicellular development, photosensory and thermosensory signal transduction. It also causes the cells to support greater proliferation of the intracellular bacterial pathogen, Legionella pneumophila. Notably however, phagocytic and macropinocytic nutrient uptake are impervious both to AMPK signalling and to these types of mitochondrial dysfunction. Surprisingly, a Complex I-specific deficiency (midA knockout) not only causes the foregoing AMPK-mediated defects, but also produces a dramatic deficit in endocytic nutrient uptake accompanied by an additional secondary defect in growth. More restricted and specific phenotypic outcomes are produced by knocking out genes for nuclear-encoded mitochondrial proteins that are not required for respiration. The Dictyostelium model for mitochondrial disease has thus revealed consistent patterns of sublethal dysregulation of intracellular signalling pathways that are produced by different types of underlying mitochondrial dysfunction. Submitted by Paul Fisher [P.Fisher@latrobe.edu.au] -------------------------------------------------------------------------------- The NDR family kinase NdrA of Dictyostelium localizes to the centrosome and is required for efficient phagocytosis Peter M. Kastner, Michael Schleicher, Annette MŸller-Taubenberger Institute for Anatomy and Cell Biology, Ludwig Maximilian University of Munich, Schillerstr. 42, 80336 Munich, Germany Traffic, in press Dictyostelium discoideum cells are professional phagocytes that provide an easily accessible system to gain insights into the mechanisms and the regulatory machinery controlling phagocytosis. Here, we describe a novel function for NDR (nuclear Dbf2-related) family kinases in phagocytosis of D. discoideum. Deletion of one of the four NDR kinases of D. discoideum, NdrA, resulted in impaired cell growth caused by reduced phagocytosis rates. Detailed analysis of NdrA-null cells revealed that the formation of phagocytic cups was delayed. Microscopic investigations showed that NdrA localizes to centrosomes, and NdrA was also identified in isolated centrosome preparations. The localization of NdrA is regulated during the cell cycle. In prophase, NdrA disappears from the centrosome and forms a cloud-like structure around the spindle, which is totally absent during later stages until completion of mitosis. Our results suggest that a signal which originates from the NdrA kinase at the centrosome affects the efficiency of phagocytosis. We assume that in NdrA-null cells the defects in phagocytosis may be caused by an impairment of vesicle trafficking which is involved in providing new membrane at the sites of particle uptake. Submitted by Annette MŸller-Taubenberger [amueller@lrz.uni-muenchen.de] -------------------------------------------------------------------------------- Primitive agriculture in a social amoeba Debra A. Brock, Tracy E. Douglas, David C. Queller, and Joan E. Strassmann Department of Ecology and Evolutionary Biology, Rice University, 6100 Main Street, Houston, Texas 77005, USA Nature, in press Agriculture has been a large part of the ecological success of humans. A handful of animals, notably the fungus growing ants, termites, and ambrosia beetles, have advanced agriculture that involves dispersing and seeding of food propagules, cultivation of the crop, and sustainable harvesting. More primitive examples, which could be called husbandry because they involve fewer adaptations, include marine snails farming intertidal fungi and damselfish farming algae. Recent work has shown that microorganisms are surprisingly like animals in having sophisticated behaviours such as cooperation, communication, and recognition, as well as many kinds of symbioses. We now show that the social amoeba Dictyostelium discoideum exhibits a primitive farming symbiosis that includes dispersal and prudent harvesting of the crop. About a third of wild-collected clones engage in husbandry of bacteria. Instead of consuming all bacteria in their patch, they stop feeding early and incorporate bacteria into their fruiting bodies. They then carry bacteria during spore dispersal and can seed a new food crop, a major advantage if edible bacteria are lacking at the new site. However, if they arrive at sites already containing appropriate bacteria, the costs of early feeding cessation are not compensated, which may account for the dichotomous nature of this farming symbiosis. The striking convergent evolution between bacterial husbandry in social amoebas and fungus farming in social insects makes sense because multigenerational benefits of farming go to already-established kin groups. Submitted by Debbie Brock [dbrock@rice.edu] ============================================================== [End dictyNews, volume 35, number 17]