dictyNews Electronic Edition Volume 39, number 24 August 23 2013 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 ========= How do amoebae swim and crawl? Jonathan D. Howe, Nicholas P. Barry, Mark S. Bretscher* Cell Biology Division, Medical Research Council Laboratory of Molecular Biology, Cambridge, Cambridgeshire, United Kingdom. PLoS ONE, in press. The surface behaviour of swimming amoebae was followed in cells bearing a cAR1-paGFP (cyclicAMP receptor fused to a photoactivatable-GFP) construct. Sensitized amoebae were placed in a buoyant medium where they could swim toward a chemoattractant cAMP source. paGFP, activated at the cell's front, remained fairly stationary in the cell's frame as the cell advanced; the label was not swept rearwards. Similar experiments with chemotaxing cells attached to a substratum gave the same result. Furthermore, if the region around a lateral projection near a crawling cell's front is marked, the projection and the labelled cAR1 behave differently. The label spreads by diffusion but otherwise remains stationary in the cell's frame; the lateral projection moves rearwards on the cell (remaining stationary with respect to the substrate), so that it ends up outside the labelled region. Furthermore, as cAR1-GFP cells move, they occasionally do so in a remarkably straight line; this suggests they do not need to snake to move on a substratum. Previously, we suggested that the surface membrane of a moving amoeba flows from front to rear as part of a polarised membrane trafficking cycle. This could explain how swimming amoebae are able to exert a force against the medium. Our present results indicate that, in amoebae, the suggested surface flow does not exist: this implies that they swim by shape changes. Submitted by Mark Bretscher [msb@mrc-lmb.cam.ac.uk] --------------------------------------------------------------------------- A new kind of membrane-tethered eukaryotic transcription factor that shares an auto-proteolytic processing mechanism with bacteriophage tail-spike proteins Hiroshi Senoo, Tsuyoshi Araki+, Masashi Fukuzawa and Jeffrey G. Williams+* Department of Biology, Faculty of Agriculture and Life Science, Hirosaki University, Hirosaki, Aomori 036-8561, Japan + College of Life Sciences, Welcome Trust Building, University of Dundee, Dow St., Dundee DD1 5EH UK * corresponding author: tel 44 1382 385823 fax 44-1382 344211 j.g.williams@dundee.ac.uk Journal of Cell Science, in press MrfA, a transcription factor that regulates Dictyostelium prestalk cell differentiation, is an orthologue of the animal Myelin-gene Regulatory Factor (MRF) proteins. We show that the MRFs contain a predicted trans- membrane domain, suggesting that they are synthesized as membrane- tethered proteins that are then proteolytically released. We confirm this for MrfA but report a radically different mode of processing from that of paradigmatic tethered transcriptional regulators; which are cleaved within the trans-membrane domain by a dedicated protease. Instead an auto- proteolytic cleavage mechanism, previously only described for the intramolecular chaperone domains of bacteriophage tail-spike proteins, processes MrfA and, by implication, the metazoan MRF proteins. We also present evidence that the auto-proteolysis of MrfA occurs rapidly and constitutively in the ER and that its specific role in prestalk cell differentiation is conferred by theregulated nuclear translocation of the liberated fragment. Submitted by Jeff Williams [j.g.williams@dundee.ac.uk] --------------------------------------------------------------------------- Iron metabolism and resistance to infection by invasive bacteria in the social amoeba Dictyostelium discoideum Salvatore Bozzaro*, Simona Buracco and Barbara Peracino Department of Clinical and Biological Sciences, University of Torino, Orbassano, Italy Frontiers Cell Infect Microbiol, in press Dictyostelium cells are forest soil amoebae, which feed on bacteria and proliferate as solitary cells until bacteria are consumed. Starvation triggers a change in life style, forcing cells to gather into aggregates to form multicellular organisms capable of cell differentiation and morphogenesis. As a soil amoeba and a phagocyte that grazes on bacteria as the obligate source of food, Dictyostelium could be a natural host of pathogenic bacteria. Indeed, many pathogens that occasionally infect humans are hosted for most of their time in protozoa or free-living amoebae, where evolution of their virulence traits occurs. Due to these features and its amenability to genetic manipulation, Dictyostelium has become a valuable model organism for studying strategies of both the host to resist infection and the pathogen to escape the defence mechanisms. Similarly to higher eukaryotes, iron homeostasis is crucial for Dictyostelium resistance to invasive bacteria. Iron is essential for Dictyostelium, as both iron deficiency or overload inhibit cell growth. The Dictyostelium genome shares with mammals many genes regulating iron homeostasis. Iron transporters of the Nramp (Slc11A) family are represented with two genes, encoding Nramp1 and Nramp2. Like the mammalian ortholog, Nramp1 is recruited to phagosomes and macropinosomes, whereas Nramp2 is a membrane protein of the contractile vacuole network, which regulates osmolarity. Nramp1 and Nramp2 localization in distinct compartments suggests that both proteins synergistically regulate iron homeostasis. Rather than by absorption via membrane transporters, iron is likely gained by degradation of ingested bacteria and efflux via Nramp1 from phagosomes to the cytosol. Nramp gene disruption increases Dictyostelium sensitivity to infection, enhancing intracellular growth of Legionella or Mycobacteria. Generation of mutants in other "iron genes" will help identify genes essential for iron homeostasis and resistance to pathogens. Submitted by Salvatore Bozzaro [salvatore.bozzaro@unito.it] ============================================================== [End dictyNews, volume 39, number 24]