dictyNews Electronic Edition Volume 28, number 3 January 26, 2007 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. ========= Abstracts ========= Localisation and organization of protein factors involved in chromosome inheritance in Dictyostelium discoideum Markus Kaller, Balint Foeldesi and Wolfgang Nellen*, Abteilung Genetik, FB 18, Universitaet Kassel, Heinrich-Plett-Str. 40. 34132 Kassel, Germany Biol. Chemistry, in press The members of the heterochromatin protein 1 (HP1) family are highly conserved heterochromatin components required for a various processes that ensure genomic integrity. We have previously shown that the two expressed HP1 isoforms in Dictyostelium, HcpA and HcpB, are mainly localized to (peri-)centromeric heterochromatin and have largely overlapping functions. However, they cause distinct phenotypes when overexpressed. We now show that the two isoforms display quantitative differences in dimerisation behaviour and that these are conferred by the C-terminal hinge and chromo shadow domains. Both Hcp proteins are targeted to distinct subnuclear regions by different chromo shadow domain-dependent and independent mechanisms in vivo. In addition, both proteins bind to DNA and RNA in vitro in an apparently sequence-nonspecific manner and binding is independent on the chromo shadow domain. Thus, this DNA/RNA binding activity may contribute to targeting the proteins to heterochromatin. To further characterize heterochromatic regions of the Dictyostelium genome, we cloned the homologue of the origin recognition complex subunit 2 (Orc2). OrcB localized to distinct subnuclear foci that were also targeted by HcpA, but was, in addition, associated with the centrosome throughout the cell cycle. The results indicate that, similar to Orc2 homologues from other organisms, OrcB is required for different processes in chromosome inheritance. Submitted by Wolfgang Nellen [Nellen@uni-kassel.de] -------------------------------------------------------------------------------- Chemotaxis in shallow gradients is mediated independently of PtdIns 3-kinase by biased choices between random protrusions Natalie Andrew and Robert H. Insall Nature Cell Biol. in press Current models of eukaryotic chemotaxis propose that directional sensing causes localized generation of new pseudopods. However, quantitative analysis of pseudopod generation suggests a fundamentally different mechanism for chemotaxis in shallow gradients: first, pseudopodia in multiple cell types are usually generated when existing ones bifurcate and are rarely made de novo; second, in Dictyostelium cells in shallow chemoattractant gradients, pseudopodia are made at the same rate whether cells are moving up or down gradients. The location and direction of new pseudopodia are random within the range allowed by bifurcation and are not oriented by chemoattractants. Thus, pseudopod generation is controlled independently of chemotactic signalling. Third, directional sensing is mediated by maintaining the most accurate existing pseudopod, rather than through the generation of new ones. Finally, LY294002 affects the frequency of pseudopod generation, but not the accuracy of selection, suggesting that phosphatidylinositol 3-kinase (PI(3)K) regulates the underlying mechanism of cell movement, rather than control of direction. Submitted by Robert Insall [R.H.Insall@bham.ac.uk] -------------------------------------------------------------------------------- Rap1 controls cell adhesion and cell motility through the regulation of myosin II Taeck J. Jeon, Dai-Jen Lee, Sylvain Merlot, Gerald Weeks, and Richard A. Firtel J. Cell Biology, in press We have investigated the role of Rap1 in controlling chemotaxis and cell adhesion in Dictyostelium. Rap1 is activated rapidly in response to chemoattractant stimulation and activated Rap1 is preferentially found at the leading edge of chemotaxing cells. Cells expressing constitutively active Rap1 are highly adhesive and exhibit strong chemotaxis defects, partially due to an inability to spatially and temporally regulate myosin assembly and disassembly. We demonstrate that the kinase Phg2, a putativeRap1 effector, colocalizes with Rap1-GTP at the leading edge and is requiredin an in vitro assay for myosin II (MyoII) phosphorylation, which disassembles MyoII and facilitates F-actin-mediated leading edge protrusion. We suggest Rap1/Phg2 plays a role in controlling leading edge MyoII disassembly while passively allowing MyoII assembly along the lateral sides and posterior of the cell. Submitted by: Rick Firtel [rafirtel@ucsd.edu] -------------------------------------------------------------------------------- Big roles for small GTPases in the control of directed cell movement Pascale G. Charest and Richard A. Firtel Biochem. J., in press Small GTPases are involved in the control of diverse cellular behaviors, including cellular growth, differentiation, and motility. In addition, recent studies have revealed new roles for small GTPases in the regulation of eukaryotic chemotaxis. Efficient chemotaxis results from coordinated chemoattractant gradient sensing, cell polarization, and cellular motility, and accumulating data suggest that small GTPase signaling plays a central role in each of these processes, as well as in signal relay. This review summarizes these recent findings, which shed light on the molecular mechanisms by which small GTPases control directed cell migration. Submitted by: Rick Firtel [rafirtel@ucsd.edu] -------------------------------------------------------------------------------- Exploitation of other social amoebae by Dictyostelium caveatum Clement Nizak1, 3, Robert J. Fitzhenry2, Richard H. Kessin2 1 Living Matter Laboratory, Center for Physics and Biology, Rockefeller University, 1230 York Avenue, Box 34, New York, NY, 10021, U.S.A. 2 Department of Anatomy and Cell Biology, Columbia University, 630 West 168th Street, New York, NY, 10032, U.S.A. 3 present address: Laboratoire de Spectromtrie Physique, CNRS, BP87, 38402 Saint Martin d'Hres Cedex, France, clement.nizak@ujf-grenoble.fr 3 Corresponding author: clement.nizak@mail.rockefeller.edu PLoS ONE, in press Dictyostelium amoebae faced with starvation trigger a developmental program during which many cells aggregate and form fruiting bodies that consist of a ball of spores held aloft by a thin stalk. This developmental strategy is open to several forms of exploitation, including the remarkable case of Dictyostelium caveatum, which, even when it constitutes 1/10e3 of the cells in an aggregate, can inhibit the development of the host and eventually devour it. We show that it accomplishes this feat by inhibiting a region of cells, called the tip, which organizes the development of the aggregate into a fruiting body. We use live-cell microscopy to define the D. caveatum developmental cycle and to show that D. caveatum amoebae have the capacity to ingest amoebae of other Dictyostelid species, but do not attack each other. The block in development induced by D. caveatum does not affect the expression of specific markers of prespore cell or prestalk cell differentiation, but does stop the coordinated cell movement leading to tip formation. The inhibition mechanism involves the constitutive secretion of a small molecule by D. caveatum and is reversible. Four Dictyostelid species were inhibited in their development, while D. caveatum is not inhibited by its own compound(s). D. caveatum has evolved a predation strategy to exploit other members of its genus, including mechanisms of developmental inhibition and specific phagocytosis. Submitted by: Clement Nizak [clement.nizak@mail.rockefeller.edu] ============================================================ [End dictyNews, volume 28, number 3]