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dictyNews Volume 28 Number 03
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.
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Abstracts
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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 Ðindependent
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]
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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]
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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.Ê Rap1 is activated rapidly in response to
chemoattractant stimulation and activated Rap1 is preferentially found at
the leading edge of chemotaxing cells.Ê 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.Ê We demonstrate that the kinase Phg2, aÊ
putativeRap1 effector, colocalizes with Rap1-GTP at the leading edge andÊ
is requiredin an in vitro assay for myosin II (MyoII) phosphorylation, which
disassembles MyoII and facilitates F-actin-mediated leading edge protrusion.Ê
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]
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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]
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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]
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[End dictyNews, volume 28, number 3]
