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dictyNews Volume 17 Number 13
Dicty News
Electronic Edition
Volume 17, number 13
December 1, 2001
Please submit abstracts of your papers as soon as they have been
accepted for publication by sending them to dicty@northwestern.edu.
Back issues of Dicty-News, the Dicty Reference database and other useful
information is available at DictyBase--http://dictybase.org.
=========================
Published Book Review
=========================
The Whole Organism, and nothing but the Organism
Robert H. Insall
School of Biosciences, Birmingham University, Edgbaston, Birmingham
B15 2TT, United Kingdom
Cell, Vol 107, 279-281, November 2001
A review of Rich Kessin's Dicty Monograph
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Abstracts
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A cell number-counting factor regulates the cytoskeleton and cell motility
in Dictyostelium
Lei Tang2, Tong Gao1, Catherine McCollum2, Wonhee Jang2, Michael G. Vicker3,
Robin R. Ammann1 and Richard H. Gomer1,2 1Howard Hughes Medical Institute
and 2Department of Biochemistry and Cell Biology, MS-140, Rice University,
6100 S. Main Street, Houston, TX 77005-1892 3Department of Biology/Chemistry
University of Bremen 28359 Bremen, Germany
PNAS in press
Abstract
Little is known about how a morphogenetic rearrangement of a tissue
is effected by the individual cells. Starving Dictyostelium discoideum cells
aggregate to form dendritic streams which then break up into groups of
~2 x 104 cells. Cell number is sensed at this developmental stage using
counting factor (CF), a secreted complex of polypeptides. A high
extracellular concentration of CF indicates that there is a large number
of cells, which then causes the aggregation stream to break up. Computer
simulations indicated that stream breakup could be caused by CF decreasing
cell-cell adhesion and/or increasing cell motility, and we observed that CF
does indeed decrease cell-cell adhesion. We find here that CF increases
cell motility. In Dictyostelium, motility is mediated by actin and myosin.
CF increases the amounts of polymerized actin and the ABP-120 actin-
crosslinking protein. Partially inhibiting motility using drugs which
interfere with actin polymerization reduces stream dissipation, resulting
in fewer stream breaks and thus larger groups. CF also potentiates the
phosphorylation and redistribution of myosin, while repressing its basal
level of assembly. The computer simulations indicated that a narrower
distribution of group sizes results when a secreted factor modulates both
adhesion and motility. CF thus appears to induce the morphogenesis of
streams into evenly-sized groups by increasing actin polymerization, ABP-120
levels, and myosin phosphorylation, and decreasing adhesion and myosin
polymerization.
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RNAi in Dictyostelium: the role of RdRPs and dsRNase
Henrik Martens, Jindrich Novotny, Jrgen Oberstrass, Theodore L. Steck1,
Pamela Postlethwait1 and Wolfgang Nellen*
Abt. Genetik, Universitt Kassel, Heinrich-Plett-Str. 40, D-34132 Kassel,
Germany, 1 Department of Biochemistry and Molecular Biology, The University
of Chicago, 920 East 58th Street, Chicago, IL 60637-1432
Mol. Biol. Cell, in press
Abstract
We show that in Dictyostelium discoideum an endogenous gene as well as a
transgene can be silenced by introduction of a gene construct which is
transcribed into a hairpin RNA (RNAi). Gene silencing was accompanied by
the appearance of sequence specific RNA ~23mers and seemed to have a
limited capacity. The three Dictyostelium homologs of the RNA directed RNA
polymerase gene (RrpA, RrpB and DosA) all contain an N-terminal helicase
domain homologous to the one in the dicer nuclease, suggesting exon
shuffling between RdRP and the dicer homolog. Only the knock-out of rrpA
resulted in a loss of the RNAi effect and simultaneously in a loss of
detectable ~23mers. However, ~23mers were still generated by the
Dictyostelium dsRNase in vitro with extracts from rrpA-, rrpB- and DosA-
cells. Both RrpA and a target gene were required for production of
detectable amounts of ~23mers suggesting that target sequences are involved
in ~23mer amplification.
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Membrane sorting in the endocytic and phagocytic pathway of Dictyostelium
discoideum
Kissia Ravanel1, Benoit de Chassey2, Sophie Cornillon1, Mohammed Benghezal1,
Laurence Zulianello1, Leigh Gebbie1, Franois Letourneur2, Pierre Cosson1*
1 Universit de Genve, Centre Mdical Universitaire, Dpartement de
Morphologie, 1 rue Michel Servet, CH-1211 Genve 4, Switzerland.
2 Institut de Biologie et de Chimie des Protines, UMR 5086 CNRS, Universit
Lyon I, 7 passage du Vercors, 69367 Lyon Cedex 07, France.
European J. Cell Biol., in Press
ABSTRACT
To study sorting in the endocytic pathway of a phagocytic and macropinocytic
cell, monoclonal antibodies to membrane proteins of Dictyostelium discoideum
were generated. Whereas the p25 protein was localized to the cell surface,
p80 was mostly present in intracellular endocytic compartments as observed
by immunofluorescence as well as immunoelectron microscopy analysis. The p80
gene was identified and encodes a membrane protein presumably involved in
copper transport. Expression of chimeric proteins revealed that the
cytoplasmic domain of p80 was sufficient to cause constitutive endocytosis
and localization of the protein to endocytic compartments. Dileucine and
tyrosine-based endocytic signals described previously in mammalian systems
were also capable of targeting chimera to endocytic compartments. In
phagocytosing cells no membrane sorting was observed during formation of the
phagosome. Both p25 and p80 were incorporated non-selectively in nascent
phagosomes, and then retrieved shortly after phagosome closure. Our results
emphasize the fact that very active membrane traffic takes place in
phagocytic and macropinocytic cells. This is coupled with precise membrane
sorting to maintain the specific composition of endocytic compartments
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Two members of the beige/CHS (BEACH) family are involved at different
stages in the organization of the endocytic pathway in Dictyostelium.
Sophie Cornillon1, Annick Dubois2, Franz Brckert3, Yaya Lefkir2, Anna
Marchetti1, Mohammed Benghezal1, Arturo De Lozanne4, Franois Letourneur2,
and Pierre Cosson1, 5
1 Universit de Genve, Centre Mdical Universitaire, Dpartement de
morphologie, 1 Rue Michel Servet, CH1211 Genve 4, Switzerland;
2 Institut de Biologie et de Chimie des Protines, UMR5086-CNRS, Universit
Lyon I, 7 Passage du Vercors, 69367 Lyon cedex 07, France
3 Laboratoire de Biochimie et Biophysique des Systmes Intgrs, UMR314 CNRS,
CEA, 38054 Grenoble, France
4 Section of Molecular Cell and Developmental Biology and Institute for
Cellular and Molecular Biology, 241 Patterson Bldg., Mail code C0930,
University of Texas, Austin, TX 78712, USA
J. Cell Sci., in press
SUMMARY
Proteins of the Chediak-Higashi/Beige (BEACH) family have been implicated in
the function of lysosomes as well as in signal transduction but their molecular
role is still poorly understood. In Dictyostelium at least six members of the
family can be identified. Here cells mutated in two of these, LVSA and LVSB,
were analyzed. Interestingly both mutants exhibited defects in the
organization of the endocytic pathway, albeit at distinct stages. In lvsB
mutant cells the regulated secretion of lysosomal enzymes was enhanced, a
phenotype reminiscent of the Chediak-Higashi syndrome. LvsA mutant cells
exhibited alterations in the organization and function of the early endocytic
and phagocytic pathway. The LvsA protein may participate in the signaling
pathway which links adhesion of a particle to the subsequent formation of a
phagocytic cup. Further genetic analysis will be necessary to determine if
other members of the BEACH family of proteins are also involved in
controlling the organization of the endocytic pathway.
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QUANTITATIVE ANALYSIS OF THE BEHAVIOR OF DICTYOSTELIUM DISCOIDEUM AMOEBAE:
STRINGENCY OF PTERIDINE RECEPTION.
Jared L. Rifkin, Biology Department, Queens College of CUNY, Flushing,
NY 11367-1597
Cell Motility and the Cytoskeleton, in press
Abstract
A convenient, sensitive, quantitative assay for the measurement of
chemotaxis of populations of D. discoideum vegetative amoebae is presented.
A strategy for determining the boundary of the bulk of a population of
migrating amoebae was devised and is described. This assay employs a dynamic
gradient and is independent of deaminase activity. Measurements of
chemoattractant capabilities of various pteridines, folates, and mixtures
of folate fragments are reported. 2-Amino 4-quinazolinone, a pterin analog
without the pyrazine ring nitrogens, is chemotactic. Lumazine, deaminated
pterin, inhibits chemotaxis towards pterin but not towards folic acid.
Deaminofolic acid is a chemoattractant as are mixtures of lumazine plus
aminobenzoylglutamic acid or deaminopteroic acid plus various amino acids.
Separately, the components of these mixtures exhibit no ability to stimulate
chemotaxis. These mixtures are of fragments that together comprise most of
the folate structure. Our results are in accord with separate receptors for
pterin vs. folic acid and with a high stringency for pterin reception but a
relative tolerance for folate reception. The possibility of using such
mixtures to investigate the requirements of various parts of the folate
structure for competent signalling is discussed.
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[End Dicty News, volume 17, number 13]
