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dictyNews Volume 23 Number 17
Dicty News
Electronic Edition
Volume 23, number 17
November 19, 2004
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 Dicty-News, the Dicty Reference database and other
useful information is available at dictyBase - http://dictybase.org.
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Abstracts
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Actin-activation of myosin heavy chain kinase A in Dictyostelium: A
biochemical mechanism for the spatial regulation of myosin II filament
disassembly
Thomas T. Egelhoff, Daniel Croft, and Paul A. Steimle
Department of Biology, University of North Carolina at Greensboro,
Greensboro, NC 27402
Corresponding Author: Paul Steimle (p_steiml@uncg.edu)
Journal of Biological Chemistry
Published online ahead of print November 14, 2004
10.1074/jbc.M410803200
Studies in Dictyostelium discoideum have established that the cycle of
myosin II bipolar filament assembly and disassembly controls the temporal
and spatial localization of myosin II during critical cellular processes,
such as cytokinesis and cell locomotion. Myosin heavy chain kinase A (MHCK A)
is a key enzyme regulating myosin II filament disassembly through myosin
heavy chain phosphorylation in Dictyostelium. Under various cellular
conditions, MHCK A is recruited to actin-rich cortical sites and is
preferentially enriched at sites of pseudopod formation; and thus is
proposed to play a role in regulating localized disassembly of myosin II
filaments in the cell. MHCK A possesses an aminoterminal coiled-coil domain
that participates in the oligomerization, cellular localization, and actin
binding activities of the kinase. In the current study, we show that the
interaction between the coiled-coil domain of MHCK A and filamentous-actin
leads to an approximately 40-fold increase in the initial rate of kinase
catalytic activity. Actin-mediated activation of MHCK A involves increased
rates of kinase autophosphorylation and requires the presence of the
coiled-coil domain. Structure-function analyses revealed that the
coiled-coil domain alone binds to actin filaments (apparent KD= 0.9 uM)
thus mediates the direct interaction with F-actin required for MHCK A
activation. Collectively, these results indicate that MHCK A recruitment
to actin-rich sites could lead to localized activation of the kinase via
direct interaction with actin filaments; and thus may represent an
important mechanism by which the cell achieves localized disassembly of
myosin II filaments required for specific changes in cell shape.
Submitted by: Paul Steimle [p_steiml@uncg.edu]
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A cost to chimerism in Dictyostelium discoideum on natural substrates
David I. Castillo, Ghislaine T. Switz, Kevin R. Foster, David C. Queller,
Joan E. Strassmann
Evolutionary Ecology Research
Most multicellular organisms go through a single cell bottleneck in
development, a process that ensures subsequent clonality of the cells within
the individual. Selection for clonality among cells could reduce costly
intra-organismal conflicts that would occur in mixtures of unrelated cells
(chimeras). In Dictyostelium discoideum, the usually solitary amoebae
aggregate with nearby cells when starving to form a motile, multicellular
slug that may be clonal or chimeric. This slug migrates to the soil surface
and forms a ball of spores held aloft by a stalk of dead cells. Previous
work on D. discoideum has shown that uniclonal slugs migrate further than
chimeric slugs of the same size across agar, indicating a functional cost
to chimerism. Here we test whether this cost to chimerism results in a
fitness cost under more natural conditions. First, we examine migration of
slugs across decaying leaves or soil. Second, we examine migration up
through layers of these substrates, which most closely reflects the natural
migration of D. discoideum slugs to the soil surface. In most trials,
chimeras performed worse than single clones. Our results indicate that
chimerism in D. discoideum has a real fitness cost in the wild, likely to
be compensated only by the larger size chimeras can attain in nature.
Submitted by: David I. Castillo [Navais@rice.edu]
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A mitotic kinesin-like protein required for normal karyokinesis, myosin
localization to the furrow, and cytokinesis in Dictyostelium
Gandikota S. Lakshmikanth, Hans M. Warrick, and James A. Spudich *
Department of Biochemistry, Stanford University School of Medicine,
Stanford, CA 94305
Published online before print November 16, 2004
Proc. Natl. Acad. Sci. USA, 10.1073/pnas.0407304101
OPEN ACCESS ARTICLE
Dictyostelium mitotic kinesin Kif12 is required for cytokinesis. Myosin II
localization to the cleavage furrow is severely depressed in Kif12-null
(dkif12) cells, which accounts in part for the cytokinesis failure. Myosin
II-null cells, however, undergo mitosis-coupled cytokinesis when adhering
to a surface, whereas the dkif12 cells cannot. During mitosis, the rate of
change of internuclear separation in dkif12 cells is reduced compared with
wild-type cells, indicating multiple roles of this molecular motor during
mitosis and cytokinesis. GFP-Kif12, which rescues wild-type behavior when
expressed in the dkif12 strain, is concentrated in the nucleus in interphase
cells, translocates to the cytoplasm at the onset of mitosis, appears in the
centrosomes and spindle, and later is concentrated in the spindle midbody.
Given these results, we hypothesize a mechanism for myosin II translocation
to the furrow to set up the contractile ring.
Submitted by: Lakshmikanth gandikota [lakshmi@pmgm2.stanford.edu]
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[End Dicty News, volume 23, number 17] 
