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dictyNews Volume 21 Number 02
Dicty News
Electronic Edition
Volume 21, number 2
July 18, 2003
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.
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Abstracts
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Dictyostelium SAPKalpha, a novel stress-activated MEK kinase-like kinase, is
important for the proper regulation of the cytoskeleton
Binggang Sun, Hui Ma, and Richard A. Firtel
Section of Cell and Developmental Biology Division of Biological
Sciences and Center for Molecular Genetics
University of California, San Diego
9500 Gilman Drive
La Jolla CA 92093-0634
Molecular Biology of the Cell, in press
MAP kinase cascades regulate various cellular functions including growth,
cell differentiation, development, and stress responses. We have identified
a new Dictyostelium kinase (SAPKalpha), which is related to members of the MLK
class of MEK kinases. SAPKalpha is activated by osmotic stress, heat shock,
and detachment from the substratum and by a membrane-permeable cGMP analog,
a known regulator of stress responses in Dictyostelium. SAPKalpha is important
for cellular resistance to stresses, as SAPKalpha null cells exhibit reduced
viability in response to osmotic stress. We found that SAPKalpha mutants affect
cellular processes requiring proper regulation of the actin cytoskeleton,
including cell motility, morphogenesis, cytokinesis, and cell adhesion.
Overexpression of SAPKalpha results in highly elevated basal and
chemoattractant-stimulated F-actin levels and strong aggregation and
developmental defects, including a failure to polarize and chemotax and
abnormal morphogenesis. These phenotypes require a kinase-active SAPKalpha.
SAPKalpha null cells exhibit reduced chemoattractant-stimulated F-actin levels,
cytokinesis, developmental and adhesion defects, and a motility defect that
is less severe than that exhibited by SAPKalpha overexpressing cells. SAPKalpha
co-localizes with F-actin in F-actin-enriched structures, including membrane
ruffles and pseudopodia during chemotaxis. Although SAPKalpha is required for
these F-actin-mediated processes, it is not detectably activated in response
to chemoattractant stimulation.
Submitted by: Rick Firtel [rafirtel@ucsd.edu]
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Feedback control in intracellular signaling pathways: Regulating chemotaxis
in Dictyostelium discoideum
Pablo A. Iglesias
Electrical & Computer and Biomedical Engineering
The Johns Hopkins University,
3400 N. Charles Street, Baltimore, MD 21218
pi@jhu.edu
European J. Control, in press
Positive and negative feedback loops are used throughout engineering
to tradeoff between amplification and robustness. We show how the
social amoebae Dictyostelium discoideum uses these same
principles to aggregate during starvation, allowing it to survive
difficult environmental conditions.
Submitted by: Pablo Iglesias [pi@jhu.edu]
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PIR121 Regulates Pseudopod Dynamics and SCAR Activity in Dictyostelium
Simone L. Blagg, Michael Stewart, Christine Sambles and Robert H. Insall*
Current Biology, in press.
School of Biosciences,
University of Birmingham,
Edgbaston,
Birmingham B15 2TT
UK
BACKGROUND
The WASP/SCAR family of adaptor proteins coordinates actin reorganization by
coupling different signalling molecules, including Rho-family GTPases, to
the activation of the Arp2/3 complex. WASP binds directly to Cdc42 through
its GTPase-binding domain (GBD), but SCAR does not contain a GBD and no
direct binding has been found. However, SCAR has recently been found to
copurify with four other proteins in a complex. One of these, PIR121, binds
directly to Rac.
RESULTS
We have identified four of the members of this complex in Dictyostelium, and
disrupted the pirA gene, which encodes PIR121. The resulting mutant cells
are unusually large, maintain an excessive proportion of their actin in a
polymerized state, and display severe defects in movement and chemotaxis.
They also continually extend new pseudopods by widening and splitting existing
leading edges, rather than by initiating new pseudopods. Comparing these
cells to scar null mutants shows behaviour that is broadly consistent with
overactivation of SCAR. Deletion of the pirA gene in a scar- mutant resulted
in cells resembling their scar- parents with no obvious changes, confirming
that PIR121 mainly acts through SCAR in vivo. Surprisingly, given their
hyperactive phenotype, we find that pirA- mutants contain very little intact
SCAR protein, despite normal levels of mRNA, suggesting a post-transcriptional
downregulation of activated SCAR.
CONCLUSIONS
Our results demonstrate a genetic connection between the pirA and scar genes.
PIR121 appears to inhibit the activity of SCAR in the absence of activating
signals. The location of the newly formed protrusions indicates that
unregulated SCAR is acting at the edges of existing pseudopods, not elsewhere
in the cell. We suggest that active SCAR protein released from the inhibitory
complex is rapidly removed, and this is an important and novel mechanism for
controlling actin dynamics.
Submitted by: Simone Blagg [slb761@bham.ac.uk]
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[End Dicty News, volume 21, number 2]
