dictyNews
Electronic Edition
Volume 40, number 10
April 4, 2014
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Abstracts
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The IQGAP-related protein DGAP1 mediates signaling to the actin
cytoskeleton as an effector and a sequestrator of Rac1 GTPases
Vedrana Filic, Maja Marinovic, Jan Faix, Igor Weber
Cellular and Molecular Life Sciences, in press
http://link.springer.com/article/10.1007%2Fs00018-014-1606-3
Proteins are typically categorized into protein families based on their
domain organization. Yet, evolutionarily unrelated proteins can also
be grouped together according to their common functional roles.
Sequestering proteins constitute one such functional class, acting as
macromolecular buffers and serving as an intracellular reservoir
ready to release large quantities of bound proteins or other molecules
upon appropriate stimulation. Another functional protein class
comprises effector proteins, which constitute essential components of
many intracellular signal transduction pathways. For instance, effectors
of small GTP-hydrolases are activated upon binding a GTP-bound
GTPase and thereupon participate in downstream interactions. Here
we describe a member of the IQGAP family of scaffolding proteins,
DGAP1 from Dictyostelium, which unifies the roles of an effector and
a sequestrator in regard to the small GTPase Rac1. Unlike classical
effectors, which bind their activators transiently leading to short-lived
signaling complexes, interaction between DGAP1 and Rac1-GTP is
stable and induces formation of a complex with actin-bundling proteins
cortexillins at the back end of the cell. An oppositely localized Rac1
effector, the Scar/WAVE complex, promotes actin polymerization at the
cell front. Competition between DGAP1 and Scar/WAVE for the common
activator Rac1-GTP might provide the basis for the oscillatory re-
polarization typically seen in randomly migrating Dictyostelium cells. We
discuss the consequencesof the dual roles exerted by DGAP1 and Rac1
in the regulation of cell motility and polarity, and propose that similar
signaling mechanisms may be of general importance in regulating
spatiotemporal dynamics of the actin cytoskeleton by small GTPases.
Submitted by Vedrana Filic [[log in to unmask]]]
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Pore-forming toxins from pathogenic amoebae.
Leippe M.
Appl Microbiol Biotechnol. 2014 Mar 28.
[Epub ahead of print], DOI: 10.1007/s00253-014-5673-z
Some amoeboid protozoans are facultative or obligate parasites in
humans and bear an enormous cytotoxic potential that can result in
severe destruction of host tissues and fatal diseases. Pathogenic
amoebae produce soluble pore-forming polypeptides that bind to
prokaryotic and eukaryotic target cell membranes and generate pores
upon insertion and oligomerization. This review summerizes the
current knowledge of such small protein toxins from amoebae,
compares them with related proteins from other species, focuses on
their three-dimensional structures, and gives insights into divergent
activation mechanisms. The potential use of pore-forming toxins in
biotechnology will be briefly outlined.
Submitted by Matthias Leippe [[log in to unmask]]
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Requirements for Hirano Body formation.
Paul Griffin, Ruth Furukawa, Cleveland Piggott, Andrew Maselli,
and Marcus Fechheimer.
Eukaryotic Cell, In press
Hirano bodies are paracrystalline F-actin-rich structures associated with
diverse conditions including neurodegeneration and aging. Generation
of model Hirano bodies using altered forms of Dictyostelium 34 kDa actin-
bundling protein allows studies of their physiological function and
mechanism of formation. We describe a novel 34 kDa protein mutant,
E60K, with a point mutation within the inhibitory domain of 34 kDa protein.
Expression of E60K in Dictyostelium induces formation of model Hirano
bodies. The E60K protein has activated actin binding and is calcium-
regulated unlike other forms of the 34 kDa protein that induce Hirano
bodies that have activated actin binding but lack calcium-regulation. Actin
filaments in the presence of E60K in vitro show enhanced resistance to
disassembly induced by latrunculin B. Actin filaments in model Hirano
bodies are also protected from latrunculin induced depolymerization.
We used nocodazole and blebbistatin to probe the role of the microtubules
and myosin II, respectively, in formation of model Hirano bodies. In the
presence of these inhibitors, model Hirano bodies can form, but are smaller
than control at early times of formation. The ultrastructure of model Hirano
bodies did not reveal any major difference in the structure and organization
in the presence of inhibitors. In summary, these results support the
conclusion that formation of model Hirano bodies is promoted by gain-of-
function actin filament bundling which enhances actin filament stabilization.
Microtubules and myosin II contribute to but are not required for formation
of model Hirano bodies.
Submitted by Ruth Furukawa [[log in to unmask]]
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The association of myosin IB with actin waves in Dictyostelium requires both
the plasma membrane-binding site and actin-binding region in the myosin tail
Hanna Brzeska1*, Kevin Pridham1, Godefroy Chery1, Margaret A. Titus2
and Edward D. Korn1
1 Laboratory of Cell Biology, National Heart, Lung, and Blood Institute, National
Institutes of Health, Bethesda, Maryland, United States of America,
2 Department of Genetics, Cell Biology and Development, University of Minnesota,
Minneapolis, Minnesota, United States of America.
*E-mail: [log in to unmask]
PLOS ONE, accepted
F-actin structures and their distribution are important determinants of the dynamic
shapes and functions of eukaryotic cells. Actin waves are F-actin formations that
move along the ventral cell membrane driven by actin polymerization.
Dictyostelium myosin IB is associated with actin waves but its role in the wave is
unknown. Myosin IB is a monomeric, non-filamentous myosin with a globular
head that binds to F-actin and has motor activity, and a non-helical tail comprising
a basic region, a glycine-proline-glutamine-rich region and an SH3-domain. The
basic region binds to acidic phospholipids in the plasma membrane through a
short basic-hydrophobic site and the Gly-Pro-Gln region binds F-actin. In the
current work we found that both the basic-hydrophobic site in the basic region and
the Gly-Pro-Gln region of the tail are required for the association of myosin IB with
actin waves. This is the first evidence that the Gly-Pro-Gln region is required for
localization of myosin IB to a specific actin structure in situ. The head is not
required for myosin IB association with actin waves but binding of the head to
F-actin strengthens the association of myosin IB with waves and stabilizes waves.
Neither the SH3-domain nor motor activity is required for association of myosin IB
with actin waves. We conclude that myosin IB contributes to anchoring actin
waves to the plasma membranes by binding of the basic-hydrophobic site to
acidic phospholipids in the plasma membrane and binding of the Gly-Pro-Gln
region to F-actin in the wave.
Submitted by Hanna Brzeska [[log in to unmask]]
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Protection of spores from ultraviolet-C irradiation by auto-fluorescent substances
in the spore mass of the cellular slime mold Dictyostelium discoideum
Saburo Uchiyama and Ikuo Hatakeyama
Graduate School of Education, Iwate University, Morioka 020-8550, Japan
Pteridines, accepted
In this study, native spores surrounded by fluorescent substances in the spore
mass of Dictyostelium discoideum were found to be resistant to relatively strong
ultraviolet-C (UV-C) irradiation (2880 J/m2). The remaining emergency activity of
the native mass of spores was over 80% even after exposure to strong UV-C
irradiation (2880 J/m2). On the other hand, the washed spores were very sensitive
to weak UV-C irradiation (144 J/m2). The mass of spores in the fruiting body
formed by amoebae with a low concentration of fluorescent substances was less
resistant to UV-C compared to that in the fruiting body formed by normally grown
amoebae. Based on the remaining emergency activity of washed spores with
appropriate lumazine solution, the concentration of fluorescent substances in the
native mass of spores was estimated to be equivalent to approximately
5 mmol/L lumazine.
Submitted by Saburo Uchiyama [[log in to unmask]]
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[End dictyNews, volume 40, number 10]
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