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dictyNews Volume 39 Number 18
dictyNews
Electronic Edition
Volume 39, number 18
June 21, 2013
Please submit abstracts of your papers as soon as they have been
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or by using the form at
http://dictybase.org/db/cgi-bin/dictyBase/abstract_submit.
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Abstracts
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Lipid composition of multilamellar bodies secreted by Dictyostelium
discoideum reveals their amoebal origin.
Paquet VE, Lessire R, Domergue F, Fouillen L, Filion G, Sedighi A,
Charette SJ.
Institut de Biologie Integrative et des Systemes, Universite Laval,
Quebec City, Quebec, Canada, G1V 0A6.
Eukaryot Cell. 2013 Jun 7. [Epub ahead of print]
When they are fed with bacteria, Dictyostelium discoideum amoebae
produce and secrete multilamellar bodies (MLBs), which are composed
of membranous material. It has been proposed that MLBs are a waste
disposal system allowing D. discoideum to eliminate undigested
bacterial remains. However, the real function of MLBs remains unknown.
Determining the biochemical composition of MLBs, especially lipids,
represents a way to gain information about the role of these structures.
To allow these analyses, a protocol involving various centrifugation
procedures has been developed to purify secreted MLBs from
amoebae-bacteria co-cultures. The purity of MLB preparation was
confirmed by transmission electron microscopy and by
immunofluorescence using H36, an antibody that binds to MLBs.
The lipid and fatty acid compositions of pure MLBs were then analyzed
by high performance thin layer chromatography (HPTLC) and gas
chromatography (GC), respectively, and compared to those of amoebae
as well as bacteria used as food source. While the bacteria were devoid
of phosphatidylcholine (PC) and phosphatidylinositol (PI), these two polar
lipid species were major classes of lipids in MLBs and amoebae. Similarly,
the fatty acid composition of MLBs and amoebae was characterized by the
presence of polyunsaturated fatty acids while cyclic fatty acids were found
only in bacteria. These results strongly suggest that the lipids constituting
the MLBs rather originate from the amoebal metabolism rather than from
undigested bacterial membranes. This opens the possibility that MLBs,
instead of being a waste disposal system, have unsuspected roles in
D. discoideum physiology.
Submitted by Steve Charette [steve.charette@bcm.ulaval.ca]
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Laurence Aubry & Gerard Klein
True Arrestins and Arrestin-Fold Proteins: A Structure-Based Appraisal.
Progress in Molecular Biology and Translational Science, Louis M. Luttrell,
editor, Vol. 118, Burlington: Academic Press, 2013, pp. 21-56.
ISBN: 978-0-12-394440-5
Arrestin-clan proteins are folded alike, a feature responsible for their recent
grouping in a single clan. In human, it includes the well-characterized visual
and beta-arrestins, the arrestin domain-containing proteins (ARRDCs),
isoforms of the retromer subunit VPS26, and DSCR3, a protein involved in
Down syndrome. A new arrestin-fold-predicted protein, RGP1, described
here may join the clan. Unicellular organisms like the yeast Saccharomyces
cerevisiae or the amoeba Dictyostelium discoideum harbor VPS26, DSCR3,
and RGP1 isoforms as well as arrestin-related trafficking adaptors or ADCs,
but true arrestins are missing. Functionally, members of the arrestin clan
have generally a scaffolding role in various membrane protein trafficking
events. Despite their similar structure, the mechanism of cargo recognition
and internalization and the nature of recruited partners differ for the different
members. Based on the recent literature, true arrestins (visual and
beta-arrestins), ARRDCs, and yeast ARTS are the closest from a functional
point of view.
Submitted by Laurence Aubry [laubry@cea.fr]
---------------------------------------------------------------------------
Analysis of phenotypic evolution in Dictyostelia highlights developmental
plasticity as a likely consequence of colonial multicellularity
Maria Romeralo1,2, Anna Skiba1, Alejandro Gonzalez-Voyer3,
Christina Schilde1, Hajara Lawal1, Sylwia Kedziora1, Jim C. Cavender4,
Gernot Gloeckner5,6, Hideko Urushihara7 and Pauline Schaap1
1College of Life Sciences, University of Dundee, Dundee, UK
2Department of Systematic Biology, Uppsala University, Uppsala, Sweden
3Department of Integrative Ecology, Estacion Biologica de Donana
(EBD-CSIC), Sevilla, Spain
4Department of Environmental and Plant Biology, Ohio University,
Athens, OH, USA
5Department of Genomics, Leibniz-Institute of Freshwater Ecology
and Inland Fisheries, IGB, Berlin, Germany
6Institute for Biochemistry I, University of Cologne, Cologne, Germany
7Faculty of Life and Environmental Sciences, University of Tsukuba,
Tsukuba-shi, Japan
Proc. R. Soc. B., in press
Colony formation was the first step towards evolution of multicellularity
in many macroscopic organisms. Dictyostelid social amoebas have
used this strategy for over 600 Myr to form fruiting structures of
increasing complexity. To understand in which order multicellular
complexity evolved, we measured 24 phenotypic characters over 99
dictyostelid species. Using phylogenetic comparative methods, we
show that the last common ancestor (LCA) of Dictyostelia probably
erected small fruiting structures directly from aggregates. It secreted
cAMP to coordinate fruiting body morphogenesis, and another
compound to mediate aggregation. This phenotype persisted up to
the LCAs of three of the four major groups of Dictyostelia. The
group 4 LCA co-opted cAMP for aggregation and evolved much
larger fruiting structures. However, it lost encystation, the survival
strategy of solitary amoebas that is retained by many species in
groups 1-3. Large structures, phototropism and a migrating
intermediate 'slug' stage coevolved as evolutionary novelties within
most groups. Overall, dictyostelids show considerable plasticity in
the size and shape of multicellular structures, both within and
between species. This probably reflects constraints placed by colonial
life on developmental control mechanisms, which, depending on local
cell density, need to direct from 10 to a million cells into forming a
functional fructification.
Submitted by Pauline Schaap [p.schaap@dundee.ac.uk]
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Two distinct functions for PI3-kinases in macropinocytosis
Oliver Hoeller, Parvin Bolourani, Jonathan Clark, Len R. Stephens,
Phillip T. Hawkins, Orion D. Weiner, Gerald Weeks & Robert R. Kay
Journal of Cell Science, in press
Class-1 PI3-kinases are major regulators of the actin cytoskeleton,
whose precise contributions to chemotaxis, phagocytosis and
macropinocytosis remain unresolved. We use systematic genetic
ablation to examine this question in growing Dictyostelium cells.
Mass-spectroscopy shows that a quintuple mutant lacking the entire
genomic complement of class-1 PI3-kinases retains only 10% of
wild-type PIP3 levels. Chemotaxis to folate and phagocytosis of
bacteria proceed normally in the quintuple mutant but
macropinocytosis is abolished. In this context PI3-kinases show
specialized functions, only one of which is directly linked to gross
PIP3 levels: Macropinosomes originate in patches of PIP3, with
associated F-actin rich ruffles, both of which depend on PI3K1/2
but not PI3K4, whereas conversion of ruffles into vesicles requires
PI3K4. A biosensor derived from the Ras-binding domain of PI3K1
suggests that Ras is activated throughout vesicle formation.
Binding assays show that RasG and RasS interact most strongly
with PI3K1/2 and PI3K4 and single mutants of either Ras have
severe macropinocytosis defects. Thus, the fundamental function
of PI3-kinases in growing Dictyostelium cells is in macropinocytosis
where they have two distinct functions, supported by at least two
separate Ras proteins.
Submitted by Oliver Hoeller [oliver.hoeller@ucsf.edu]
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[End dictyNews, volume 39, number 18]
