Copy Link
Add to Bookmark
Report
dictyNews Volume 38 Number 06
dictyNews
Electronic Edition
Volume 38, number 6
February 24, 2012
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 dictyNews, the Dicty Reference database and other
useful information is available at dictyBase - http://dictybase.org.
Follow dictyBase on twitter:
http://twitter.com/dictybase
=========
Abstracts
=========
A surface glycoprotein indispensable for gamete fusion in the
social amoeba Dictyostelium discoideum
Yoshinori Araki*, Hideki D. Shimizu*, Kentaro Saeki, Marina
Okamoto, Lixy Yamada, Kentaro Ishida, Hitoshi Sawada,
and Hideko Urushihara
Eukaryotic Cell, in press
Sexual reproduction is essential for the maintenance of species
in a wide variety of multicellular organisms, and even unicellular
organisms that normally proliferate asexually possess a sexual
cycle because of its contribution to increased genetic diversity.
Information concerning the molecules involved in fertilization is
accumulating in many species of the metazoan, plant, and fungal
lineages, and the evolutionary consideration of sexual reproduction
systems is now an interesting issue. Macrocyst formation in the
social amoeba Dictyostelium discoideum is a sexual process in
which cells become sexually mature under dark and submerged
conditions and fuse with complementary mating-type cells. In the
present study, we isolated D. discoideum insertional mutants
defective in sexual cell fusion and identified the relevant gene,
macA, which encodes a highly glycosylated, 2041-amino-acid
membrane protein (MacA). Although its overall similarity is
restricted to proteins of unknown function within dictyostelids, it
contains LamGL and discoidin domains that are implicated in cell
adhesion. Growth and development of macA-null mutants were
indistinguishable from those of the parental strain. The
overexpression of macA using the V18 promoter in a macA-null
mutant completely restored its sexual defects. Although the macA
gene encoded exactly the same protein in a complementary
mating-type strain, it was expressed at a much lower level. These
results suggest that MacA is indispensable for gamete interactions
in D. discoideum, probably via cell adhesion. There is a possibility
that it is controlled in a mating-type-dependent manner.
Submitted by: Hideko Urushihara [hideko@biol.tsukuba.ac.jp]
--------------------------------------------------------------------------------------
Pseudopod growth and evolution during cell movement is
controlled through SCAR/WAVE dephosphorylation.
Seiji Ura, Alice Y. Pollitt, Douwe M. Veltman, Nicholas A. Morrice,
Laura M. Machesky, & Robert H. Insall
Current Biology, in press
Background
SCAR/WAVE is a principal regulator of pseudopod growth in crawling
cells. It exists in a stable pentameric complex, which is regulated at
multiple levels that are only beginning to be understood. SCAR/WAVE
is phosphorylated at multiple sites, but how this affects its biological
activity is unclear. Here we show that dephosphorylation of Dictyostelium
SCAR controls normal pseudopod dynamics.
Results
We demonstrate that the C-terminal acidic domain of most Dictyostelium
SCAR is basally phosphorylated at four serine residues. A small amount
of singly phosphorylated SCAR is also found. SCAR phosphorylation site
mutants cannot replace SCARÕs role in the pseudopod cycle, though they
rescue cell size and growth. Unphosphorylatable SCAR is hyperactive Ð
excessive recruitment to the front gives large pseudopods that fail to
bifurcate because they continually grow forwards. Conversely,
phosphomimetic SCAR is weakly active, causing frequent small,
disorganised pseudopods.
Even in its regulatory complex, SCAR is normally held inactive by an
interaction between the phosphorylated acidic and basic domains. Loss
of basic residues complementary to the acidic phosphosites yields a
hyperactive protein similar to unphosphorylatable SCAR.
Conclusions
Regulated dephosphorylation of a fraction of the cellular SCAR pool
is a key step in SCAR activation during pseudopod growth.
Phosphorylation increases autoinhibition of the intact complex.
Dephosphorylation weakens this interaction and facilitates SCAR
activation, but also destabilizes the protein. We show that SCAR is
specifically dephosphorylated in pseudopods, increasing activation by
Rac and lipids and supporting positive feedback of pseudopod growth.
Submitted by Robert Insall [r.insall@beatson.gla.ac.uk]
==============================================================
[End dictyNews, volume 38, number 6]
