dictyNews
Electronic Edition
Volume 38, number 6
February 24, 2012

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=========
Abstracts
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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 [[log in to unmask]]
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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 SCAR’s role in the pseudopod cycle, though they 
rescue cell size and growth. Unphosphorylatable SCAR is hyperactive – 
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 [[log in to unmask]]
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[End dictyNews, volume 38, number 6]