Dicty News
Electronic Edition
Volume 21, number 5
August 22, 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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A PTEN-related 5' phosphatidylinositol phosphatase localized in the Golgi
Sylvain Merlot, Ruedi Meili, David J. Pagliarini, Tomohiko Maehama, 
Jack E. Dixon, and Richard A. Firtel

J. Biol. Chem., in press

SUMMARY
Phosphoinositides play important roles as signaling molecules in different 
cellular compartments by regulating the localization and activity of 
proteins through their interaction with specific domains.  The activity of 
these lipids depends on which sites on the inositol ring are phosphorylated.  
Signaling pathways dependent on phosphoinositides phosphorylated on the D3 
position of this ring (3'-phosphoinositides) are negatively regulated by 
3'-phosphoinositide-specific phosphatases that include PTEN and myotubularin.  
Using the conserved PTEN catalytic core motif, we have identified a new 
protein in the Dictyostelium genome called PLIP that defines a new subfamily 
family of phosphoinositide phosphatases, clearly distinct from PTEN or other 
closely related proteins.  We show that PLIP is able to dephosphorylate a 
broad spectrum of phosphoinositides, including 3'-phosphoinositides.  In 
contrast to previously characterized phosphoinositide phosphatases, PLIP has 
a strong preference for phosphatidylinositol-5-phosphate , a newly discovered 
phosphoinositide.  We found that PLIP is localized in the Golgi with its 
phosphatase domain facing the cytoplasmic compartment.  PLIP null cells 
created via homologous recombination are unable to effectively aggregate to 
form multicellular organisms at low cell densities.  PLIP's presence in the 
Golgi suggests that it may be involved in membrane trafficking.

Submitted by: Rick Firtel [rafirtel@ucsd.edu]

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The novel ankyrin-repeat containing kinase ARCK-1 acts as a suppressor of the 
Spalten signaling pathway during Dictyostelium development

Laurence Aubry, Susan Lee, Kissia Ravanel, and Richard A. Firtel

Devel. Biol., in press

ABSTRACT
Spalten (Spn), a member of the PP2C family of Ser/Thr protein phosphatases, is 
required for Dictyostelium cell-type differentiation and morphogenesis.  We 
have identified a new protein kinase, ARCK-1, through a second site suppressor 
screen for mutants that allow spn null cells to proceed further through 
development.  ARCK-1 has a C-terminal kinase domain most closely related to 
Ser/Thr protein kinases and an N-terminal putative regulatory domain with 
ankyrin repeats, a 14-3-3 binding domain, and a C1 domain, which is required 
for binding to RasBGTP in a two-hybrid assay.  Disruption of the gene encoding 
ARCK-1 results in weak, late developmental defects.  However, overexpression of 
ARCK-1 phenocopies the spn null phenotype, consistent with Spn and ARCK-1 being 
on the same developmental pathway.  Our previous analyses of Spn and the present 
analysis of ARCK-1 suggest a model in which Spn and ARCK-1 differentially control 
the phosphorylation state of a protein that regulates cell-type differentiation.  
Dephosphorylation of the substrate by Spn is required for cell-type 
differentiation.  Control of ARCK-1 and Spn activities by upstream signals is 
proposed to be part of the developmental regulatory program mediating cell-fate 
decisions in Dictyostelium.

Submitted by: Rick Firtel [rafirtel@ucsd.edu]

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[End Dicty News, volume 21, number 5]