dictyNews Volume 28 Number 13

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dictyNews 
Electronic Edition 
Volume 28, number 13 
May 18, 2007 

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. 

 
========= 
Abstracts 
========= 

 

A CHS/Beige homologue is involved in biogenesis of Dictyostelium secretory  
lysosomes 

Steve J. Charette and Pierre Cosson 

Universite de Geneve, Centre Medical Universitaire, Departement de  
Physiologie Cellulaire et Metabolisme, 1 rue Michel Servet, CH-1211 Geneva 4,  
Switzerland. 

Journal of Cell Science, in press 

The Chediak-Higashi syndrome (CHS) is characterized at the cellular level by  
a defect in the ability of cells to secrete lysosomes. However the precise  
step affected in the secretion process is unclear to date. This question was  
addressed by characterizing Dictyostelium discoideum cells where LVSB, the  
homologue of the CHS gene, was mutated. As observed in mammalian cells,  
secretion of lysosome-derived compartments was affected in lvsB mutant cells.  
This defect was mirrored by a decrease in the number of fusion-competent  
post-lysosomal compartments, which in Dictyostelium can be clearly  
distinguished from lysosomes. In addition, the transfer of endocytosed  
particles from lysosomes to post-lysosomes was strongly diminished in lvsB  
mutant cells compared to wild-type cells. These results suggest that LvsB is  
primarily involved in transport from lysosomes to post-lysosomes, and thus  
plays a critical role in the maturation of lysosomes into fusion-competent  
post-lysosomal compartments.  

 
Submitted by: Steve Charette [steve.charette@medecine.unige.ch] 
-------------------------------------------------------------------------------- 

 
GrlJ, a Dictyostelium GABAB-like receptor with roles in post-aggregation  
development 

Yogikala Prabhu 1,4, Rolf Mueller 1, Christophe Anjard 3 and  
Angelika A.Noegel 1,2* 

1Institute of Biochemistry I, Centre for Biochemistry, Medical Faculty,  
University of Cologne, Joseph-Stelzmann-Str. 52, D-50931 Kšln, Germany  
2Centre for Molecular Medicine Cologne, University of Cologne,  
Joseph-Stelzmann-Str. 52, 50931 Kšln, Germany 
3Center for Molecular Genetics, Division of Biological Sciences, University  
of California San Diego, La Jolla, CA 92093-0368, USA 
4Present address: Cell Biology and Metabolism Branch, National Institute of  
Child Health and Human Development, National Institutes of Health,  
Bethesda, MD 20892, USA 

 
BMC Developmental Biology, in press  

Background: The G-protein-coupled receptor (GPCR) family represents the  
largest and most important group of targets for chemotherapeutics. They are  
extremely versatile receptors that transduce signals as diverse as biogenic  
amines, purins, odorants, ions and pheromones from the extracellular  
compartment to the interior via biochemical processes involving GTP-binding  
proteins. Until recently, the cyclic AMP receptors (cARs) were the only known  
G protein coupled receptors in Dictyostelium discoideum. The completed genome  
sequence revealed the presence of several families of GPCRs in Dictyostelium,  
among them members of the family 3 of GPCRs, the GABAB/ glutamate like  
receptor family, which in higher eukaryotes is involved in neuronal signaling.  

Results: D. discoideum has seventeen Family 3 members of GPCRs, denoted GrlA  
through GrlR. Their transcripts are detected throughout development with  
increased levels during early and late development. We have examined here  
GrlJ. GFP-tagged GrlJ localises to the plasmamembrane and to internal  
membranes. Inactivation of the grlJ gene leads to precocious development,  
and the mutant completes development ~6 hours earlier. Alterations were also  
noted at the slug stage and in spore formation. grlJ- slugs were longer  
and broke apart several times on their way to culmination forming smaller  
but proportionate fruiting bodies. Spores from grlJ- fruiting bodies were  
malformed and less viable, although the spore differentiation factors were  
synthesized and sensed normally. Expression of a GFP-tagged full length GrlJ  
rescued the phenotype. 

Conclusion: Our data suggest that GrlJ acts at several stages of  
Dictyostelium development and that it is a negative regulator in Dictyostelium  
development.  

 
Submitted by: Angelika A.Noegel [noegel@uni-koeln.de] 
-------------------------------------------------------------------------------- 

 
Possible roles of the endocytic cycle in cell motility 

David Traynor and Robert R. Kay 

J. Cell Sci., in press 

 
Starving, highly motile Dictyostelium cells maintain an active endocytic  
cycle, taking up their surface about every 11 minutes.  Cell motility  
depends on a functional NSF protein Ð also essential for endocytosis and  
membrane trafficking generally Ð and we therefore investigated possible  
ways in which the endocytic cycle might be required for cell movement.   
First, NSF, and presumably membrane trafficking, are not required for the  
initial polarization of the leading edge in a cyclic-AMP gradient.  Second,  
we can detect no evidence for membrane flow from the leading edge, as  
photobleached or photoactivated marks in the plasma membrane move forward  
roughly in step with the leading edge, rather than backwards from it.  Third,  
we find that the surface area of a cell Ð measured from confocal  
reconstructions Ð constantly fluctuates during movement as the cell projects  
pseudopodia and otherwise changes shape; increases of 20-30% can often occur  
over a few minutes.  These fluctuations cannot be explained by reciprocal  
changes in filopodial surface area and they substantially exceed the 2 Ð3%  
by which membranes can stretch.  We propose that the endocytic cycle has a  
key function in motility by allowing adjustment of cell surface area to  
match changes in shape; and that without this function, movement is  
severely impaired. 

 
Submitted by: Rob Kay [rrk@mrc-lmb.cam.ac.uk] 
-------------------------------------------------------------------------------- 

 
Locally controlled inhibitory mechanisms are involved in eukaryotic 
GPCR-mediated chemosensing 

Xuehua Xu1, Martin Meier-Schellersheim2, Jianshe Yan1 and Tian Jin1* 

 
J. Biol. Chem., in press 

GPCR signaling mediates a balance of excitatory and inhibitory 
activities that regulate Dictyostelium chemosensing to cAMP. The 
molecular nature and kinetics of these inhibitors are unknown. We report 
that transient cAMP stimulations induce PIP3 responses without a 
refractory period, suggesting that GPCR-mediated inhibition accumulates 
and decays slowly. Moreover, exposure to cAMP gradients leads to 
asymmetric distribution of the inhibitory components. The gradients 
induce a stable accumulation of the PIP3 reporter PHCrac-GFP in the 
front of cells near the cAMP source. Rapid withdrawal of the gradient 
led to the re-association of G-protein subunits, and the return of the 
PIP3 phosphatase PTEN and PHCrac-GFP to their pre-stimulus distribution. 
Reapplication of cAMP stimulation produces a clear PHCrac-GFP 
translocation to the back but not to the front, indicating that a 
stronger inhibition is maintained in the front of a polarized cell. Our 
study demonstrates a novel spatiotemporal feature of currently unknown 
inhibitory mechanisms acting locally on the PI3K activation pathway. 

 
Submitted by: Tian Jin [TJIN@niaid.nih.gov] 
============================================================== 
[End dictyNews, volume 28, number 13]