dictyNews Volume 25 Number 11

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Dicty News

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Dicty News 
Electronic Edition 
Volume 25, number 11 
November 11, 2005 

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 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 novel Dictyostelium RasGEF required for chemotaxis and development    
         
Maddalena Arigoni1, Enrico Bracco1, Daniel F.Lusche2, Helmut Kae3, Gerald 
Weeks3 and Salvatore Bozzaro1§ 

1 Department of Clinical and Biological Sciences, University of Torino, 
Regione Gonzole 10, 10043 Orbassano, Italy; 2 Faculty of Biology, University 
of Konstanz, 78457 Konstanz, Germany; 3 Dept.Microbiology and Immunology, 
University of British Columbia, Canada V6T1Z3 

 
BMC Cell Biology, in press 

Background 
Ras proteins are guanine-nucleotide-binding enzymes that couple cell surface 
receptors to intracellular signaling pathways controlling cell proliferation 
and differentiation, both in lower and higher eukaryotes. They act as 
molecular switches by cycling between active GTP and inactive GDP-bound 
states, through the action of two classes of regulatory proteins: a) guanine 
nucleotide exchange factor (GEFs) and b) GTP-ase activating proteins (GAPs). 
Genome wide analysis of the lower eukaryote Dictyostelium discoideum 
revealed a surprisingly large number of Ras Guanine Nucleotide Exchange 
Factors (RasGEFs). RasGEFs promote the activation of Ras proteins by 
catalyzing the exchange of GDP for GTP, thus conferring to RasGEFs the role 
of main activator of Ras proteins.  Up to date only four RasGEFs, which are 
all non- redundant either for growth or development, have been characterized 
in Dictyostelium. We report here the identification and characterization of 
a fifth non-redundant GEF, RasGEFM. 

Results 
RasGEFM is a multi-domain protein containing six poly-proline stretches, a 
DEP, RasGEFN and RasGEF catalytic domain. The rasGEFM gene is differentially 
expressed during growth and development. Inactivation of the gene results in 
cells that form small, flat aggregates and fail to develop further. 
Expression of genes required for aggregation is delayed. Chemotaxis towards 
cAMP is impaired in the mutant, due to inability to inhibit lateral 
pseudopods. Endogenous cAMP accumulates during early development to a much 
lower extent than in wild type cells. Adenylyl cyclase activation in 
response to cAMP pulses is also strongly reduced, by contrast guanylyl 
cyclase is stimulated to higher levels than in the wild type. The actin 
polymerization response to cAMP is also altered in the mutant. Cyclic AMP 
pulsing for several hours partially rescues the mutant.  In vitro 
experiments suggest that RasGEFM acts downstream of the cAMP receptor but 
upstream of the G protein. 

Conclusion 
The data indicate that RasGEFM is involved in the establishment of the cAMP 
relay system. We propose that RasGEFM is a component of a Ras regulated 
pathway, which integrate signals acting as positive regulator for adenylyl 
cyclase and negative regulator for guanylyl cyclase. Altered guanylyl 
cyclase, combined with defective regulation of actin polymerization, results 
in altered chemotaxis. 

 
Submitted by: Salvatore Bozzaro [salvatore.bozzaro@unito.it] 

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[End Dicty News, volume 25, number 11]