The aimless RasGEF is required for processing of chemotactic signals through G-protein-coupled receptors in Dictyostelium.
BACKGROUND
Ras proteins are small GTP-binding proteins that play an essential role in a wide range of processes, particularly in mammalian growth control. They act as molecular switches, being inactive when GDP is bound, and active when associated with GTP. Activation is accomplished by guanine nucleotide exchange factors (RasGEFs); when RasGEFs interact with Ras proteins, GDP is allowed to escape, and is replaced by GTP. Dictyostelium responds to chemoattractants through typical seven transmembrane domain receptors and heterotrimeric G proteins. There are at least five different Dictyostelium Ras genes, whose functions are not yet known.


RESULTS
We have isolated the aimless gene, which encodes the Dictyostelium homologue of RasGEFs, during a screen for insertional mutants that fail to aggregate. We found that aimless null mutants grew at a normal rate, but were severely impaired in both chemotaxis and activation of adenylyl cyclase, both of which are critical for the early stages of development. Although coupling between receptors and their G proteins is unaffected, and several cyclic AMP (cAMP)-mediated responses appear normal, activation of adenylyl cyclase by receptors and GTP gamma S (a non-hydrolyzable GTP analogue) is reduced by up to 95%. The motility of mutant cells appears normal, suggesting a true defect in gradient sensing.


CONCLUSIONS
The discovery of the aimless gene adds an interesting new member to the family of RasGEFs. Our data suggest an unforeseen role for a RasGEF, and therefore presumably a complete Ras pathway, in the processing of chemotactic signals through G-protein-coupled receptors.
Insall
R H
RH
MRC Laboratory for Molecular Cell Biology, University College London, UK. dmcbrob@ucl.ac.uk
Borleis
J
J
Devreotes
P N
PN
D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D002454:Cell Differentiation; D002455:Cell Division; D002633:Chemotaxis; D003001:Cloning, Molecular; D000242:Cyclic AMP / Q000378:metabolism*; D004023:Dictyostelium / Q000235:genetics / Q000378:metabolism*; D019204:GTP-Binding Proteins / Q000235:genetics* / Q000378:metabolism*; D015870:Gene Expression; D020662:Guanine Nucleotide Exchange Factors; D006801:Humans; D008969:Molecular Sequence Data; D016296:Mutagenesis; D011506:Proteins / Q000235:genetics / Q000378:metabolism; D015800:Protozoan Proteins; D011956:Receptors, Cell Surface / Q000378:metabolism*; D017386:Sequence Homology, Amino Acid; D015398:Signal Transduction; D020663:ras Guanine Nucleotide Exchange Factors
D020662:Guanine Nucleotide Exchange Factors; D011506:Proteins; D015800:Protozoan Proteins; D011956:Receptors, Cell Surface; C104358:aimless protein, Dictyostelium; D020663:ras Guanine Nucleotide Exchange Factors; D000242:Cyclic AMP; D019204:GTP-Binding Proteins