A novel Dictyostelium RasGEF is required for normal endocytosis, cell motility and multicellular development.
BACKGROUND
Dictyostelium possesses a surprisingly large number of Ras proteins and little is known about their activators, the guanine nucleotide exchange factors (GEFs). It is also unclear, in Dictyostelium or in higher eukaryotes, whether Ras pathways are linear, with each Ras controlled by its own GEF, or networked, with multiple GEFs acting on multiple Ras proteins.


RESULTS
We have identified the Dictyostelium gene that encodes RasGEFB, a protein with homology to known RasGEFs such as the Son-of-sevenless (Sos) protein. Dictyostelium cells in which the gene for RasGEFB was disrupted moved unusually rapidly, but lost the ability to perform macropinocytosis and therefore to grow in liquid medium. Crowns, the sites of macropinocytosis, were replaced by polarised lamellipodia. Mutant cells were also profoundly defective in early development, although they eventually formed tiny but normally proportioned fruiting bodies. This defect correlated with loss of discoidin Igamma mRNA, a starvation-induced gene, although other genes required for development were expressed normally or even precociously. RasGEFB was able to rescue a Saccharomyces CDC25 mutant, indicating that it is a genuine GEF for Ras proteins.


CONCLUSIONS
RasGEFB appears to be the principal activator of the RasS protein, which regulates macropinocytosis and cell speed, but it also appears to regulate one or more other Ras proteins.
Wilkins
A
A
MRC Laboratory for Molecular Cell Biology, University College London, Gower Street, WC1E 6BT, London, UK.
Chubb
J R
JR
Insall
R H
RH
D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D018797:Cell Cycle Proteins / Q000235:genetics / Q000378:metabolism; D002465:Cell Movement; D003001:Cloning, Molecular; D018076:DNA, Complementary; D004023:Dictyostelium / Q000235:genetics / Q000254:growth & development / Q000378:metabolism / Q000502:physiology; D004705:Endocytosis / Q000502:physiology*; D005656:Fungal Proteins / Q000235:genetics / Q000378:metabolism; D015870:Gene Expression; D018390:Gene Targeting; D008969:Molecular Sequence Data; D016296:Mutagenesis; D010587:Phagocytosis / Q000502:physiology; D012441:Saccharomyces cerevisiae; D020663:ras Guanine Nucleotide Exchange Factors / Q000235:genetics / Q000378:metabolism* / Q000502:physiology; D020666:ras-GRF1 / Q000235:genetics / Q000378:metabolism
D018797:Cell Cycle Proteins; D018076:DNA, Complementary; D005656:Fungal Proteins; D020663:ras Guanine Nucleotide Exchange Factors; D020666:ras-GRF1