The cytohesin paralog Sec7 of Dictyostelium discoideum is required for phagocytosis and cell motility

Background: Dictyostelium harbors several paralogous Sec7 genes
that encode members of three subfamilies of the Sec7 superfamily of
guanine nucleotide exchange factors. One of them is the cytohesin
family represented by three members in D. discoideum, SecG, Sec7
and a further protein distinguished by several transmembrane
domains. Cytohesins are characterized by a Sec7-PH tandem domain
and have roles in cell adhesion and migration. Results: We study
here Sec7. In vitro its PH domain bound preferentially to
phosphatidylinositol 3,4-bisphosphate (PI(3,4) P-2),
phosphatidylinositol 4,5-bisphosphate (PI(4,5)P-2) and
phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P-3). When
following the distribution of GFP-Sec7 in vivo we observed the
protein in the cytosol and at the plasma membrane. Strikingly, when
cells formed pseudopods, macropinosomes or phagosomes, GFP-Sec7 was
conspicuously absent from areas of the plasma membrane which were
involved in these processes. Mutant cells lacking Sec7 exhibited an
impaired phagocytosis and showed significantly reduced speed and
less persistence during migration. Cellular properties associated
with mammalian cytohesins like cell-cell and cell-substratum
adhesion were not altered. Proteins with roles in membrane
trafficking and signal transduction have been identified as
putative interaction partners consistent with the data obtained
from mutant analysis. Conclusions: Sec7 is a cytosolic component
and is associated with the plasma membrane in a pattern distinctly
different from the accumulation of PI(3,4,5)P-3. Mutant analysis
reveals that loss of the protein affects cellular processes that
involve membrane flow and the actin cytoskeleton.