The Rab7 GTPase regulates late endosome trafficking of the Epidermal Growth Factor Receptor (EGFR) to the lysosome for degradation. However, little is known about how Rab7 activity, functioning late in the endocytic pathway, affects EGFR signaling. We are using C. elegans vulva cell fate induction, a paradigm for genetic analysis of EGFR/Receptor Tyrosine Kinase (RTK) signaling, to assess the genetic requirements for
rab-7. Using a
rab-7 deletion mutant,
ok511, we found that RAB-7 antagonizes LET-23 EGFR signaling to a similar extent as previously described negative regulators. Epistasis analysis places
rab-7 upstream or in parallel to
lin-3 EGF-like and
let-23 EGFR. However,
rab-7 expression in the Vulva Presursor Cells (VPCs) is sufficient to rescue the
rab-7(
ok511) VPC induction phenotypes indicating that RAB-7 functions in the signal receiving cell and suggests that RAB-7 regulates ligand-dependent signaling. Components of the Endosomal Sorting Complex Required for Transport (ESCRT)-0, and -I, complexes,
hgrs-1 Hrs, and
vps-28, also antagonize signaling, suggesting that LET-23 EGFR likely transits through Multivesicular Bodies (MVBs) en route to the lysosome. Consistent with RAB-7 regulating LET-23 EGFR trafficking, LET-23::GFP is present in larger puncta in the VPCs of
rab-7 mutant animals. Our data imply that Rab7, by mediating EGFR trafficking and degradation, plays an important role in downregulation of EGFR signaling. Failure to downregulate EGFR signaling contributes to oncogenesis, and thus Rab7 could possess tumor suppressor activity in humans.
rab-7(
ok511) strongly suppresses the
lin-2(-) Vulvaless phenotype, but would not have been identified in previous
lin-2(-) suppressor screens due to its maternal effect lethal phenotype. We performed a clonal
lin-2(-) suppressor screen to identify additional maternal effect lethal suppressors. We identified two suppressors,
vh4 and
vh22, which map to distinct regions of chromosome I.
vh4 fails to complement
agef-1(-) and
agef-1(RNAi) suppresses
lin-2(-), however no lesion was identified in the
agef-1 coding sequence. AGEF-1 is a putative Guanine nucleotide Exchange Factor for ARF-1/-3 GTPases that could function with UNC-101 and the AP-1 adaptor complex to modulate LET-23 EGFR signaling. We are using whole genome sequencing to identify the causative DNA lesions for
vh4 and
vh22.