Three C. elegansRAC proteins (CED-10, MIG-2 and RAC-2) are functionally redundant in cell migration and axon pathfinding (e.g. single mutants show few defects whereas double mutants display strong axon pathfinding and cell migration defects). In addition,
mig-2;
ced-10double mutants display maternal-effect embryonic lethality.
mig-2;
ced-10double mutants harboring an extrachromosomal array that supplies
ced-10(+) activity were viable and fertile whereas animals that did not inherit the array were embryonic lethal (as opposed to maternal-effect lethal because the array did not provide maternal
ced-10(+) activity). We took advantage of this functional redundancy to screen for new mutations that display synthetic lethality with
ced-10(
n1993)and might define genes that act in parallel to
ced-10.Three new
ced-10 synthetic lethal mutations called
lq13,
lq17and
lq20were isolated. We found that
lq13was a new allele of
mig-2,an expected result of the screen. Interestingly, we found that
lq20was a new gain-of-function allele of
ced-10itself. Further, we found that
mig-2(
lq13)and the existing
ced-10(
n3246)allele (Reddien and Horvitz, 2000) were gain-of-function alleles as well. All three gain-of-function mutations affected residues near the switch 1 and switch 2 regions of the GTPases, which mediate interaction of the GTPases with Dbl-homology GTP exchange factors (DH-GEFs). Possibly,
lq13,
lq20and
n3246are dominant-negative mutations that perturb interaction of the GTPases with their GEFs.We found that
lq17was a hypomorphic mutation in
unc-34,which encodes an Enabled-like protein involved in cytoskeletal signaling and which has previously been shown to affect axon pathfinding on its own (Withee et al., 2004). However,
unc-34(
lq17)mutants had few defects in axon pathfinding and cell migration whereas double mutants with
ced-10,
mig-2and rac-2had strong defects, indicating that
unc-34acts in parallel to Rac signaling in axon pathfinding. These data indicate that UNC-34 and Rac signaling have overlapping roles in axon pathfinding and cell migration and are consistent with previous results suggesting that CED-10 and UNC-34 act in parallel pathways downstream of the UNC-40 guidance receptor (Gitai et al., 2003). The actin-binding protein UNC-115 abLIM was previously shown to act downstream of RAC-2 and possibly CED-10 in axon pathfinding. Indeed, we found that
unc-115;
unc-34double mutants displayed synthetic defects in axon pathfinding. Possibly, UNC-115 modulates the growth cone actin cytoskeleton in response to Rac signaling and in parallel to UNC-34.