The distal tip cells (DTCs) migrate in a defined pattern to specify the shape of the hermaphrodite germ line; only rarely is there deviation in this migration. Unlike many other cells, the path of DTCs migration can be readily visualized without the aid of time-lapse (or 4D) microscopy, as it forms the shape of the developed hermaphrodite gonad. Along with a clear structure that makes fixation irrelevant, the ability to assay movement without time-lapse microscopy makes the DTCs a very nice model system to study cell migration. The migration of the DTCs depends on the redundant activity of two Rac orthologues, CED-10 and MIG-2, as well as a central signaling pathway that involves CED-2, CED-5, and CED-12. In contrast, the engulfment of apoptotic cells requires only
ced-2, -5, -10, and -12 and does not require the second Rac isoform,
mig-2. It has been a mystery for quite some time as to how
ced-10 is activated in response to an eat me signal without a GEF. Recently, however, it has been published that members of the CDM (
ced-5 Dock180 and myoblast city) family of assembly proteins have an intrinsic RacGEF activity that is mediated by a novel (but conserved) domain in the C-terminus of the molecule called the DOCKER domain. This domain preferentially binds to nucleotide-free Rac and probably stabilizes the transition state so that it can acquire a GTP molecule. Thus, it is quite possible that CED-5 provides the GEF activity for CED-10 during the process of engulfment and DTC migration. We have used the CED-5 homologue DOCK180 to assay the involvement of this DOCKER domain in cell migration in vivo. While DOCK180(WT) partially rescues the DTC migration defect of
ced-5 mutant worms, overexpressing a DOCK180 that had a mutant DOCKER domain caused a dominant negative effect. To further characterize this dominant negative (dn) effect, the DOCK180(dn) array was crossed into several mutant backgrounds. Enhanced mismigration was seen in
ced-10 and
ced-12 mutant worms, but not
mig-2 mutants. This is an interesting result because it suggests that DOCK180 mediates rescue of DTC migration solely via the Rac orthologue MIG-2 rather than both MIG-2 and CED-10, and provides a reasoning for the inability of DOCK180 to rescue the engulfment defect of
ced-5 mutant worms.