During cell migration, the lipid signalling molecule phosphatidylinositol 3,4,5 trisphosphate (PtdIns(3,4,5)P₃) is generated specifically at the leading edge, and is responsible for recruiting the actin polymerization machinery here. How this lipid signal is coupled to actin regulators is only now being revealed: this month in Nature Cell Biology, Côté et al. demonstrate that a direct interaction between the Rac regulatory protein DOCK180 and PtdIns(3,4,5)P₃ provides one means to ensure actin polymerization at the right place.

In mammalian cells, the Rac guanine nucleotide-exchange factor DOCK180 functions in a complex with CrkII and ELMO, and expression of all three proteins is required for cell migration. So far, studies of DOCK180 have focused on its DOCK homology region (DHR)-2 and found that it is essential for Rac activation. In this new study, Côté et al. set out to test the role of another conserved domain in DOCK180 — the DHR-1 domain. Interestingly, expression of DOCK180 lacking the DHR-1 domain did not interfere with DOCK180-mediated activation of Rac or its ability to bind CrkII and ELMO, but it did disrupt cell spreading and motility.

How might the DHR-1 domain affect DOCK180 function? We know that during chemoattraction, DOCK180 translocates to the membrane in response to PtdIns(3,4,5)₃ production. Given that the DHR-1 region may contain a C2 motif — implicated in phosphoinositide binding — Côté et al. decided to investigate whether the DHR-1 domain might provide a direct link to lipids at the cell's leading edge. Indeed, a liposome competition assay revealed that DHR-1 specifically binds to PtdIns(3,4,5)P₃. To determine the biological significance of this finding, the effects of the DHR-1 domain on DOCK180 membrane targeting were examined. Removal of the DHR-1 domain prevented the translocation of DOCK180 to the membrane upon stimulation of PtdIns(3,4,5)P₃ production, indicating that DHR-1 is not only critical for DOCK180 membrane translocation, but may also be important to ensure the localized activation of Rac.

If the main function of the DHR-1 domain is to make sure DOCK180 gets to the leading edge, its requirement should be bypassed by artificially targeting DOCK180 to the membrane. Côté et al. tested this idea by replacing the DHR-1 domain with a PtdIns(3,4,5)P₃-binding pleckstrin homology domain. This DOCK180 construct retained the ability to activate Rac and form a complex with CrkII and ELMO but, more importantly, it could also translocate to the cell membrane upon PtdIns 3-kinase activation. Moreover, it fully recapitulated the function of wildtype DOCK180 in migration assays, implying that membrane targeting is the primary role of the DHR-1 domain.

This new link between the DHR-1 domain and PtdIns(3,4,5)P₃ reveals how DOCK180 is able to activate Rac locally at the onset of migration. The findings described by Côté et al. suggest that the main role of DHR-1 is to regulate the translocation of the DOCK180-CrkII-ELMO complex to the leading edge; whether this interaction can also regulate phosphoinositide signalling and whether CrkII and ELMO also have membrane-targeting activity, remains to be investigated.

Author: Monica Hoyos-Flight