WASP restricts active rac to maintain cells' front-rear polarization

Summary Efficient motility requires polarized cells, with pseudopods at the front and a retracting rear. Polarization is maintained by restricting the pseudopod catalyst, active Rac, to the front. Here, we show that the actin nucleation-promoting factor Wiskott-Aldrich syndrome protein (WASP) contributes to maintenance of front-rear polarity by controlling localization and cellular levels of active Rac. Dictyostelium cells lacking WASP inappropriately activate Rac at the rear, which affects their polarity and speed. WASP’s Cdc42 and Rac interacting binding (“CRIB”) motif has been thought to be essential for its activation. However, we show that the CRIB motif’s biological role is unexpectedly complex. WASP CRIB mutants are no longer able to restrict Rac activity to the front, and cannot generate new pseudopods when SCAR/WAVE is absent. Overall levels of Rac activity also increase when WASP is unable to bind to Rac. However, WASP without a functional CRIB domain localizes normally at clathrin pits during endocytosis, and activates Arp2/3 complex. Similarly, chemical inhibition of Rac does not affect WASP localization or activation at sites of endocytosis. Thus, the interaction between small GTPases and WASP is more complex than previously thought—Rac regulates a subset of WASP functions, but WASP reciprocally restricts active Rac through its CRIB motif.
Graphical Abstract
Highlights • WASP exploits its CRIB motif to remove active Rac from the membrane via endocytosis • WASP is recruited to clathrin-coated pits (CCPs) independently of small GTPases • WASP triggers actin polymerization at CCPs independently of small GTPase activation • WASP maintains homeostasis in Dictyostelium by controlling the level of active Rac
Amato et al. have discovered a mechanism that contributes to front-rear polarization in migrating cells. During clathrin-mediated endocytosis, Dictyostelium WASP interacts with active Rac via its CRIB motif. This interaction leads to incorporation of active Rac within endocytic vesicles, which facilitates its removal from inappropriate locations.