Ubiquitylation of active Rac1 by the E3 ubiquitin-ligase HACE1

Abstract

Rho GTPases undergo ubiquitylation and degradation via the ubiquitin-proteasome pathway. We now report in the November issue of Developmental Cell that the E3 ubiquitin-ligase HACE1 catalyzes the ubiquitylation of GTP-bound Rac1. Depletion of HACE1 leads to an increase of Rac1 activity. We have proposed that HACE1 limits Rac1 activity in cells, a regulation that is usurped by some pathogenic bacteria for efficient invasion of host cell monolayers. We here review these findings in parallel with the regulation of RhoA by the ubiquitin and proteasome system (UPS) and discuss the impact of these regulations on the capacity of Rho GTPases to signal.

Figure 1. Rho protein ubiquitylation. (A) HACE1 HECT-domain containing protein E3 ubiquitin-ligase binds preferentially to the GTP-bound form of Rac1, and to an ubiquitin conjugated to an E2 enzyme. Ubiquitin molecules are next transferred to a conserved cysteine residue (C876 of HACE1) of the HECT-domain prior to conjugation on Rac1 to form K48-poly-ubiquitin chain, a signal for targeting to the proteasomal machinery. (B) RhoA is a target of Smurf1 HECT-domain containing E3 ubiquitin-ligase. (C) The GDP-bound form of RhoA is recognized by a multi-subunit E3 ubiquitin-ligase homologous to SCF (for Skp1-Cullin-1 and RING-finger), which contains Cullin-3, as scaffold protein, and the BTB-domain containing protein BACURD for specific binding to RhoA. This allows a direct conjugation of ubiquitin to RhoA for subsequent proteasomal destruction.