Regulation of catalytic activities of HECT ubiquitin ligases

Abstract

Studies in yeast and mammalian cells over the past decade have shown that HECT domain ubiquitin ligases (HECT E3 enzymes) are involved in diverse physiological pathways. Many substrates of specific HECT E3s have been identified, as well as many adaptor proteins that aid in defining substrate specificity or intra-cellular localization of HECT E3s. Here we review some recently discovered mechanisms for regulation of the catalytic activities of HECT E3s, including regulation at the level of E2 recruitment, phosphorylation-dependent relief of inhibitory intra-molecular interactions, and regulation by association with a deubiquitinating enzyme.

Figure 1

Modes of regulating or modulating the catalytic activities of C2-WW-HECT E3s. A. Some C2-WW-HECT E3s, such as Rsp5, appear to be constitutively active and can be charged with ubiquitin in the presence of only ATP, E1 enzyme (not shown), and an appropriate E2 enzyme. The arrow indicates ubiquitin transfer (a transthiolation reaction) from the E2 active-site cysteine to the E3 active-site cysteine. B. Smad7 activates Smurf2 by recruitment of the E2 enzyme, UbcH7. The PY motif of Smad7 interacts with the WW domain region of Smurf2, while the NTD interacts with both the HECT domain and the E2. These interactions increase the affinity of Smurf2 for UbcH7. C. Itch is a C2-WW-HECT E3 that adopts a “closed” inactive conformation due to intramolecular interactions between the WW-PRR region and the HECT domain. In response to signaling events, JNK1 is recruited by a D domain within the HECT domain and then phosphorylates multiple sites within the PRR region. This relieves the inhibitory interactions, leading to activation of the E3. D. While Rsp5 appears to be constitutively active, its activity is modulated following ubiquitination by Rup1- and Ubp2-dependent deubiquitination. Rup1 and Ubp2 (a ubiquitin-specific cysteine protease) are stably associated with Rsp5, but do not inhibit substrate association.