BH3‐ligand regulates access of MCL‐1 to its E3 ligase

Abstract

A genome wide search for new BH3-containing Bcl-2 family members was conducted using position weight matrices (PWM) and identified a large (480 kDa), novel BH3-only protein, originally called LASU1 (now also known as Ureb-1, E3 histone , ARF-BP1, and Mule). We demonstrated that LASU1 is an E3 ligase that ubiquitinated Mcl-1 in vitro and was required for its proteasome-dependent degradation in HeLa cells. Of note, the BH3 domain of LASU1 interacted with Mcl-1 but not with Bcl-2 or Bcl-Xl. A competing BH3-ligand derived from Bim interacted with Mcl-1 and prevented its interaction with LASU1 in HeLa cells, causing elevation of the steady-state levels of Mcl-1. This suggests that the unliganded form of Mcl-1 is sensitive to LASU1-mediated degradation of Mcl-1.

Activation of the E3 ubiquitin ligase Itch through a phosphorylation-induced conformational change

“…Smurf2, a HECT E3 ligase involved in TGF-␤ signaling, was shown to be regulated by a mechanism involving an interaction with phosphorylated Smad7, which in turn facilitates the recruitment of Ubc7 (24). In addition, the activity of the recently described HECT E3 ligase Mule͞ARF-BP1 is negatively regulated by its binding partner ARF (25), and we speculate that phosphorylation may modulate the substrate specificity of this enzyme, thereby explaining its diametrically opposed, substrate-dependent effects on cell survival (25)(26)(27). Therefore, understanding the mechanisms through which protein phosphorylation and other modifications control the activity of HECT E3 ligases has become of great importance in cellular regulation.…”

Differential effects of anti-apoptotic Bcl-2 family members Mcl-1, Bcl-2, and Bcl-xL on Celecoxib-induced apoptosis

“…It preferentially binds to Mcl-1 and A1, but not to Bcl-2 or BclxL, to neutralize their anti-apoptotic effects [13]. In addition, Noxa might also regulate Mcl-1 protein stability [34,35] since Noxa expression inversely correlated with Mcl-1 protein levels in some studies [36,37]. In contrast, in our cell system, silencing of Noxa did not increase Mcl-1 expression levels arguing against a role of Noxa for Mcl-1 degradation in our system.…”

Unraveling MCL-1 degradation

“…18 Surprisingly, MULE possesses a well-conserved BH3 domain similar to that of proapoptotic BAK that allows it to selectively target MCL-1. 17,19 Of note, while MULE may target MCL-1 for degradation it is not only specific for MCL-1 as several other groups independently identified the same E3 ligase and have implicated it in the proteasome degradation of p53, E3 Histone , and c-Myc. [20][21][22] Future work will be necessary to unravel how this E3 ligase can regulate such a wide variety of substrates and how this dynamic network is regulated.…”

“…promoting BAK activation and allowing MCL-1 to be targeted by MULE for proteasome-dependent degradation. 17,19,25 Interestingly, Green and colleagues demonstrate that withdrawal of IL-3 combined with inhibition of GSK-3, promoted the interaction between MCL-1 and proapoptotic BIM. 23 These data imply that phosphorylation of MCL-1 (at human serine-159) may hinder its interaction with proapoptotic BCL-2 family members perhaps further sensitizing cells to apoptotic stimuli.…”