MSL2 Promotes Mdm2-independent Cytoplasmic Localization of p53

Abstract

Although it was originally thought of as a passive way to block the nuclear function of p53, accumulating evidence suggests that cytoplasmic localization of p53 plays an active role in p53-mediated functions such as apoptosis and autophagy. Previous studies by us and others demonstrated that Mdm2-mediated p53 ubiquitination induces both degradation and cytoplasmic localization. Here we describe MSL2, a novel E3 ligase for p53 that promotes ubiquitin-dependent cytoplasmic p53 localization. Unlike Mdm2 or most other p53 E3 ligases, MSL2-mediated p53 ubiquitination does not affect the stability of p53. Moreover, the MSL2-mediated effect on p53 is Mdm2-independent. Thus, our study identifies an important ubiquitin-ligase for modulating p53 subcellular localization.The function of the tumor suppressor p53 as a sequencespecific transcription factor controlling the expression of numerous target genes is critical for the regulation of cellular senescence, cell-cycle arrest, and apoptosis (1-3). During normal homeostasis, p53 is localized predominantly in the nucleus and is maintained at low levels via ubiquitination-mediated targeting for proteasomal degradation (4). Both protein levels and transcriptional activity increase dramatically in response to stress through an array of critical post-translational modifications (4, 5). Polyubiquitination of C-terminal lysines of p53 by Mdm2 (6 -8) and other ubiquitin-protein isopeptide ligases (E3) 2 such as Pirh2 (9), COP-1 (10), and Arf-BP1 (11) controls p53 levels by targeting p53 for proteasomal degradation in unstressed cells or after the cellular stress is resolved (4, 12). The stress-induced p53-dependent apoptotic response consists of transcription-dependent and -independent functions of p53 (13-17). Although transactivation of pro-apoptotic target genes such as PUMA, BAX, and PIG3 requires p53 to act as a transcription factor in the nucleus, cytoplasmic p53 can elicit an apoptotic response by localizing to the mitochondria and activating a direct mitochondrial death program (13, 18 -26). Mdm2-mediated p53 monoubiquitination, occurring when Mdm2 activity levels are low, promotes p53 nuclear export and the generation of a cytoplasmic p53 pools (27-32). Once p53 localizes to the mitochondria, both directly activated and enabled pathways are utilized to induce apoptosis (18). p53 interacts with anti-apoptotic members of the Bcl family such as BclXl and Bcl2 at the outer mitochondrial membrane to release and allow the oligomerization of the pro-apoptotic factors Bak and Bax. These in turn promote the formation of pores in the mitochondrial membrane resulting in the release of cytochrome c and other apoptotic activators from the mitochondria (21,22,(33)(34)(35). Alternatively, p53 can interact directly with Bak, releasing Bak from its inhibitory interaction with Mcl1 and thereby directly activating Bak-induced apoptosis (23, 36). Recently, a role for cytoplasmic p53 in autophagy was described, providing further evidence that changes on subcellular localization of p53 have pro...

Control of p53 multimerization by Ubc13 is JNK-regulated

“…The likely candidates are Hdm2, which has been directly implicated in the regulation of p53 mRNA translation (13), PARC, which regulates p53 stability within large molecular weight complexes in the cytosol (15), and MSL2 and WWP1, which were shown to promote p53 ubiquitination without affecting its stability (16,17).…”

p53-family proteins and their regulators: hubs and spokes in tumor suppression

“…107 The E3 ligases WWP1 and MSL2 are also involved in p53 nuclear export, indicating that ubiquitination is a pivotal mechanism for p53 cytosolic localization. 108 Notably, in some instances ubiquitination can have the opposite effect. In fact, the zincfinger protein E4F1 binds p53 and promotes its ubiquitination on an atypical lysine residue; this modification does not induce nuclear export but stimulates p53 recruitment on chromatin and expression of a subset of target genes.…”

Protein monoubiquitylation: targets and diverse functions

“…Given that p53 hyperactivation as well as p53 loss is detrimental to cells, the amount and activity of this protein are tightly regulated by transcriptional and post‐transcriptional mechanisms including monoubiquitylation‐induced nuclear exclusion (Li et al . 2003; Kruse & Gu 2009). MDM2 and MSL2 each mediate monoubiquitylation of the COOH‐terminal region of p53 (at K370, K372, K373, K381, K382, or K386 by MDM2, and at K351 or K357 by MSL2), which results in a conformational change of the protein that exposes its nuclear export signal (Nie et al .…”