Site-specific monoubiquitination downregulates Rab5 by disrupting effector binding and guanine nucleotide conversion.

Rab GTPases, which are involved in intracellular trafficking pathways, have recently been reported to be ubiquitinated. However, the functions of ubiquitinated Rab proteins remain unexplored. Here we show that Rab5 is monoubiquitinated on K116, K140, and K165. Upon co-transfection with ubiquitin, Rab5 exhibited abnormalities in endosomal localization and EGF-induced EGF receptor degradation. Rab5 K140R and K165R mutants restored these abnormalities, whereas K116R did not. We derived structural models of individual monoubiquitinated Rab5 proteins (mUbRab5s) by solution scattering and observed different conformational flexibilities in a site-specific manner. Structural analysis combined with biochemical data revealed that interactions with downstream effectors were impeded in mUbRab5K140, whereas GDP release and GTP loading activities were altered in mUbRab5K165. By contrast, mUbRab5K116 apparently had no effect. We propose a regulatory mechanism of Rab5 where monoubiquitination downregulates effector recruitment and GDP/GTP conversion in a site-specific manner.

Autophagy inhibits cell death induced by the anti-cancer drug morusin.

Autophagy is a cellular process by which damaged organelles and dysfunctional proteins are degraded. Morusin is an anti-cancer drug isolated from the root bark of Morus alba. Morusin induces apoptosis in human prostate cancer cells by reducing STAT3 activity. In this study, we examined whether morusin induces autophagy and also examined the effects of autophagy on the morusin-induced apoptosis. Morusin induces LC3-II accumulation and ULK1 activation in HeLa cells. In addition, we found that induction of ULK1 Ser317 phosphorylation and reduction of ULK1 Ser757 phosphorylation occurred simultaneously during morusin-induced autophagy. Consistently, morusin induces autophagy by activation of AMPK and inhibition of mTOR activity. Next, we investigated the role of autophagy in morusin-induced apoptosis. Inhibition of autophagy by treating cells with the 3-methyladenine (3-MA) autophagic inhibitor induces high levels of morusin-mediated apoptosis, while treatment of cells with morusin alone induces moderate levels of apoptosis. Cell survival was greatly reduced when cells were treated with morusin and 3-MA. Taken together, morusin induces autophagy, which is an impediment for morusin-induced apoptosis, suggesting combined treatment of morusin with an autophagic inhibitor would increase the efficacy of morusin as an anti-cancer drug.

SERBP1 affects homologous recombination-mediated DNA repair by regulation of CtIP translation during S phase.

DNA double-strand breaks (DSBs) are the most severe type of DNA damage and are primarily repaired by non-homologous end joining (NHEJ) and homologous recombination (HR) in the G1 and S/G2 phase, respectively. Although CtBP-interacting protein (CtIP) is crucial in DNA end resection during HR following DSBs, little is known about how CtIP levels increase in an S phase-specific manner. Here, we show that Serpine mRNA binding protein 1 (SERBP1) regulates CtIP expression at the translational level in S phase. In response to camptothecin-mediated DNA DSBs, CHK1 and RPA2 phosphorylation, which are hallmarks of HR activation, was abrogated in SERBP1-depleted cells. We identified CtIP mRNA as a binding target of SERBP1 using RNA immunoprecipitation-coupled RNA sequencing, and confirmed SERBP1 binding to CtIP mRNA in S phase. SERBP1 depletion resulted in reduction of polysome-associated CtIP mRNA and concomitant loss of CtIP expression in S phase. These effects were reversed by reconstituting cells with wild-type SERBP1, but not by SERBP1 ΔRGG, an RNA binding defective mutant, suggesting regulation of CtIP translation by SERBP1 association with CtIP mRNA. These results indicate that SERBP1 affects HR-mediated DNA repair in response to DNA DSBs by regulation of CtIP translation in S phase.

WIP1, a homeostatic regulator of the DNA damage response, is targeted by HIPK2 for phosphorylation and degradation.

WIP1 (wild-type p53-induced phosphatase 1) functions as a homeostatic regulator of the ataxia telangiectasia mutated (ATM)-mediated signaling pathway in response to ionizing radiation (IR). Here we identify homeodomain-interacting protein kinase 2 (HIPK2) as a protein kinase that targets WIP1 for phosphorylation and proteasomal degradation. In unstressed cells, WIP1 is constitutively phosphorylated by HIPK2 and maintained at a low level by proteasomal degradation. In response to IR, ATM-dependent AMPKα2-mediated HIPK2 phosphorylation promotes inhibition of WIP1 phosphorylation through dissociation of WIP1 from HIPK2, followed by stabilization of WIP1 for termination of the ATM-mediated double-strand break (DSB) signaling cascade. Notably, HIPK2 depletion impairs IR-induced γ-H2AX foci formation, cell-cycle checkpoint activation, and DNA repair signaling, and the survival rate of hipk2+/- mice upon γ-irradiation is markedly reduced compared to wild-type mice. Taken together, HIPK2 plays a critical role in the initiation of DSB repair signaling by controlling WIP1 levels in response to IR.