DNA damage-induced histone H1 ubiquitylation is mediated by HUWE1 and stimulates the RNF8-RNF168 pathway

Drosophila HUWE1 Ubiquitin Ligase Regulates Endoreplication and Antagonizes JNK Signaling During Salivary Gland Development

The HECT-type ubiquitin ligase HECT, UBA and WWE Domain Containing 1, (HUWE1) regulates key cancer-related pathways, including the Myc oncogene. It affects cell proliferation, stress and immune signaling, mitochondria homeostasis, and cell death. HUWE1 is evolutionarily conserved from Caenorhabditis elegance to Drosophila melanogaster and Humans. Here, we report that the Drosophila ortholog, dHUWE1 (CG8184), is an essential gene whose loss results in embryonic lethality and whose tissue-specific disruption establishes its regulatory role in larval salivary gland development. dHUWE1 is essential for endoreplication of salivary gland cells and its knockdown results in the inability of these cells to replicate DNA. Remarkably, dHUWE1 is a survival factor that prevents premature activation of JNK signaling, thus preventing the disintegration of the salivary gland, which occurs physiologically during pupal stages. This function of dHUWE1 is general, as its inhibitory effect is observed also during eye development and at the organismal level. Epistatic studies revealed that the loss of dHUWE1 is compensated by dMyc proeitn expression or the loss of dmP53. dHUWE1 is therefore a conserved survival factor that regulates organ formation during Drosophila development.

PARP1-dependent recruitment of the FBXL10-RNF68-RNF2 ubiquitin ligase to sites of DNA damage controls H2A.Z loading

The mammalian FBXL10-RNF68-RNF2 ubiquitin ligase complex (FRRUC) mono-ubiquitylates H2A at Lys119 to repress transcription in unstressed cells. We found that the FRRUC is rapidly and transiently recruited to sites of DNA damage in a PARP1- and TIMELESS-dependent manner to promote mono-ubiquitylation of H2A at Lys119, a local decrease of H2A levels, and an increase of H2A.Z incorporation. Both the FRRUC and H2A.Z promote transcriptional repression, double strand break signaling, and homologous recombination repair (HRR). All these events require both the presence and activity of the FRRUC. Moreover, the FRRUC and its activity are required for the proper recruitment of BMI1-RNF2 and MEL18-RNF2, two other ubiquitin ligases that mono-ubiquitylate Lys119 in H2A upon genotoxic stress. Notably, whereas H2A.Z is not required for H2A mono-ubiquitylation, impairment of the latter results in the inhibition of H2A.Z incorporation. We propose that the recruitment of the FRRUC represents an early and critical regulatory step in HRR.

CRL4BRBBP7 targets HUWE1 for ubiquitination and proteasomal degradation

The E3 ubiquitin ligase HUWE1/Mule/ARF-BP1 plays an important role in diverse biological processes including DNA damage repair and apoptosis. Our previous study has shown that in response to DNA damage HUWE1 was downregulated in CUL4B-mediated ubiquitination and subsequent proteasomal degradation, and CUL4B-mediated regulation of HUWE1 was important for cell survival upon DNA damage. CUL4B is a core component of the CUL4B Ring ligase complexes containing ROC1, DDB1 and a DDB1-Cullin Associated Factors (DCAFs), the latter of which are DDB1-binding WD40 adaptors critical for substrate recognition and recruitment. However, the identity of DCAF in CRL4B that mediates degradation of HUWE1 remains elusive. Here we report that RBBP7 is the DCAF in the CRL4B complex bridging the DDB1-CUL4B-ROC1 to HUWE1. Loading of HUWE1 to the E3 ubiquitin ligase complex resulted in its polyubiquitination, and consequently its proteasome mediated degradation. Overexpression of RBBP7 promoted HUWE1 protein degradation, while depletion of RBBP7 stabilized HUWE1, and hence accelerated the degradation of MCL-1 and BRCA1, two substrates of HUWE1 that are critical in apoptosis and DNA damage repair. Taken together, these data reveal CRL4B^RBBP7 is the E3 ligase responsible for the proteasomal degradation of HUWE1, and further provide a potential strategy for cancer therapy by targeting HUWE1 and the CUL4B E3 ligase complex.