The heterodimeric ubiquitin conjugating enzyme (E2) UBC13-UEV mediates polyubiquitylation through lysine 63 of ubiquitin (K63), rather than lysine 48 (K48). This modification does not target proteins for proteasome-dependent degradation. Searching for potential regulators of this variant polyubiquitylation we have identified four proteins, namely RNF8, KIA00675, KF1, and ZNRF2, that interact with UBC13 through their RING finger domains. These domains can recruit, in addition to UBC13, other E2s that mediate canonical (K48) polyubiquitylation. None of these RING finger proteins were known previously to recruit UBC13. For one of these proteins, RNF8, we show its activity as a ubiquitin ligase that elongates chains through either K48 or K63 of ubiquitin, and its nuclear co-localization with UBC13. Thus, our screening reveals new potential regulators of non-canonical polyubiquitylation.

RNF8 is a ubiquitin ligase with a FHA domain near its N terminus, and a RING-finger domain at its C terminus, through which it recruits several ubiquitin-conjugating enzymes. In metazoans, only the mitotic checkpoint regulator CHFR shares this domain architecture. Here we show that RNF8 is a nuclear protein that follows a cellcycle-dependent turnover, reaching its highest levels in mitosis, followed by a strong decline in late mitotic stages. Overexpression of RNF8 caused a delay in cytokinesis and the frequent appearance of aberrant mitotic figures. These effects were dependent on the ubiquitin ligase activity of RNF8, since they were significantly attenuated when a RING-finger mutant, inactive as an E3, was overexpressed. Depletion of RNF8 also caused a delay in the exit from the mitotic arrest induced by nocodazole, associated with a reduced turnover of the APC/C substrate cyclin B1. These observations suggest that RNF8 regulates the rate of exit from mitosis and cytokinesis.

BackgroundSeveral pathways that control cell survival under stress, namely RNF8-dependent DNA damage recognition and repair, PCNA-dependent DNA damage tolerance and activation of NF-κB by extrinsic signals, are regulated by the tagging of key proteins with lysine 63-based polyubiquitylated chains, catalyzed by the conserved ubiquitin conjugating heterodimeric enzyme Ubc13-Uev.Methodology/Principal FindingsBy applying a selection based on in vivo protein-protein interaction assays of compounds from a combinatorial chemical library followed by virtual screening, we have developed small molecules that efficiently antagonize the Ubc13-Uev1 protein-protein interaction, inhibiting the enzymatic activity of the heterodimer. In mammalian cells, they inhibit lysine 63-type polyubiquitylation of PCNA, inhibit activation of NF-κB by TNF-α and sensitize tumor cells to chemotherapeutic agents. One of these compounds significantly inhibited invasiveness, clonogenicity and tumor growth of prostate cancer cells.Conclusions/SignificanceThis is the first development of pharmacological inhibitors of non-canonical polyubiquitylation that show that these compounds produce selective biological effects with potential therapeutic applications.

The CLC gene family encodes Cl(-) channels or Cl(-)/H(+) exchangers. While our understanding of their structure-function relationship has greatly benefited from the crystal structure of bacterial homologues, human inherited diseases and knock-out mice were crucial in deciphering their physiological roles. Several vesicular CLC Cl(-)/H(+) exchangers are expressed in the proximal tubule (PT). ClC-5 mutations cause Dent's disease which is associated with low molecular weight proteinuria and kidney stones. ClC-5 knock-out mice revealed impaired endocytosis as the primary defect in Dent's disease. It extends to receptor-mediated and fluid-phase endocytosis and entails changes in calciotropic hormones that result in kidney stones. No renal functions could be assigned so far to ClC-3 and ClC-4, which are also expressed in PTs. Loss of ClC-7 or its beta-subunit Ostm1 entails lysosomal storage in the PT, in addition to the neuronal lysosomal storage and osteopetrosis that are the hallmarks of ClC-7/Ostm1 loss in mice and men.

No abstract