E4 ligase–specific ubiquitination hubs coordinate DNA double-strand-break repair and apoptosis

Multiple protein ubiquitination events at DNA double-strand breaks (DSBs) regulate damage recognition, signaling and repair. It has remained poorly understood how the repair process of DSBs is coordinated with the apoptotic response. Here, we identified the E4 ubiquitin ligase UFD-2 as a mediator of DNA-damage-induced apoptosis in a genetic screen in Caenorhabditis elegans. We found that, after initiation of homologous recombination by RAD-51, UFD-2 forms foci that contain substrate-processivity factors including the ubiquitin-selective segregase CDC-48 (p97), the deubiquitination enzyme ATX-3 (Ataxin-3) and the proteasome. In the absence of UFD-2, RAD-51 foci persist, and DNA damage-induced apoptosis is prevented. In contrast, UFD-2 foci are retained until recombination intermediates are removed by the Holliday-junction-processing enzymes GEN-1, MUS-81 or XPF-1. Formation of UFD-2 foci also requires proapoptotic CEP-1 (p53) signaling. Our findings establish a central role of UFD-2 in the coordination between the DNA-repair process and the apoptotic response

CUL-2^LRR-1 and UBXN-3 drive replisome disassembly during DNA replication termination and mitosis

Replisome disassembly is the final step of DNA replication in eukaryotes, involving the ubiquitylation and CDC48-dependent dissolution of the CMG helicase (CDC45-MCM-GINS). Using Caenorhabditis elegans early embryos and Xenopus laevis egg extracts, we show that the E3 ligase CUL-2(LRR-1) associates with the replisome and drives ubiquitylation and disassembly of CMG, together with the CDC-48 cofactors UFD-1 and NPL-4. Removal of CMG from chromatin in frog egg extracts requires CUL2 neddylation, and our data identify chromatin recruitment of CUL2(LRR1) as a key regulated step during DNA replication termination. Interestingly, however, CMG persists on chromatin until prophase in worms that lack CUL-2(LRR-1), but is then removed by a mitotic pathway that requires the CDC-48 cofactor UBXN-3, orthologous to the human tumour suppressor FAF1. Partial inactivation of lrr-1 and ubxn-3 leads to synthetic lethality, suggesting future approaches by which a deeper understanding of CMG disassembly in metazoa could be exploited therapeutically

p97 complexes as signal integration hubs

<p>Abstract</p> <p>In the ubiquitin-proteasome system, a subset of ubiquitylated proteins requires the AAA+ ATPase p97 (also known as VCP or Cdc48) for extraction from membranes or protein complexes before delivery to the proteasome for degradation. Diverse ubiquitin adapters are known to link p97 to its client proteins, but two recent papers on the adapter protein UBXD7, including one by Bandau <it>et al</it>. in <it>BMC Biology</it>, suggest that rather than simply linking p97 to ubiquitylated proteins, this adapter may be essential to coordinate ubiquitylation and p97-mediated extraction of the proteasome substrate. These findings add to growing indications of richly diverse roles of adapters in p97-mediated signaling functions.</p> <p>See research article: <url>http://www.biomedcentral.com/1741-7007/10/36</url></p

A comprehensive guide to pilus biogenesis in Gram-negative bacteria

Pili are crucial virulence factors for many Gram-negative pathogens. These surface structures provide bacteria with a link to their external environments by enabling them to interact with, and attach to, host cells, other surfaces or each other, or by providing a conduit for secretion. Recent high-resolution structures of pilus filaments and the machineries that produce them, namely chaperone-usher pili, type IV pili, conjugative type IV secretion pili and type V pili, are beginning to explain some of the intriguing biological properties that pili exhibit, such as the ability of chaperone-usher pili and type IV pili to stretch in response to external forces. By contrast, conjugative pili provide a conduit for the exchange of genetic information, and recent high-resolution structures have revealed an integral association between the pilin subunit and a phospholipid molecule, which may facilitate DNA transport. In addition, progress in the area of cryo-electron tomography has provided a glimpse of the overall architecture of the type IV pilus machinery. In this Review, we examine recent advances in our structural understanding of various Gram-negative pilus systems and discuss their functional implications