α/β subdomains, and a core β-ladder domain that contributes nine rungs to the extended central β-sheet.The connector subdomain and β-roll form a hydrophobic surface that extends from the dimer and is a good candidate for membrane interactions.Mutations within the connector subdomain permitted liposome remodeling but affected viral replication, suggesting that interactions with the viral replicative complex were compromised.NS1 crystallized as trimer of dimers, and the resulting hexamer fits well with two-dimensional EM class averages of NS1.In the hexamer, the β-rolls line an interior central cavity, and antibody epitopes map to the exterior surface in the wing and β-ladder domains.Although the structures reveal distinct regions for membrane association and immune system interactions, insight into how these features contribute to pathogenicity await further study.

Na+/H+ antiporters are found in the plasma membrane of every cell and are essential for maintaining pH and sodium levels within the cell as well as cell volume. Similarly to other secondary active transporters, Na+/H+ antiporters are believed to operate through an alternating-access mechanism of transport. Many Na+/H+ antiporters are…

Topoisomerase I (TOP1) relaxes DNA supercoiling, permitting the progression of replication forks in proliferating cells. The TOP1 inhibitor camptothecin (CPT) and its derivatives exhibit toxicity toward replicating cancer cells by trapping the TOP1 cleavage complex on DNA, resulting in stalled forks and eventually double-stranded breaks (DSBs). Although the heterodimeric nuclease complex Mus81– Eme1 has been shown to play a role in replication fork rescue and homologous recombination, whether it functions in processing stalled forks due to blocked TOP1 was unclear. Now Pommier and colleagues have shown that Mus81 is involved in alleviating the toxicity of TOP1 inhibition. Cells lacking Mus81 were more sensitive to CPT, but surprisingly, instead of excising inhibited TOP1 from DNA, Mus81 promoted DSB formation at replication foci. The generation of DSBs correlated with Chk2 phosphorylation, indicating activation of the DNA damage response, which would promote repair of the DSBs. These Mus81 effects were specific for replicating cells, and DNA combing of CPT-treated cells showed that Mus81 helps to partially restore the velocity of stalled replication forks. As TOP1 inhibition is known to increase supercoiling of DNA ahead of replication forks, the authors propose that one role of Mus81 is to alleviate the supercoiling, allowing stalled forks to proceed and helping cells to withstand such genotoxic insults. (J. Cell Biol. 195, 739–749, 2011) SM

Small rnA-mediated DnA repairThe repair of DNA double-strand breaks (DSBs) is a complex, tightly regulated process.Small noncoding RNAs, such as miRNAs and siRNAs, play many diverse regulatory roles, but their functions in DSB repair have remained largely unexplored.Qi and colleagues have now examined the repair of Arabidopsis thaliana DSBs and report that small RNAs of ~21 nucleotides in length, derived from both sense and antisense sequences flanking DSBs, are upregulated following DNA damage.Termed DSB-induced small RNAs (diRNAs), their production depends on the kinase ATR, which promotes DNA repair in response to single-stranded DNA, and RNA polymerase IV, which transcribes siRNAs in plants.RNA-dependent RNA polymerases that produce double-stranded RNA (dsRNA) from single-stranded RNA are also necessary for diRNA production, as are Dicer-like proteins, which cleave dsRNAs into shorter fragments.Argonaute 2 (AGO2), a component of the RNA-induced silencing complex, binds mature diRNAs, and loss of AGO2 significantly reduces diRNA levels and impedes DSB repair.However, diRNA expression does not affect RNA-directed DNA methylation, histone H2AX phosphorylation, or the levels of key DNA repair proteins, so the mechanism for their effects on DNA repair is still unclear.Importantly, diRNAs were also detected in human cells, where DSB repair depends on Dicer and AGO2.This suggests that diRNAs have an evolutionarily conserved role in DNA repair, and the authors propose their function may be to recruit specific repair factors.

cellular membranes.The VLPs could induce an interferon response in both mouse and human cells without causing an inflammatory response.Using VLPs that lacked specific envelope glycoproteins, the authors established that viral membrane fusion is required to induce the interferon response.Other membrane-fusion events, such as cell-cell fusion or cationic liposome fusion, could cause a similar response.Using liposomes with different lipid composition, the authors could see a direct correlation between fusogenic potential and the level of cellular responses.They next investigated the signaling events involved in the cellular response to membrane fusion.Cells from STING-deficient mice did not show interferon responses

Like many other viruses, adenovirus undergoes replication inside the host cell nucleus. The viral genome is packaged within a capsid that protects it from degradation and from detection by cellular defense systems in the cytoplasm, but the capsid needs to be disassembled before replication can occur. The viral capsid is far too large to pass through the nuclear pore, and how exactly the viral genome gets uncoated from the capsid and enters the nucleus is not clear. Now Greber and colleagues report that adenoviruses use a cellular motor, kinesin-1, to facilitate both processes. Using fluorescence and transmission electron microscopy analyses of adenovirus-infected cells, the authors observed that the disassembled viral capsids are associated with nucleoporins at the cytoplasm periphery. Because kinesin-1 is involved in anterograde transport of cellular and viral cargos, the authors tested kinesin-1 components and found that the light chains Klc1 and Klc2 and heavy chain Kif5C associate with adenoviral capsids and are required for their disassembly at the nuclear envelope and hence for viral infection. Kif5C was previously known to interact with the nuclear pore, resulting in activation of the ATP turnover by Kif5C in the presence of microtubules. Although the interaction of Kif5C with the nuclear pore complex was required for viral infection, it was not necessary for capsid association with the nuclear envelope. The specific interactions of viral capsid proteins with kinesin-1 and with nuclear pore components were mapped. The action of kinesin-1 on viral capsids docked to the nuclear pore resulted in disruption of the nuclear pore complex. The authors propose a model whereby kinesin-1 motor activity pulls apart the viral capsid; this separates the capsid from the genome and disrupts the nuclear pore complex, thereby facilitating entry of the adenoviral DNA into the nucleus. (Cell Host Microbe 10, 210–223, 2011) IC