Weaving a Cellular Web the Replication Connection

DNA REPAIR The replication connection The Mre11 complex — containing Mre11, Rad50 and Nbs1 — promotes the repair of double-strand DNA breaks (DSBs). One way to do this is by homologous recombination — a process that is intrinsically linked to DNA replication. Because of this connection, the Mre11 complex has been implicated in replication, and two new papers support this idea. CELL POLARITY Jean Gautier and colleagues report in Molecular Cell the The authors conclude that the cloning and purification of the Xenopus Mre11 homologue. SAPK pathway is involved in control- They show that X-Mre11 is phosphorylated in response to Weaving a ling this actin-dependent mitotic DSBs, and that this modification stimulates the 3′–5′ checkpoint, which probably “ensures exonuclease activity of the X-Mre11 complex (the function of cellular web mitotic spindles are properly oriented which is currently a mystery). before anaphase is allowed to take Gautier and colleagues asked whether X-Mre11 is The intricate web of protein–protein place”. This checkpoint might be phosphorylated in the absence of DSBs, using purified sperm interactions in the cell is now reveal- especially important during the nuclei that had been incubated in extracts to allow chromatin ing its secrets to us, due to the devel- development of multicellular organ- assembly and DNA replication. The authors detected opment of high-throughput two- isms, when the establishment of phosphorylated X-Mre11 in the nuclear fraction during hybrid screening. Reporting in the asymmetric cell fates can depend on replication, indicating that active X-Mre11 can associate with Journal of Cell Biology, Drees and col- the orientation of the spindle during chromatin during normal DNA synthesis. They then used leagues now describe the results of mitosis. several techniques to show that DSBs arise during replication, their screen for cell polarity proteins. Alison Mitchell and that they accumulate when Mre11 is not present. The To target this type of protein, References and links implication, they say, is that DSBs arise during normal Drees and colleagues used as bait 68 ORIGINAL RESEARCH PAPER Gachet, Y. et al. replication and that the X-Mre11 complex is involved in their proteins known to mediate cell polar- A MAP kinase-dependent actin checkpoint ensures proper spindle orientation in fission yeast. repair. ity, ranging from Rho-type GTPases Nature 412, 352–355 (2001) In Molecular and Cellular Biology, John Petrini and to proteins involved in secretion. The FURTHER READING Nakaseko, Y. & Yanagida. M. Cytoskeleton in the cell cycle. Nature 412, colleagues describe their investigations into the Nbs1 result was 128 new protein–protein 291–292 (2001) component of the Mre11 complex. Phosphorylation of Nbs1 is interactions, 44 of which involve pro- required for activation of the S-phase checkpoint, which stalls teins of unknown function. So what replication in response to DNA damage. However, cells does this tell us about how the polari- This serine lies in a consensus containing an aberrant Nbs1 that lacks the amino terminus ty factors are weaved together? phosphorylation site for cannot activate the S-phase checkpoint, even when One important outcome is that calcium–calmodulin-dependent phosphorylated. new connections were made, both protein kinase II (CaMKII), and Given that the amino-terminal region of Nbs1 seems to be between signalling pathways — for purified brain CaMKII indeed essential for checkpoint activation, Petrini and colleagues example, the Rho1 and Cdc42 effector phosphorylates myosin-V at this investigated it further using a two-hybrid screen and discovered pathways — and between distinct site. Moreover, inhibitors of that it interacts with the transcription factor E2F1. Within 400 processes, such as the assembly of CaMKII block myosin-V base pairs of the Epstein–Barr virus latent origin of replication actin and the morphogenesis check- phosphorylation during mitosis, (oriP) are two E2F-binding sites, and the authors confirmed not point. To confirm some of these inter- preventing the release of the motor from melanosomes. only that E2F–Nbs1 binds near oriP, but also that binding actions, the authors fused factors to So it seems that CaMKII is increased as cells progressed through S phase. Finally they yellow fluorescent protein and looked responsible for the specific release showed that the Mre11 complex co-localizes with DNA at their subcellular localization. of myosin-V from melanosomes replication foci containing a replication protein — the Given that many of these factors during mitosis. It will be equally proliferating cell nuclear antigen (PCNA) — throughout S have mammalian homologues, these interesting to next find out how phase. These results support the idea that the Mre11 complex interactions have implications for these organelles recruit their influences the progression of S phase by acting not only at polarity in diverse cell types, but motor again at the end of mitosis replication origins (through the interaction with E2F1) but also there is still much to learn. The next to resume their place in traffic. at the replication fork. step will be to use genetic and bio- Raluca Gagescu Alison Mitchell chemical tools to ask when and where these factors meet in the cell References and links References and links and how the information flows ORIGINAL RESEARCH PAPER Karcher, R. ORIGINAL RESEARCH PAPERS Costanzo, V. et al. Mre11 protein complex prevents L. et al. Cell cycle regulation of myosin-V by double-strand break accumulation during chromosomal DNA replication. Mol. Cell 8, through this complex web. calcium–calmodulin-dependent protein 137–147 (2001) | Maser, R. S. et al. Mre11 complex and DNA replication: linkage to E2F Alison Schuldt, and sites of DNA synthesis. Mol. Cell. Biol. 21, 6006–6016 (2001) kinase II. Science 293, 1317–1320 (2001) Associate Editor, Nature Cell Biology FURTHER READING Rogers, S. L. et al. FURTHER READING D’Amours, D. & Jackson, S. P. The yeast XRS2 complex functions Regulation of melanosome movement in the in S phase checkpoint regulation. Genes Dev. (in the press) | Hopfner, K. P. et al. Structural References and links cell cycle by reversible association with biochemistry and interaction architecture of the DNA double-strand break repair Mrell ORIGINAL RESEARCH PAPER Drees, B. L. et al. myosin V. J. Cell Biol. 146, 1265–1276 (1999) nuclease and Rad50 ATPase. Cell 105, 473–485 (2001) A protein interaction map for cell polarity WEB SITE Gautier lab | Petrini lab development. J. Cell Biol. 154, 549–571 (2001)