DNA replication

Live-cell imaging reveals replication of individual replicons in eukaryotic replication factories. Kitamura, E. et al. Cell 125, 1297–1308 (2006)

The authors developed a new assay to study the dynamics of DNA replication in single cells using time-lapse microscopy. Their findings support the long-standing hypothesis that sister replication forks that are generated from the same origin remain associated with each other in a replication factory during replication. In addition, the formation of replication factories was found to depend on Cdc6, which is required for DNA replication initiation. The authors therefore propose that replication-factory formation is a consequence of DNA replication.

DNA repair

Histone modification-dependent and -independent pathways for recruitment of checkpoint protein Crb2 to double-strand breaks. Du, L.-L. et al. Dev. Cell 20, 1583–1596 (2006)

Live immunofluorescence-microscopy studies were combined with genetic analysis to investigate the role of the Schizosaccharomyces pombe protein Crb2, a mediator of the DNA-damage response. Du and co-workers show that Crb2 relocalizes to sites of DNA double-strand breaks and that this relocalization requires the modification of histones H2 and H4. However, neither of these modifications is required for Crb2 to carry out its mediator function. A second recruitment mechanism is independent of histone modifications, and such dual recruitment mechanisms are thought to be a common feature of DNA-damage-checkpoint mediators.

Morphogenesis

Coordinated cell-shape changes control epithelial movement in zebrafish and Drosophila. Köppen, M. et al. Development 133, 2671–2681 (2006)

In this study, the authors show that the process of spreading the outer epithelium of the zebrafish embryo over the yolk cell surface and the underlying cells requires the localized recruitment of actin and myosin-2 in the yolk cytoplasm. This process also requires the Ste20-like kinase Msn1, an orthologue of Drosophila melanogaster Misshapen. Similarly, these three proteins are needed during dorsal closure in D. melanogaster. Together, these data hint at the existence of a conserved mechanism that coordinates cell-shape changes during epithelial morphogenesis.

Nuclear Transport

Flexible phenylalanine–glycine nucleoporins as entropic barriers to nucleocytoplasmic transport. Lim, R. Y. H. et al. Proc. Natl Acad. Sci. USA 103, 9512–9517 (2006)

The nature of the selective barrier-gate of nuclear pore complexes (NPCs) is poorly understood. About 30% of the nucleoporins, which make up the NPC, contain a domain that is enriched in Phe–Gly (FG) repeats. Tethering of the FG domain of nucleoporin Nup153 to the surface of so-called nanodots showed that this domain is unfolded and flexible. Atomic force microscopy analysis showed that it forms brush-like conformations, which function as a repulsive entropic barrier at the nuclear pore.