Lamellae lead the way

Lamellae lead the way pen any cell biology text and it is clear that lamellipodia, the ruffled edges of motile cells, initiate movement. So it may come as a surprise that Gupton et al. can disrupt this domain without blocking cell movement (page 619). In fact, cells move faster without lamellipodia. The driving force for movement appears to come from the lamella, the structure that lies just proximal to the lamellipodium. Previously, the team examined the lamella and lamellipodia using quantitative fluorescent speckle microscopy (qFSM), a technique that allows for deconvolution of images of physically overlapping cell domains. The domains have distinct molecular components , filament–assembly kinetics, and motion. Moreover, although the lamellipodia moved forward and backward repeatedly in the axis of cell movement, the team saw hints that cells made forward progress only when the lamella moved forward. To find out if lamellipodia are required for movement, the team injected skeletal muscle tropomyosin into epithelial cells in a wound-healing model. Tropomyosin normally resides in lamellae and is excluded from lamelli-podia. Increasing the amount of tropomyosin O A stalled fork stalls recombination omologous recombination does not rescue stalled replication forks during S phase, report Meister et al. on page 537. Rather they find that replication and recombination are separated in terms of when and where they occur. If inappropriate recombination does occur when replication is stalled, replication forks fall apart. Researchers have debated whether homologous recombination, which is required for double-strand break repair, is also used to resolve stalled replication forks. However, because the Sand G2/M-phase checkpoints share molecular components in budding yeast, the question has been difficult to answer. Now, Meister et al. test the question in fission yeast, where the checkpoints are molecularly distinct and therefore can be separated. When they added hydroxyurea to wild-type cells, thus depleting DNA precursors and stalling replication, the recom-bination protein Rad22 did not aggregate on the chromosomes. However, Rad22 foci did appear on chromosomes in cells deficient for the S-phase checkpoint component cds1. Furthermore, transferring the cells to fresh media rescued wild-type cells, but not cds1 mutants. 2D gel analysis of the replication forks showed that the fork structure decayed in cds1 mutant cells, but maintained normal shapes in H The separation of replication (green) and recombination (red) is lost in cells deficient in the S-phase checkpoint (shown). wild-type cells. In both experiments, cells carrying mutations in the recombination protein rhp51 behaved …