Where mtDNA is made

eukocytes have a better chance of sticking to neighboring cells with the right ligands if they rotate on their way by, based on results from Dwir et al. (page 649). Flowing leukocytes are captured via L-selectin interactions with carbohydrate ligands on vessel walls and subsets of other leukocytes. L-selectin is an unusual adhesive molecule—shear stress improves its adherance to ligands, whereas most adhesive bonds are destabilized by shear forces. Previous experiments have shown that L-selectin tethers are not formed below a critical shear threshold. But using new high temporal resolution videomicroscopy, Dwir et al. show that very short-lived bonds, too transient to detect previously, are indeed formed below this threshold. Above the shear threshold, leukocyte tethers were stabilized more than 10-fold. Stabilization was not induced when higher viscosity was used to raise the shear force without raising the flow rate. Shear may stabilize adhesive tethers because initially bound cells can rotate past the substrate fast enough to form additional L-selectin bonds before the first bond is broken. So, although individual bonds may be destabilized by higher shear force, the benefits obtained by multiple bonds are much larger. ᭿ L Rotation favors the formation of multiple L-selectin bonds. n page 661, Holmbeck et al. identify a new mechanism of cartilage remodeling. This quick remodeling system bypasses time-consuming steps to bone formation that are required in the previously known pathway. In the well-known pathway to bone formation, a cartilage scaffold must be mineralized before it is degraded by osteoclasts and replaced with bone. But some bones seem to be formed without cartilage mineralization. Holmbeck et al. find that this process relies on the matrix metalloprotease MT1-MMP, which degrades unmineralized cartilage. The authors examined bone formation in MT1-MMP–deficient mice, which have skulls that are misshapen by cartilage. They find that in wild-type mice this same cartilage is not mineralized, but rather expresses MT1-MMP before O Where mtDNA is made n page 503, Meeusen and Nunnari show that yeast mito-chondria harbor self-sustaining DNA replication factories. The mitochondrial genome (mtDNA) is packaged into nucleoids, some of which attach to the mitochondrial membrane at sites that contain the outer membrane protein Mmm1. Mmm1 binds to the actin cytoskeleton, and mitochondrial movement depends on actin, so one obvious hypothesis is that this DNA–protein structure segregates mtDNA into buds. But Meeusen and Nunnari suggest that Mmm1 and associated proteins replicate, rather than actively segregate, the genome. The authors …