Cohesin gets pushed around

Cohesin gets pushed around ings of cohesin hold together sister chromatids. Armelle and colleagues now suggest that cohesin rings get pushed to their final resting places by transcription complexes. Cohesins are known to bind a heterochromatin protein at centromeres, but previous reports of cohesin localization on chromosome arms were incomplete. The authors did a comprehensive survey of four budding yeast chromosomes and two fission yeast chromosomes using chromatin immuno-precipitation. In budding yeast, 91% of cohesin sites were between converging genes, and cohesins were bound to 84% of the 328 convergent intergene regions. MO) and colleagues gathered similar data for budding yeast. They suggest that transcription displaces cohesin from the DNA of active genes, and that the chromatin in convergent intergenic regions may be a stickier substrate for cohesin. R Cohesin binds more (black) between convergent genes but less (gray) in the middle of active genes. Flat-out pushing by actin amellipodia are almost flat, whereas many model systems for actin-based propulsion involve the convex surfaces of beads, vesicles, or bacteria such as Listeria. Thus, it is reassuring that actin pushing against a flat surface can lead to productive movement even in these model systems, as demonstrated by Ian Schwartz, James McGrath, and colleagues (University of Rochester, NY) using squashed beads. Actin at the leading edge of moving cells has been suggested to work as a tethered ratchet. Thermal fluctuations lead to the bending of actin filaments away from the cell surface, freeing them for lengthening by polymerization. As these bent, and thus strained, longer filaments relax by straightening, they exert a forward force on the cell surface. Bead-based experiments have, by contrast, led to the elastic propulsion model. In this L But Uhlmann's group favors the sliding model. Looking earlier, they saw that cohesins initially load at sites defined by the loading protein Scc2. These sites are most often in highly transcribed regions, but soon after loading the cohesin moves toward the intergenic regions. This would get the cohesin away from the loading proteins, which may have an opening activity that can also promote unloading. Cohesin that was seen in the middle of several dormant genes later moved downstream when the genes were turned on during either meiosis or heat shock. Passive pushing is conceivable, says Uhlmann, because proteins bound to RNA polymerase and its nascent transcript " make the transcription machinery quite enormous. " Pushing would prevent transcripts from getting …