Pack loose, grow long

MukBs cooperate to loop DNA W hen it comes to corralling DNA, MukB is the little condensin that could, say Yuanbo Cui, Aoya Petrushenko, and Valentin Rybenkov (University of Oklahoma, Norman, OK). Small in size but with a big effect, condensins are believed to stabilize large DNA loops, thereby giving order to an otherwise tangled mess of chromatin. To investigate the mechanics of the E. coli condensin MukB, the authors stretched DNA between a glass capillary and a magnetic bead and monitored MukB-induced DNA condensation. The DNA compacted in a series of discrete steps, indicating that multiple MukBs are at work. The process could be reversed by applying excess force in the opposite direction, but the longer a condensed complex was allowed to sit, the more force was required, suggesting that MukBs link up over time. A small decrease in MukB concentration led to a dramatic decline in condensation rate. The slowed rate was primarily due to a longer lag before condensation began, which indicates that multiple MukBs must fi rst come together in a nucleation step. ATP stimulated this nucleation but had no subsequent effect, and faster hydrolysis did not lead to faster nucleation. ATP thus seems to be a structural, rather than energetic, component. According to Rybenkov, MukBs are acting as clamps to hold loops together. “I think we now have a biochemical explanation for how stable loops appear in such a big unruly molecule as DNA,” he says. Cui, Y., et al. 2008. Nat. Struct. Mol. Biol. doi:10.1038/nsmb.1410. L onger cytoskeletal rods have more empty space between them, and that space helps drive amoeboid cell motility, say Thomas Roberts (Florida State University, Tallahassee, FL) and colleagues. Theories of amoeboid-like cell movement have largely relied on cytoskeleton polymerization to explain membrane protrusion. But in studying fi laments of the worm major sperm protein (MSP) using electron tomography, the authors noted that fi laments became more loosely packed as they elongated. “Polymer physicists have known for a long time that shorter rods pack more tightly,” says Roberts. By contrast, longer rods, regardless of composition, enclose more empty space. Measurements from the new images were consistent with this effect, and simulations confi rmed that growing rods exhibited the same length-dependent space constraints as static ones. An MSP mutant that polymerized 2.4 times more slowly reduced the protrusion rate by 3.4-fold, indicating that both polymerization and packing-based expansion contribute to protrusion. “It’s plausible that this effect may occur in actin systems as well,” says Roberts, although actin-based protrusion differs in its molecular details, including the use of cross-linking proteins.